Category Archives: AC & Thermal Management

Fault diagnosis

Cooling performance check

Every workshop needs, besides special tools, technical knowledge to do a proffesional job. This can be purchased by special training. This applies particularly to air-conditioning systems. The following instructions can merely serve as guide due to the different systems in use.

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It is very important to correctly interpret the pressure gauge reading. Some typical examples:

Screen Shot 2017-05-18 at 16.42.32

Ambient temp= surrounding environment temperature.
Tests should not be carried out in extreme cold or warm environmental conditions.

Compressors without magnetic clutches

General

Screen Shot 2017-05-18 at 15.27.04For some years, so-called clutchless, externally triggered, variable compressors have been used (Fig. 1). All well-known compressor manufacturers use a great variety of different basic types. Here is a list of the most common types on the market: Denso, with the types 6SEU & 7SEU, Sanden, with the types PXE 13 & PXE 16. However, Delphi/Harrison is also represented via the CVC7 series, the design of which is very similar to the V5 compressor. This generation of compressors is used by almost all vehicle manufacturers (Audi, BMW, Citroen, Seat, VW, Opel). Externally triggered means that the stroke volume of the compressor is determined by a built-in control valve that is triggered by the air-conditioning control unit, depending on a wide variety of system parameters such as outside/desired temperature, high/low pressure, rotational speed, engine load. Clutchless means that the compressor no longer has an electromagnetic clutch. This means that the compressor is permanently driven via the pulley and works even if the air-conditioning system is switched off. However, in relation to this, the power is regulated down to a few percent.

Screen Shot 2017-05-18 at 15.28.06Operation

The pulley unit of the compressor consists, for example, of a drive plate and the actual pulley (drawing). The drive plate consists of a rubber element and forms the connection between the pulley and the compressor shaft. It acts as a vibration damper and also protects the compressor or the other driven units from overload or damage. If the compressor should lock up for example, the transmission forces between the pulley and the drive plate increase considerably in the area of the rubber element.

Depending on compressor manufacturer or type, the connection is interrupted by the deformation of the rubber element or by the triggering of an overload protection . The pulley then simply runs freely. This prevents damage to the belt or other units driven by the belt.Screen Shot 2017-05-18 at 15.36.42

The control valve (Fig. 2) is located in the compressor and receives its pulse-width modulated signals (PWM) from the air-conditioning control unit. The current which is fed from the control unit to the control valve, and which ultimately determines the power of the compressor, can be displayed as a measured-value block with the aid of diagnosis equipment. Clutchless compressors also have a safety valve (Fig. 3), which is intended to protect the compressor and the other
components of the air-conditioning system from excessive pressure. The valve usually triggers at a pressure between 35 and 45 bar (depending on the compressor manufacturer). The valve opens only until the overpressure has reduced. Afterwards, it closes again, so as not to release the entire quantity of refrigerant into the atmosphere. If the film of the valve is damaged, it can be assumed that the valve has triggeredScreen Shot 2017-05-18 at 15.37.23

Diagnosis

The pulleys and their rubber elements that are designed as overload protection are,
depending on the compressor type, designed differently. Depending on the type, it is possible in different ways to determine whether or not the overload protection has
been triggered:

  • Rubber abrasion parts are visible on the inside of the pulley (Fig. 4). The compressor shaft is no longer driven. Provided that the compressor can be turned easily, the pulley and the rubber element can be replaced.Screen Shot 2017-05-18 at 15.39.23
  • The over load protection has triggered the drive plate (Fig. 5). The drive plate and the rubber element can be replaced individually.
    Prerequisite: The compressor can be turned easily.Screen Shot 2017-05-18 at 15.39.58
  • A triggered torque limiter cannot necessarily be detected visually. In order to check whether the limiter has triggered, the compressor shaft must be held using a suitable tool (Fig. 6), and, at the same time, the pulley must be turned to the left. If the pulley can be turned to the left, the limiter has triggered and the compressor must be replaced. In the case of compressor types Sanden PXE 13 and PXE 16, replacement of the torque limiter is not possible.

Screen Shot 2017-05-18 at 15.45.22
In the case of the Audi A3 for example, at the lowest temperature setting, the maximum current that is fed from the control unit to the control valve is approximately 0.65 A. In relation to this, the compressor reaches its maximum power. In regulating operation, an average current of 0.3 A flows. In the case of newer vehicles, however, the problem is that diagnosis outside the engine-management area is not yet possible with many testers. Here, ideally, the use of an oscilloscope is helpful. With the aid of suitable test peaks, the PWM signal at the plug connection of the compressor can be recorded. In relation to this, the oscilloscope should be set to 5 V/div and 0.5 ms/div. With the engine running, the individual modes of operation can now be depicted on the screen of the oscilloscope. In the case of the lowest temperature setting ( Lo ) a square wave signal with a duty factor of approximately  75% (Fig. 7) is shown. The duty factor results from the ratio of pulse width -B- and the signal distance -C- (in this case 75% operating time, 25% switch-off time). At the same time, a readout of the level of the vehicle-electric-system voltage (approximately 13.5 V) can be taken on the basis of the volt divisions (A=5 V). The voltage value(9.8 V) displayed as a number is simply an average value. The pulse width depends on the desired cooling performance and the vehicle-electric-system voltage. Over the distance of the area -B-, the current to the control valve from the control unit is regulated . Depending on the setting of the control panel and ambient influences (e.g. outside temperature) the pulse width of the square wave signal is changed in such a way, or the control valve is triggered in such a way, that the compressor power necessary to reach the desired temperature is produced. Fig. 8 shows how, in the case of the temperature setting High, the compressor is regulated downwards. Fig. 9 was recorded in Econ operation (compressor off) and shows no signal. On the basis of these methods, it can be determined to what extent asignal change takes places through the control unit. If a plausible change of the signals takes place, but no change in the blow temperature or lowering of the interior temperature results, the compressor is probably faulty.Screen Shot 2017-05-18 at 15.45.52

