Category Archives: Hella

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.


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


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.

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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.


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


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)

Manifold air pressure sensor (MAP)

Screen Shot 2017-04-27 at 17.04.19General

The manifold air pressure sensor measures the vacuum in the induction pipe after the throttle valve. The readings taken by the MAP sensor and the intake air sensor are used to compute the volume of air intake. The result is combined with the lambda probe signal to make up a closed control loop. The MAP sensor installs either immediately in the induction pipe or somewhere outside in the near vicinity.


The MAP sensor housing contains the logic analyser and a measuring head. Inside the measuring head there is a diaphragm which encloses a reference pressure chamber carrying four bridge-connected strain resistors. Two of these strain resistors are located at the centre of the diaphragm to act as measuring resistors. The other two resistors are installed on the outside of the diaphragm where they serve as reference resistors for temperature compensation. Pressure changes produce changes in the diaphragm shape which, in turn, modifies the conductivity of the measuring resistors and with it the measuring voltage. The logic analyser processes the measuring voltage and outputs it to the engine control device.

Effects of a failure

The following signs may be indicative of a MAP sensor failure:

  • Massive loss of power
  • Misfiring during acceleration
  • Instable idle speed
  • Lighting up of the engine control lamp

The following may be causes of MAP sensor failures:

  • Damaged measuring elements
  • Internal short circuits
  • No power supply, earth connection
  • Vacuum connector clogged up; vacuum pipe ripped or damaged.


Troubleshooters should check the following points:

  • Visual inspection: check if all connectors are properly plugged in and all cables connected and undamaged. Check that the vacuum connector and/or vacuum tube are clean inside and properly attached.
  • In vehicles equipped with a diagnostic system connector, check the error memory and perform a comparison of target and actual values (if possible).
  • Use a suitable instrument to check the power and earth connections.

Throttle Valve Switch

GeneralScreen Shot 2017-04-27 at 16.24.54

A throttle valve switch records the position of the throttle valve. It is fitted to the throttle valve spindle and send the information about the position to the ECU to assist in the calculating the fuel injection.


Inside the throttle valve switch are two contacts which are operated by a lever mechanism. The two switches send the information about full load and idle speed of the engine to the ECU. The ECU uses these to calculate the fuel injection for each throttle position.

Effects of failure

A faulty throttle valve switch can cause the following:

  • Engine stalls at idle
  • Engine is jerky at full throttle

Cause of failure:

  • Mechanical damage (e. g. vibrations)
  • Bad electrical connections in the plug
  • Bad electrical connections on the contacts inside the switches


For fault recognition considering the following system tests:

1. Check throttle valve switch for correct fitting.

2. Check lever mechanism by operating throttle valve.

3. Check electrical lead for correct fitting and contact.

4. Check shift contacts with an ohmmeter:

  • idle speed contact closed, throttle at idle speed position: measurement between pin 1 and 3, measured value: > 30Mohm
  • idle speed contact opened, between idle and full throttle position: measurement between pin 1 and pin 3(attention- open the throttle valve during the measurement till the idle contact is open) measured value = 0 ohm
  • full load contact opened, between idle and full throttle position measurement between pin 1 and 2, measured value: >30 Mohm
  • full load contact closed, full throttle position: measurement between pin 1 and 2, measured value: =0 ohm

Pin definition

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Throttle-valve housings

Screen Shot 2017-04-27 at 15.42.04General points

Throttle valves are installed between induction bridge and load sensor. Throttle valves control the air flow suctioned in by the engine. The mixing ratio of fuel and air is changed by the throttle valve’s opening angle. The throttle valves are actuated by mechanical connection to the accelerator pedal or electrical actuating elements.


A distinction is made between the following throttle-valve housings:

Mechanical throttle-valve housings

When the accelerator pedal is pressed, the driver’s load requirement is transmitted to the throttle valve via rods or Bowden cable. A built-in potentiometer transmits the throttle valve position to the control unit. The idling speed is adapted to the required operating status the integrated idling actuator.