Screen Shot 2017-05-18 at 15.46.34
In addition, there is diagnosis equipment on the market with which it is possible to generate a PWM signal with a different pulse duration. Thus, it is possible to determine whether a triggering of the compressor leads to a change in the refrigerant pressure. This serves as the basis for being able to make a statement as to what extent the compressor still functions perfectly.Screen Shot 2017-05-18 at 15.47.30

A functional check by means of a PWM signal can also be carried out using a function generator (Fig. 10). For this, it is however essential to connect a load which corresponds to that of an electronic control valve to the control-unit side of the air-conditioning system. Otherwise, the control unit will detect a fault in the system and will store this in the fault memory. This can lead to malfunctions or to failure of the system. In this case, a readout of the fault memory must be taken and
deleted using diagnosis equipment.

Screen Shot 2017-05-18 at 15.47.51
Again and again, complaints about compressors are hastily filed in connection with noises and other problems of air- conditioning systems. In very many cases, it turns out that the compressor  is alright, or that the cause of the fault does not stem from the compressor itself. For this reason, all components of the system should always be included when troubleshooting. Noises are caused not only by the compressor, but also by its fastening, the drive, the expansion valve or the lines. An incorrect quantity of refrigerant can also be responsible for various noises. In relation to this, there is the separate technical information. Noise development. The technical information Compressor damage provides information about different types of compressor damage and their causes.
In addition, the oil provides important information about possible damage:

  • If the oil in the compressor has taken on a red colour, this may be attributable to much moisture.
  • Oil with a black colour points to a defective compressor.
  • Silver-grey oil should be checked for metal swarf. The greyish discoloration points to metal abrasion.

As the system oil quantities become ever lower (sometimes only 80 ml), the monitoring and observance of the oil quantity is of great importance (e.g. during air-conditioning servicing and when replacing components.

The repair of clutchless compressors is possible only to a limited extent. It is essential that suitable tools and repairinformation are used when carrying out such a repair. Of course, assessment of the system pressures is very important when carrying out diagnosis work. In relation to this, the specified values of the vehicle manufacturer should be consulted.

This also applies to the blow temperature. Guidelines in relation to the assessment of system pressures can be taken from the table below:
Screen Shot 2017-05-18 at 15.56.31

Compressors without magnetic clutches

General

Screen Shot 2017-05-18 at 15.27.04For some years, so-called clutchless, externally triggered, variable compressors have been used (Fig. 1). All well-known compressor manufacturers use a great variety of different basic types. Here is a list of the most common types on the market: Denso, with the types 6SEU & 7SEU, Sanden, with the types PXE 13 & PXE 16. However, Delphi/Harrison is also represented via the CVC7 series, the design of which is very similar to the V5 compressor. This generation of compressors is used by almost all vehicle manufacturers (Audi, BMW, Citroen, Seat, VW, Opel). Externally triggered means that the stroke volume of the compressor is determined by a built-in control valve that is triggered by the air-conditioning control unit, depending on a wide variety of system parameters such as outside/desired temperature, high/low pressure, rotational speed, engine load. Clutchless means that the compressor no longer has an electromagnetic clutch. This means that the compressor is permanently driven via the pulley and works even if the air-conditioning system is switched off. However, in relation to this, the power is regulated down to a few percent.

Screen Shot 2017-05-18 at 15.28.06Operation

The pulley unit of the compressor consists, for example, of a drive plate and the actual pulley (drawing). The drive plate consists of a rubber element and forms the connection between the pulley and the compressor shaft. It acts as a vibration damper and also protects the compressor or the other driven units from overload or damage. If the compressor should lock up for example, the transmission forces between the pulley and the drive plate increase considerably in the area of the rubber element.

Depending on compressor manufacturer or type, the connection is interrupted by the deformation of the rubber element or by the triggering of an overload protection . The pulley then simply runs freely. This prevents damage to the belt or other units driven by the belt.Screen Shot 2017-05-18 at 15.36.42

The control valve (Fig. 2) is located in the compressor and receives its pulse-width modulated signals (PWM) from the air-conditioning control unit. The current which is fed from the control unit to the control valve, and which ultimately determines the power of the compressor, can be displayed as a measured-value block with the aid of diagnosis equipment. Clutchless compressors also have a safety valve (Fig. 3), which is intended to protect the compressor and the other
components of the air-conditioning system from excessive pressure. The valve usually triggers at a pressure between 35 and 45 bar (depending on the compressor manufacturer). The valve opens only until the overpressure has reduced. Afterwards, it closes again, so as not to release the entire quantity of refrigerant into the atmosphere. If the film of the valve is damaged, it can be assumed that the valve has triggeredScreen Shot 2017-05-18 at 15.37.23

Diagnosis

The pulleys and their rubber elements that are designed as overload protection are,
depending on the compressor type, designed differently. Depending on the type, it is possible in different ways to determine whether or not the overload protection has
been triggered:

  • Rubber abrasion parts are visible on the inside of the pulley (Fig. 4). The compressor shaft is no longer driven. Provided that the compressor can be turned easily, the pulley and the rubber element can be replaced.Screen Shot 2017-05-18 at 15.39.23
  • The over load protection has triggered the drive plate (Fig. 5). The drive plate and the rubber element can be replaced individually.
    Prerequisite: The compressor can be turned easily.Screen Shot 2017-05-18 at 15.39.58
  • A triggered torque limiter cannot necessarily be detected visually. In order to check whether the limiter has triggered, the compressor shaft must be held using a suitable tool (Fig. 6), and, at the same time, the pulley must be turned to the left. If the pulley can be turned to the left, the limiter has triggered and the compressor must be replaced. In the case of compressor types Sanden PXE 13 and PXE 16, replacement of the torque limiter is not possible.

Screen Shot 2017-05-18 at 15.45.22
In the case of the Audi A3 for example, at the lowest temperature setting, the maximum current that is fed from the control unit to the control valve is approximately 0.65 A. In relation to this, the compressor reaches its maximum power. In regulating operation, an average current of 0.3 A flows. In the case of newer vehicles, however, the problem is that diagnosis outside the engine-management area is not yet possible with many testers. Here, ideally, the use of an oscilloscope is helpful. With the aid of suitable test peaks, the PWM signal at the plug connection of the compressor can be recorded. In relation to this, the oscilloscope should be set to 5 V/div and 0.5 ms/div. With the engine running, the individual modes of operation can now be depicted on the screen of the oscilloscope. In the case of the lowest temperature setting ( Lo ) a square wave signal with a duty factor of approximately  75% (Fig. 7) is shown. The duty factor results from the ratio of pulse width -B- and the signal distance -C- (in this case 75% operating time, 25% switch-off time). At the same time, a readout of the level of the vehicle-electric-system voltage (approximately 13.5 V) can be taken on the basis of the volt divisions (A=5 V). The voltage value(9.8 V) displayed as a number is simply an average value. The pulse width depends on the desired cooling performance and the vehicle-electric-system voltage. Over the distance of the area -B-, the current to the control valve from the control unit is regulated . Depending on the setting of the control panel and ambient influences (e.g. outside temperature) the pulse width of the square wave signal is changed in such a way, or the control valve is triggered in such a way, that the compressor power necessary to reach the desired temperature is produced. Fig. 8 shows how, in the case of the temperature setting High, the compressor is regulated downwards. Fig. 9 was recorded in Econ operation (compressor off) and shows no signal. On the basis of these methods, it can be determined to what extent asignal change takes places through the control unit. If a plausible change of the signals takes place, but no change in the blow temperature or lowering of the interior temperature results, the compressor is probably faulty.Screen Shot 2017-05-18 at 15.45.52

Screen Shot 2017-05-18 at 15.46.34
In addition, there is diagnosis equipment on the market with which it is possible to generate a PWM signal with a different pulse duration. Thus, it is possible to determine whether a triggering of the compressor leads to a change in the refrigerant pressure. This serves as the basis for being able to make a statement as to what extent the compressor still functions perfectly.Screen Shot 2017-05-18 at 15.47.30

A functional check by means of a PWM signal can also be carried out using a function generator (Fig. 10). For this, it is however essential to connect a load which corresponds to that of an electronic control valve to the control-unit side of the air-conditioning system. Otherwise, the control unit will detect a fault in the system and will store this in the fault memory. This can lead to malfunctions or to failure of the system. In this case, a readout of the fault memory must be taken and
deleted using diagnosis equipment.

Screen Shot 2017-05-18 at 15.47.51
Again and again, complaints about compressors are hastily filed in connection with noises and other problems of air- conditioning systems. In very many cases, it turns out that the compressor  is alright, or that the cause of the fault does not stem from the compressor itself. For this reason, all components of the system should always be included when troubleshooting. Noises are caused not only by the compressor, but also by its fastening, the drive, the expansion valve or the lines. An incorrect quantity of refrigerant can also be responsible for various noises. In relation to this, there is the separate technical information. Noise development. The technical information Compressor damage provides information about different types of compressor damage and their causes.
In addition, the oil provides important information about possible damage:

  • If the oil in the compressor has taken on a red colour, this may be attributable to much moisture.
  • Oil with a black colour points to a defective compressor.
  • Silver-grey oil should be checked for metal swarf. The greyish discoloration points to metal abrasion.

As the system oil quantities become ever lower (sometimes only 80 ml), the monitoring and observance of the oil quantity is of great importance (e.g. during air-conditioning servicing and when replacing components.

The repair of clutchless compressors is possible only to a limited extent. It is essential that suitable tools and repairinformation are used when carrying out such a repair. Of course, assessment of the system pressures is very important when carrying out diagnosis work. In relation to this, the specified values of the vehicle manufacturer should be consulted.

This also applies to the blow temperature. Guidelines in relation to the assessment of system pressures can be taken from the table below:
Screen Shot 2017-05-18 at 15.56.31

Compressor Damage

Air Conditioning No Longer Operates After Eliminating Leakage or Following Air Conditioning Service

After replacing air conditioning components and following normal service to the air conditioner, frequently the air conditioning no longer operates properly either immediately after completion of the work or a short time later.

What does customer complain about?

Originally the vehicles come to the garage with the customer indicating that “the air conditioning no longer cools properly” or “the air conditioning does not cool at all”.