Screen Shot 2017-04-27 at 15.39.58Electromotive throttle-valve housings

The throttle valve is triggered via Bowden cable and control unit. The position of the throttle valve is regulated mechanically through the accelerator pedal and transmitted to the engine control unit via the integrated electronics. The engine control unit uses the driver requirement and the current engine operating status to calculate the required throttle valve opening angle. This information is relayed back to the throttle valve by means of a control signal, and the throttle valve position is corrected by the integrated throttle valve actuator motor.

Electronic throttle-valve housings
The information from the accelerator pedal and all the other engine management data available are compared permanently. Using these data, the engine control unit calculates the respective optimum throttle valve position. The electronic throttle valve is exclusively triggered by the control signal from the engine management system. The electronic triggering of the throttle valve ensures particularly efficient engine control.

Screen Shot 2017-04-27 at 15.40.29Effects and reasons for failure

Effect of failure:

  • Loss of power
  • Misfiring during acceleration
  • Vehicle goes into emergency running mode
  • Fluctuating idling speed
  • Engine warning light comes on

Reasons for failure are:

  • Soiling through oil carbon deposits
  • Mechanical blockage through foreign particles
  • Defective actuator motor
  • Defective potentiometer


  • Readout fault store
  • Check the supply voltages and signals using a multimeter and oscilloscope
  • Visual inspection of the cabling and mechanical assemblies

Repair note

After replacement, the throttle-valve housing must be adapted to the engine control unit. The specifications issued by the respective vehicle manufacturers must be heeded in this context.

Exhaust gas recirculation system (EGR)

Screen Shot 2017-04-27 at 14.35.50General

To achieve exhaust emission reductions, some vehicles, are fitted with an exhaust gas recirculation system. This system is controlled by the ECU and the EGR valve.


Due to the recirculation of a part of the exhaust gas the NOx concentration can be reduced. The recirculated exhaust gas, supplied to the air intake, will not combust, it absorbs a part of the combustion heat and warms up. This causes a drop in the combustion temperature. A lower combustion temperature causes a lower NOx concentration. To ensure that always the right quantity is recirculated the control follows the engine performance maps of the ECU. There are two control possibilities: The direct connection between the ECU and the EGR valve or via a switching valve. In this case, the ECU controls the switching valve that open and close a vacuum line. The vacuum then opens and closes the EGR valve.

Causes of failure

A faulty exhaust recirculation system can produce the following fault symptoms:

  • Engine control light illumination, storing a fault code
  • Black smoke (diesel engine)
  • Rough idling

Causes for a faulty exhaust recirculation system:

  • EGR valve plugged or permanently open
  • Missing control of the ECU / ground
  • Faulty, plugged lines
  • Faulty, plugged vacuum lines
  • Faulty switching valve
  • Faulty wires, bad contact of the connectors

Fault diagnosis

For the fault recognition consider the following steps:

  • Visual check of all relevant components for damage
  • Check of all lines and connectors for damage, correct fitting and size
  • Read out the fault memory (if possible)
  • Check the EGR valve and lines for clogging and fouling
  • Check for supply voltage from the ECU at the switching valve and/or at the EGR valve

Visco fan

General pointsScreen Shot 2017-04-26 at 16.08.51

For heat dissipation in truck and powerful car engines, fans and fan drives which provide cooling air particularly efficiently are required in addition to powerful coolers. Visco fans (Fig. 1) are made up of a fan wheel and a Visco clutch. They are used in longitudinally mounted engines, are installed in front of the radiator (in the direction of travel) and are driven via a V-belt or directly by the engine.


The fan wheel (Fig. 2) is made mainly of plastic and is screwed to the Visco clutch. Number and position of the fan blades varies depending on the design. The housing of the Visco clutch is made of aluminium and has numerous cooling ribs (Fig. 3). The Visco fan can be regulated by a purely temperature-dependent, self-regulating bi-metal clutch. The regulating parameter here is the Screen Shot 2017-04-26 at 16.09.03ambient temperature of the coolant radiator. A further variant is the electrically driven Visco clutch. This is regulated electronically and actuated electro-magnetically. In this case, input parameters of various sensors are consulted for regulation. Further information can be found in the Technical Information sheet “Visco Clutch”.