What is done in the garage?Screen Shot 2017-05-16 at 17.02.47

In such cases, it is usual to first check the quantity of refrigerant in the circuit. Here, it is frequently noted that the quantity of refrigerant in the system is insufficient. Since up to 10% of the refrigerant can diffuse out of the air conditioning system within a year, depending on the type of system, or leakage may be present in the system, the air conditioning is filled directly with refrigerant and a leak detection additive to check for leakage or the system is checked for any possible leak with the aid of nitrogen. Then, a leaky component (Figure 1) in the refrigerant circuit is replaced or only the filter/drier element is replaced depending on the results of the test. The system is then evacuated properly and charged with refrigerant and oil in conformance with the manufacturer’s specifications.

Screen Shot 2017-05-16 at 17.03.13When the air conditioning is put back into operation, it is possible that the compressor no longer has any output. If the pressure values indicated in the service station are considered, it is noticeable that the values on the high and low pressure sides are virtually identical (Figure 2). This allows the suspicion that the flow through the refrigerant circuit is insufficient, e.g. at the expansion valve, or that the compressor is defective.

Strangely enough, there are also cases where the initial test on the air conditioning system indicate that the high and low pressure values are within the normal range and only the quantity of refrigerant is too low and that the problems will be solved after properly recharging the air conditioning system. As a result of the evacuation and recharging procedures, contamination particles or metal abrasion can become loose and deposited in the compressor control valve (Figure 3) or in the expansion valve/throttle valve (Figure 4) leading to malfunctions. This occurs particularly when the filter/drier is excessively old or the system has been “overcharged”.

Screen Shot 2017-05-16 at 17.03.39What can be done?

If problems arise, the compressor should be removed and the oil drained. If the oil has a “grey discoloration” (when contrast agent is used (grey-green or grey-yellow) resulting from fine metal particles (Figure 5), it is necessary to properly flush the refrigerant circuit due to the foreign particles, replace the expansion valve and filter/drier and evacuate the refrigerant circuit again in conformance with specifications and recharge the system with refrigerant and oil. The system should then operate properly.

Is customer sufficiently informed?

Since the garage only submitted a cost estimate for finding the leak and, where applicable, Screen Shot 2017-05-16 at 17.04.05replacing the leaky components or for air conditioner service only, difficulties can arise with the customer when attempting to justify the cost. Frequently, the customer is not willing to pay the significant additional costs for items such as replacing the compressors and flushing the system. For this reason, a detailed discussion with the customer describing the technical facts and risks is particularly important.

What are the causes for compressor failure?

The compressor is the only moving component in the refrigerant circuit and must be supplied with a sufficient quantity of oil. Another purpose of the oil in the refrigerant circuit is to cool the compressor to prevent it from overheating. If a compressor is operated over a longer period of time with too little refrigerant (e.g. due to leakage), this leads to insufficient dissipation of the heat and lubrication of the compressor components because the oil is transported through the air conditioning system by the refrigerant. The overload to the compressor components results in formation of metallic abrasion on the components leading to partial or complete clogging of the control valve contained in the compressor. This clogging of the control valve leads to the compressor no longer operating properly. Tis can only be corrected by proper replacement of the compressor which also includes “flushing” the system. Insufficient lubrication leads to damage on all types of compressors, however, output regulated compressors react particularly sensitively to lack of refrigerant or oil.
Screen Shot 2017-05-16 at 17.04.30

Note for garage and particularly for personnel receiving vehicles to be repaired

When receiving vehicles on which the customer complaints about insufficient air conditioning performance, it is best to indicate that damage could have already occurred to the compressor due to lack of lubrication resulting from an insufficient quantity of refrigerant which could make it necessary to replace the compressor. In cases of doubt, always remove the compressor and if the oil is contaminated “flush” the system before replacing the compressor. If the customer requests you to proceed in any other manner, it is advantageous for the garage to note this on the repair bill or to have it confirmed by the customer in writing.

This technical information was compiled in cooperation with the compressor manufacturer SANDEN and applies to all known compressor manufacturers and compressor models presently on the market

Air Conditioning Compressors

GeneralScreen Shot 2017-05-16 at 16.56.38

The AC compressor is driven by the engine via a poly-V-belt or multiple V-belts and compresses or pumps the refrigerant in the system. There are different types of compressors.

Function

The refrigerant coming from the evaporator is sucked into the compressor in the
gaseous state at low temperature and low pressure, compressed and then transferred to the condenser in the gaseous state at high temperature and high pressure.

Effects of Failure

A damaged or defective compressor can have the following effects:

  • Noise development
  • Refrigeration capacity poor or completely absent
  • Storage of error code (automatic air conditioning)

Failure can result from various causes:

  • Bearing damage due to defective tensioning device or wear
  • Leakage at the compressor shaft or in the housing
  • Mechanical damage to the compressor housing
  • Connections,
  • Insufficient oil

Diagnostics

Function test and system pressure test:

  • Does the compressor switch on, is the connector tight, is voltage present?
  • Ensure that the drive belt is seated properly, check for damage and tension.
  • Visual check for leakage.
  • Ensure that refrigerant lines are seated tightly.
  • Compare pressures on high and low pressure sides
  • On new systems – read out error code

Special Considerations

Before installing a new compressor it is important to check the amount of oil required.

Screen Shot 2017-05-16 at 16.57.23

Heat exchanger

General pointsScreen Shot 2017-05-16 at 16.47.33

The heat exchanger is installed in the heating box of the vehicle interior and has coolant flowing through it. The interior air is routed through the heat exchanger and thus heated up.

Structure/function

Like the coolant radiator, the heat exchanger is made up of a mechanically jointed pipe/fin system. The trend is moving to all-aluminium design here, too. Coolant flows through the heat exchanger. The flow quantity is usually controlled by mechanically or electrically controlled valves. The interior air is heated up via the cooling fins (network) of the heat exchanger. The air flow produced by the interior fan or the wind blast is routed through the heat exchanger which has hot coolant flowing through it. This heats up the air which is returned to the inside of the vehicle.

Effects of failureScreen Shot 2017-05-16 at 16.48.39

A faulty or poorly working heat exchanger can become noticeable as follows:

  • Poor heating performance
  • Loss of coolant
  • Odour build-up (sickly-sweet)
  • Fogged windows
  • Poor air flow

The following can be considered as possible causes:

  • Poor heat exchange caused by external or internal impurities (corrosion, coolant additives, dirt, limescale deposits)
  • Loss of coolant through corrosion
  • Loss of coolant through leaky connections
  • Soiled interior filter
  • Impurity/blockage in the ventilation system (leaves)
  • Faulty flap control

Troubleshooting

Test steps towards recognising faults:

  • Watch out for smells and windows fogging
  • Check interior filter
  • Check heat exchanger for leaks (hose connections, beading, network)
  • Watch out for impurities in/discolouring of the coolant
  • Check coolant flow (blockage through foreign matter, limescale deposits, corrosion)
  • Measure coolant inlet and outlet temperature
  • Watch for blockages/foreign matter in the ventilation system
  • Check flap control (recirculated air/fresh air)

Coolant radiators

General points

Screen Shot 2017-04-25 at 16.34.37Coolant radiators are installed in the air flow at the front of the vehicle, with different designs available. They
have the task of dissipating heat produced by combustion in the engine and absorbed by the coolant. Other coolers, e.g. for automatic transmission, can be found in or on the coolant radiator.

Structure/function

The most important component of a coolant module is the coolant radiator. It comprises the radiator core and water tank with all the necessary connections and attachment elements. The radiator core itself is made up of the radiator network – aScreen Shot 2017-04-25 at 16.34.47 pipe/rib system – the pipe bottoms and the sides. Conventional coolant radiators have a coolant box made of glass fibre reinforced polyamide which has a seal fitted and is beaded before being placed on the pipe bottom. The current trend is moving towards all-aluminium radiators, which stand out due to reduced weight and a slimmer design. In addition, they are 100 % recyclable.

The coolant is cooled down by means of the cooling ribs (network). The external air flowing through the radiator network withdraws heat from the coolant. In terms of design, a distinction is made between downflow and crossflow radiators. In the case of downflow radiators, the water enters the radiator at the top and emerges at the bottom. In the case of crossflow radiators, the coolant enters at one side and emerges at the other. If the input and output pipes of the crossflow radiator are on the same side, the water tank is divided. Coolant then flows through the radiator, in opposite directions in the upper and lower parts. Crossflow radiators have a lower design and are used particularly in passenger cars.

Effects of failure

A faulty radiator can become noticeable as follows:

  • Poor cooling performance
  • Increased engine temperature
  • Permanent radiator fan operation
  • Poor air conditioning system performance

Screen Shot 2017-04-25 at 16.36.36

The following can be considered as possible causes:

  • Loss of coolant caused by damage to the radiator (gravel throw, accident)
  • Loss of coolant through corrosion or leaky connections
  • Poor heat exchange caused by external or internal impurities (dirt, insects, limescale deposits)
  • Soiled or old coolantScreen Shot 2017-04-25 at 16.36.42

Troubleshooting

Test steps towards recognising faults:

  • Check the coolant radiator for outer soiling, clean with reduced compressed air pressure or a water jet if necessary. Do not get too close to the radiator lamellas
  • Check the radiator for external damage and leaks (hose connections, beading, lamellas, plastic housing)
  • Check coolant for discolouring/soiling (e.g. oil caused by faulty gasket) and check anti-freeze content
  • Check coolant flow (blockage through foreign matter, sealing agents, limescale deposits)
  • Measure the temperature of the coolant as it enters and leaves the radiator with the aid of an infrared thermometer (e.g. from Behr Hella Service, part no.: 8PE 351 228-031)

Checking the Air-Mass Sensor and Boost Pressure Control

Design, measuring principle, signal transmission

Automobile manufacturers are striving to get even more power out of diesel engines. At the same time, the pollutant emission must be reduced in order to adhere to current and future limit values. In order to be able to implement this, the precise determination of the air mass sucked in and the accurate controlling of the boost pressure is of great importance.

The air mass sucked in is one of the most important variables for calculating and controlling different functions for the engine control of current vehicles. With diesel engines, the measured value for the air mass sucked in is, amongst others, required for the exhaust gas re-circulation quantity, as correction value for the calculation of the injection time and the injection quantity, and for the regeneration of the particle filter.

From the beginning of the 1990´s, hot film air-mass sensors have mainly been used for determining the air mass sucked in. In the mean time, there are several generations of hot film air-mass sensors on the market that, in particular, differ in their type of signal output.

DesignScreen Shot 2017-04-20 at 12.11.17

The design of the hot film air-mass sensors has remained the same throughout all generations but, with view on the precision, has been continuously developed in the calculation of the air mass and sensitivity against contamination. The hot film air-mass sensors comprise mainly from a plastic measuring tube that is positioned betwen the air filter and throttle valve. The measuring tube is equipped with a flow rectifier (that is a plastic or wire grating in most cases) that prevents turbulences, and guarantee an equal flow in the measuring tube.