Effects of failure
A faulty viscous fan can become noticeable as follows:

  • Heavy noise development
  • Increased engine temperature or coolant temperature

The following can be considered as possible causes:

  • Damaged fan wheel
  • Loss of oil/leak
  • Soiling of the cooling area or bi-metal
  • Bearing damage

Screen Shot 2017-04-26 at 16.09.09Troubleshooting

Test steps towards recognising faults:

  • Check coolant level
  • Check the fan wheel for damage
  • Make sure no oil is leaking
  • Check the bearing for play and noises
  • Check the attachment of fan wheel and Visco clutch
  • Check the air baffle plates/air cover to make sure theyare present and a tight fit

Condenser fans


Screen Shot 2017-04-26 at 15.00.26To guarantee correct liquefaction of the coolant in the condenser or sufficient engine cooling, an additional or more powerful condenser fan is usually installed in vehicles with air-conditioning systems. Existing fans may also be replaced by more powerful ones.


Condenser fans are mounted as extractor fans (in the direction of motion behind the condenser) or as pressure fans (in the direction of motion in front of the condenser). They are controlled by pressure or temperature switches or by a control unit. The fact that a higher air flow passes by the condenser or the engine cooler when the fan is in operation guarantees that the coolant is optimally liquefied or the engine is sufficiently cooled in any operation condition of the vehicle.

Effects of failure

A defective condenser fan may have the following consequences:

  • Poor cooling capacity
  • Frequent on/off switching of the compressor
  • Loud noise when the fan is running
  • Increasing engine temperature (in the case of a combined fan)

Possible causes for failure or damage:

  • Contact fault at the electrical connections
  • Short-circuit/contact fault in the electric motor of the fan
  • Defective bearing
  • Impeller broken due to material fatigue or accident


The following tests should be carried out to locate the fault:

  • Visual inspection for damage
  • Acoustic check for running noise
  • Check electrical connections for correct fit and contact
  • Check power supply
  • Read error memory (if possible)

When trouble-shooting, it should be taken into account that the fan is controlled by a pressure or temperature switch or by a control unit.

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.


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


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)

Air Flow Sensor (MAF)


Screen Shot 2017-04-25 at 14.45.07The air flow sensor records the incoming air flow. It is constructed of a duct style housing with a flow rectifier, sensor protection and a sensor module. The air flow sensor is fitted into the inlet pipe between the air filter housing and inlet manifold.


Two metal film resistors, fitted on a glass membrane, situated in the air flow. The first resistor(RT) is a temperature sensor and measures the air temperature. The second resistor(RS) measures the air flow. Depending on air mass drawn into the resistor RS is cooled down. To compensate for the constant temperature difference between the resistors RT and RS the current flow through the resistor RS must be regulated. This heating current is the measured variable of the air flow drawn in by the engine. This measurement is needed by the ECU to assist in calculating the fuel injection.

Effects of failure

A faulty air flow sensor can cause the following:

  • engine stalls or the ECU switches to limp mode
  • engine warning light illumination

Causes of failure:

  • bad connection at the plug
  • damaged measuring elements
  • mechanical damaged(vibrations, accident)
  • range drift of the measurement elements(wrong scope pattern)


For fault recognition consider the following system tests:

  1. Check electrical lead for correct fitting and contact
  2. Check air flow sensor for damage
  3. Check measurement elements for damage
  4. Measurement of the operation voltage, ignition on (wiring diagram needed for pin definition), measured value: 7.5 14 V
  5. Measurement of the output voltage, engine runs(wiring diagram needed for pin definition), measured value: 0 . 5 V
  6. Check the wiring harness between the sensor plug and the removed ECU plug for short circuit to earth and continuity, measurement with an ohmmeter between sensor plug and vehicle ground, measured value: >30 Mohm, measurement between sensor and ECU plug, measured value: < 1 ohm
  7. Electronic check of the air flow sensor by the ECU. If there is a failure the ECU stores a fault/trouble code and the engine warning light is illuminated. The fault/trouble code can be read out with a code reader or a diagnostic test equipment.