Another component of the air-mass sensor is the plug-in sensor. It contains the sensor element and the evaluation electronics and is positioned in precisely defined position in the measuring tube. For measuring the air mass being sucked in, a partial air flow is guided past via a bypass duct in the plug-in sensor on the sensor element. In order to protect the sensor element against damage caused by solid particles and liquids in the intake air, the bypass duct in newer generations has been designed in such a manner that the partial air flow is guided to the sensor element at the inlet of the bypass duct in a very tight curve. The inert soiling and liquid particles cannot follow this fast deflection movement and are added to the intake air again via the separation borehole. The robustness of the sensor could be improved even more thanks to this measure.

Measuring principle

With the last hot film air-mass sensor generation, the measuring principle has almost not changed at all. The sensor element comprises a micro-mechanical sensor diaphragm whose elements are vapor deposited onto a semiconductor substrate and from an evaluation electronic. On the sensor diaphragm, the hot film is heated to a temperature of approx. 120 °C via the air temperature and kept at a constant temperature. The intake air which flows past cools down the hot film. Two temperature-dependent resistances on the sensor diaphragm record the drop in temperature on the right and left of the heating zone. The temperature difference between both measuring points serves as the variable for the air mass sucked in. The sensor in the integrated evaluation electronics converts the temperature difference at both measuring points into an analog or digital signal, that is then transferred to the control unit. In many cases, the air-mass sensor is also fitted with a separate intake air temperature sensor (NTC). Its signal is however, not required for the calculation of the air mass.

Digital signal transmission

With the first generation of the hot film air-mass sensors, the sensor signal was output in the form of electric voltage that could be between zero and five Volt depending on the air mass being supplied. With the newest generation, the signal for the air mass sucked in is transferred to the control unit in a digital manner of a variable frequency. The temperature of the intake air is usually also transferred digitally, and that being in the form of a pulse width modulation (PWM) signal. The digital signal for the air mass has, depending on the dimensioning of tScreen Shot 2017-04-20 at 12.21.07he air-mass sensor, a frequency of 1.5 to 12 kHz. The frequency increases with the increasing air mass. There are however, hot film air-mass sensors where the frequency reduces with increasing air mass. When checking the air-mass sensors, observe the specifications of the respective manufacturers.

With newer air-mass sensor generations, the digital signal for the intake-air temperature has a constant frequency of approx. 19 Hz. Only the duty cycle changes parallel to the air temperature here. The frequency of the intake-air temperature signal has an important additional function: it is used as a time basis for the signal of the air mass by the evaluation electronics in the control unit. In this manner, the signal transmission can be monitored permanently and it can therefore be ensured that the signal frequency output can be allocated to the correct air mass value.

There are many different variants of the hot film air-mass sensor in the newest generations. Depending on the requirements of the manufacturer, they are available with a different number of pins, with or without intake air temperature sensor and with analog or digital signal output.

Check the digital air-mass sensor with workshop equipment

Screen Shot 2017-04-20 at 12.28.14A defective air mass sensor cannot always be recognized straight-away. Possible error symptoms that indicate a malfunction of the air mass sensor are black smoke,
deficiency in performance, emergency run or the entry of an error code in the error memory. To check a hot film air-mass sensor with digital signal output, a diagnostic unit, a multimeter or an oscilloscope – that supports frequency measurement – and a matching test adapter is required.

Starting the troubleshooting is carried out as usual by querying the error memory. The evaluation of the parameters – in particular those of the air mass sucked in and calculated – can provide information on a malfunction of the air-mass sensor (HFM_03). If the suspicion of an error on the sensor is more specific, this can be checked safely in the workshop in just a few steps.

The following steps are necessary for this:

1. Checking of current consumption
For this purpose, one switches off the ignition and using a matching adapter, connects a multimeter into the voltage supply line serially. When the ignition is switched on, a current value of, for example, 32 to 38 mA (observe the manufacturer specifications) can be read from the multimeter. If the quiescent current consumption is less than the setpoint, a broken sensor diaphragm is most probably at hand Some sensor manufacturers specify that the voltage supply must be checked instead of the current consumption. The value measured must then correspond with the on-board voltage.

2. Measuring the zero air with stationary engine
For this purpose, the engine must be switched off and the respective line of the test adapter must be connected to the multimeter for measuring the frequency. As an alternative, diagnostics professionally can also use an oscilloscope. It is only important that during the zero air measurement, that the exhaust gas extraction system is switched off or removed. In this way, one prevents that airstreams distort the measuring result. When the ignition is switched on, a frequency between, for example, 1.76 and 1.93 kHz (observe the manufacturer specifications) must be able to be measured. The frequency when the air mass sucked in increases. This is easy to check by starting the engine after the zero air measurement and a surge of gas is initiated.

 

3. Checking the temperature signals
A frequency measurement is also carried out here with ignition switched on and stationary engine. The setpoint is, for example, 18.5 to 20.8 Hz (observe the manufacturer specifications). The frequency of the temperature signal remains constant. The duty cycle of the signal changes proportional to the intake-air temperature. In case the temperature signal is output analog, the resistor of the intake air temperature sensor can be determined in a classic manner using a multimeter. Here, a few exemplary values: at 25 °C the resistance should be about 2 kΩ and at 60 °C 564 Ω. The different test points can be set using a hot air blower. As an alternative, the intake air temperature can also be checked using a diagnostic unit. In doing so, the respective value for the intake-air temperature is read out with the parameters. It must correspond with the ambient air temperature.All test steps on the air-mass sensor can be carried out in an installed state. If the measured values deviate from the setpoints, the air-mass sensor must be renewed.

Find and avoid the cause of damageScreen Shot 2017-04-20 at 12.29.43

Hot film air mass sensors are highly sensitive sensors. Any type of impurities, being dust particles or humidity damage the sensor diaphragm and lead to incorrect measuring results.

Similar damage is also caused by oil vapor that can completely burn-in to the hot film element over the course of time. In order to avoid damage and the premature failure of the air-mass sensor, we recommend that careful handing is exercised during service and repair work in the area of the intake and air guide system. Thus, during maintenance work, the mechanic must take particular care that the air filter elements are used in original equipment quality, all air lines are absolutely tight and that the air filter housing is also cleaned with care using a lint-free cloth and, where appropriate, avacuum extraction system (no compressed air). If there is a suspicion that humidity has entered the air guide system, all air ducts must be examined additionally and the cause rectified according to manufacturer specifications.

Variants of the boost pressure control

For an optimum function of a supercharged engine, the boost pressure of the exhaust turbocharger must be adapted to the engine load and the engine speed. The simplest form of the boost pressure control is the bypass on the turbine side (bypass channel). In doing so, the turbine is selected as small as possible that the requirements on the torque behavior is fulfilled at low speeds and a good driving property of the engine is reached. With such a dimensioning, more gas is supplied to the turbine shortly before reaching the maximum torque, that is necessary for generating the boost pressure. This is why a part of the exhaust gas quantity is guided around the turbine through a bypass after reaching the the required boost pressure. The boost pressure control valve that opens and closes the bypass is controlled by a spring-loaded diaphragm depending on the boost pressure.

Screen Shot 2017-04-20 at 12.30.43
With modern passenger car diesel engines, the adjustable turbine geometry (VTH) with rotatable guide vanes is state-of-the art for the boost pressure control for many years (Picture 5). The adjustable turbine geometry allows the flow cross-section of the turbine to be adjusted depending on the engine operating point. As a result, the entire exhaust gas energy is used and the flow cross-section of the turbine can be set optimally for every operating point so that, compared with the bypass control, the efficiency of the turbocharger is improved and therefore that of the engine. The ongoing adjustment of the turbine cross-section to the driving state has an effect that the fuel consumption and emissions are reduced. That a low speed already ensure for a high torque of the engine and a carefully matched control strategy a noticeable effect on the improvement in the driving behavior.

Strategic troubleshooting

Faults on the boost pressure control can usually be recognized by a deficiency in performance. In such cases, vehicle garages
should arrange a targeted troubleshooting strategy.

In doing so, the following procedure has been proven in the process.

1. Customer conversation

In doing so, the specialist should find out when and under which conditions the error occurs. Then a test drive should be carried out together with the customer, where the error can be reconstructed. That is indispensable for further ways of troubleshooting and for the success control of the work assigned.

2. Visual inspection

With the subsequent visual inspection, all booster air lines should be checked for damage and a tight seating. This also applies to all vacuum lines, the control valves and pneumatic actuating equipment. Marten damage and porous vacuum hoses lead to malfunction of the boost pressure control. Finally, the connector and electric lines of the control valves should also be checked for a tight fit, interruption or contact resistance.

3. Self-diagnosis

Modern engine control units offer a great diversity of options that test the booster air system through its paces using a diagnostic unit. In this way, the error memory already gives the first indications of possible malfunctions. The clever evaluation of the parameters and the actuation test can limit errors in the booster air system by further options.

4. Pressure tests

Finally, diverse pressure tests on the booster air system have to be carried out for a safe diagnostic. They allow the specialist to check the results of the self-diagnostics for plausibility. The specialist therefore prevents expensive components from being replaced for nothing. Moreover, in pressure tests errors can also be detected in the booster air system that were not detected by the self-diagnostics.

Screen Shot 2017-04-20 at 13.53.23Check boost pressure

In the scope of the pressure test, the actual boost pressure should be determined first. A manometer is required for testing the boost pressure that can measure vacuum as well as overpressure. The manometer is connected to an easily accessible position on the boost pressure circuit using a T-piece and respective stage adapter.

The boost pressure is tested under load. A test drive or, where necessary, running on a dynamometer cannot be avoided for this purpose. In doing so, driving cycles with low engine speeds and higher loads must be strived for with an engine that has reached operating temperature. The boost pressure test using a manometer has many advantages for the garage specialist for troubleshooting.

The specialist can:

  • compare the actual boost pressure reached with the setpoints of the manufacturer
  • verify the parameters supplied by the control unit for plausibility. malfunctions on the boost pressure sensor can be revealed easily and quickly in this manner
  • observe the course of the boost pressure and recognize temporary fluctuations in pressure that result in sporadic drops in performance. The parameters output from the control unit frequently react too sluggish in order to detect temporary fluctuations in pressure in a safe manner.

Do not replace the turbocharger too quickly

Screen Shot 2017-04-20 at 14.03.25If the boost pressure measured does not correspond to the manufacturer specifications, the cause must be determined. Faults on
the boost pressure control, leaks on the boost air system or damage on the turbocharger may be causes.

However, before the garage specialist replaces the turbocharger, they must be able to rule out all other error sources. Being electro-pneumatic valves or actuator motors for the boost pressure control, the functions can be tested using the actuator test and if required, a pressure / vacuum pump. Leaks in the boost air pressure can be detected by sealing the air route to be tested using matching adapters and applying pressure (maximum values can be obtained in the manufacturer specifications). The leaking point can be determined by feeling and listening, or by using a leak detection spray.

A/C – A Strategy For Success

A ‘best practice’ approach to air conditioning service will help your business to thrive this summer, as Steve Hudson, Head of Business Development at BEHR HELLA SERVICE explains.

Today, virtually every vehicle presented for routine mechanical servicing or accident repair has been factory-fitted with an air conditioning (A/C) system. In fact, A/C is now such standard equipment that motor factors and technicians alike are passing up significant profit opportunities if they haven’t developed a strategy to maximise associated parts sales when various A/C components have to be replaced.

Added value sales accrue when service technicians take a ‘best practise’ approach to the A/C system as a whole, not just the principal component that is being changed. For example, it’s a matter of routine for workshops to fit a new tensioner and, perhaps, a new water pump when replacing a timing belt.

However, workshops can reap similar rewards from additional parts sales if they adopt the same approach when changing A/C components. Just as importantly, from the perspective of satisfying and retaining the customer, you can point to the fully restored efficiency, integrity and reliability of the system as justification for the expense.

How does the A/C system work?

The A/C system is formed by a closed circuit of pipes connecting the various components, with a central compressor circulating the refrigerant through the system. The compressor significantly increases the temperature of the gaseous refrigerant before it passes to a condenser, through which it is forced under high pressure, causing it to shed heat and condense from gas to liquid.

From the condenser, the refrigerant – still under high pressure – then travels to a receiver drier, which filters out moisture and other contaminants before progressing to a thermo expansion valve or fixed orifice tube which is located just ahead of an evaporator. The pressure of the refrigerant drops as it enters the evaporator, with cooling taking place as the process of evaporation changes i2ts state back to gas from liquid.

Like the condenser, the evaporator is a large surface area heat ex changer, which distributes the cold from evaporation to the surrounding area before the ‘conditioned air’ is blown from the ventilation system into the interior of the vehicle. The refrigerant then returns to the compressor for the process to
start again. When a fixed orifice tube is insitu there is a suction accumulator placed on the low side of the circuit and the receiver drier isn’t part of the vehicle A/C system.

Additional sale

1-copy-1-300x200All through the process, every part in the system represents a potential additional sale. When replacing the compressor for ex ample, look at the receiver dryer and the thermo expansion valve as well and don’t overlook all the associated ‘O’ rings, oil, dye and flushing fluid commodities. The same thoughts apply when replacing the condenser.

Every second year the receiver drier/suction accumulator should be replaced anyway and if you change any other component and open the A/C system to atmosphere in the interim, a new receiver drier/suction accumulator should certainly be fitted due to moisture going in to the system.

Stations ready

Naturally, in order to undertake service and repair work on an A/C system, technicians need to use a high quality A/C service station, which is where our sister company HELLA GUTMANN SOLUTIONS steps in. Its comprehensive ‘Husky ’ range of mobile A/C service stations for both R134a and R1234yf refrigerants provide3s the essential solution for all conventional, hybrid and electric vehicles.

The compact, entry level Husky 150 is designed for easy three step operation, but still incorporates several features commonly found on more costly units. Also for R134a refrigerant, the Husky 300 is a fully automatic station boasting leak detection, hermetically sealed oil/UV management and POE filling system functions, among other advanced features.

The range topping fully automatic Husky 3000 and Husky 3500 service stations incorporate the latest technology and industry leading features relevant to R1234yf refrigerant A/C systems, such as high precision filling, leak detection and refrigerant analysis functions, plus an integrated print-out facility to communicate results directly with the customer.

Why do hybrids use high voltage A/C compressors?

Vehicles with full-hybrid technology use high voltage compressors that are not dependent on the running of the combustion engine. This makes for greater A/C comfort in such vehicles.

A vehicle’s interior that has become overheated can now be cooled down to the desired temperature before the start of any journey by using a remote control. This stationary cooling process can only be brought about if sufficient battery capacity is available.

The compressor is controlled with the lowest possible output with the necessary A/C requirements being taken into account. In the high voltage compressors used today, the power is regulated by adjusting the rotary speed in steps of 50 min-1. It is therefore not necessary to have an internal power control.

Function and design
In contrast to the ‘swash plate’ principle, which is primarily used in the belt-driven compressor field, high voltage compressors use the ‘scroll’ principle to compress the refrigerant. The benefits are that the weight is reduced by approximately 20% and there is a reduction in the cylinder capacity of the same amount, whilst the output remains identical.

In order to generate the right amount of torque for the drive of the electric compressor, a DC voltage of over 200V is used.

Specialist subject
You should be aware that appropriate staff training is required for the servicing of hybrid vehicles, with further skills imperative for those who service and repair the complex thermal management systems found in hybrids. In Germany, for example, those employees who work on high voltage systems are obliged to attend an additional two-day course in order to qualify as an “Electrician for High Voltage Systems”.

The course teaches the technician to recognise the risks when working on systems of this kind and also how to switch off all the current to the system for the duration of the work. It is prohibited for people who have not attended specific training courses to work on high voltage systems.

Also, tasks dealing with general servicing and repairs (such as work on exhaust systems, shock absorbers, oil changes and the changing of tyres) may only be carried out by employees who have attended the same course, in order to learn about the risks associated with these types of systems.

Tools of the trade
It is also essential to use tools that comply with the specifications provided by the manufacturer of the hybrid vehicle. During the A/C check and service, steps must be taken to ensure that the electric A/C compressors ARE NOT lubricated with standard PAG oils, as these don’t have the necessary insulation properties. POE oil or a special type of PAG oil is normally used instead, as these have the required properties.

Consequently, A/C service units with an internal rinsing function and a separate fresh oil reservoir should be used for the A/C check and service in hybrid vehicles. This then ensures that any mixing of the various types of fresh oil is prevented.

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