Category Archives: Brakes

ABS Sensors

An anti-lock braking system (ABS) is a vehicle safety system that allows the wheels of a car to maintain tractive contact with the road surface while braking, preventing the wheels from locking up (ceasing rotation) and avoiding uncontrolled skidding. It is an automated system that uses the principles of cadence braking. Cadence braking is a skill practiced by skilful or professional drivers in vehicles without or prior to ABS technology. The ABS system does this at a much faster rate and with better control than many drivers could manage. ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces. However, on loose gravel, ice or snow covered surfaces, ABS can increase braking distance, although still improving vehicle steering control.

Since their introduction, anti-lock braking systems have been improved considerably in a bid to further improve driver safety and comfort. Later technology not only prevents wheel lock up under braking, but can also provide data for the on board navigation system, traction control system, emergency brake assist, hill start assist, electronic stability control and the front-to-rear brake bias. None of the above would be possible without wheel speed sensors.

The ABS, or wheel speed sensor in a relatively simple yet vitally important part of the ABS system as it is used to communicate the rotational speed of the wheel to the ABS control module.

Wheel speed sensors are installed directly above or next to the pulse wheel, these are also known as a tone wheel, but more often than not, called the ABS ring. The ring is attached to a part of the vehicle that is rotating at the same speed as the road wheel, such as the wheel hub, brake disc, CV joint or the drive shaft.

Types of ABS Sensors

ABS sensors are classified in to two different types, Passive and Active. Passive is without a power supply and Active is with a power supply.

Passive sensors

These comprise of a wire coil wound around a magnetic core and a permanent magnet. The pole pin inside the coil is connected to the magnet and the magnetic field extends to the ABS ring. The rotational movement of the ABS ring and the associated alternation of teeth and gaps effects a change in the magnetic flux through the pulse wheel and the coil. The changing magnetic field induces an alternating voltage in the coil that can be measured. The frequencies and amplitudes of the alternating voltage are related to the wheel speed. The sensor creates a AC signal that changes frequency as the wheel changes speed. The ABS control unit converts the AC signal to a digital signal for interpretation.

Structure of a passive sensor
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Output wave of a Passive sensor

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Passive sensors are larger and less accurate than active sensors and only start to operate when the wheel reaches a certain speed, therefore they have limited operation at lower speed. They are also unable to operate in reverse, so thus unable to determine the direction of travel.

Active sensors on the other hand are a lot more accurate and are able to detect speeds of less them 0.06 mph, this is vital for modern traction control systems. Some active sensors can even detect the rotation direction of the wheels. Active sensors require an external power source to operate and work in conjunction with a toothed or magnetic ABS ring. Active sensors create a digital signal which is transmitted to the control unit in the form of a current signal using pulse width modulation.

Structure of an Active sensor

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There are two types of active sensor. The Hall sensor and the Magneto-resistive sensor

Hall sensor – The hall sensor uses the Hall effect, which is a generation of a voltage (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. They react to changes in magnetic fields with a voltage difference which is send to the ABS control unit as a square wave signal. They use a semiconductor sensor coupled to an electronic circuit, protecting the sensor from possible voltage spikes, and a permanent magnet.

Output wave of an active sensor

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Hall sensors record the wheel speed via either a toothed or magnetic encoder (ABS ring) often found on the wheel hub, disc or bearing. The sensors are very accurate, but must be installed with precision.

Hall sensor with a toothed ring

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Hall sensor with a magnetic ring

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The advantage of using a magnetic ring over a toothed ring is that the sensor can be a lot smaller as there is no permanent magnet required in the sensor. It is instead located in the virtually flat ‘ABS ring.’ That magnetic ring can be located in the wheel bearing, allowing them to be used in confined spaces. The variation in the magnetic field is now created by the sections of polarity within the ring.

Magneto-resistive sensor – These sensors use a magnetic encoder ring similar in appearance to the encoder ring associated with the hall sensor. However, the encoder ring associated with this sensor has magnetic arc segments which cause a clear change in resistance when passing the sensor. It is this that enables the control unit to determine the wheel’s rotational direction. Magneto-resistive sensors are much more precise, but generally more expensive than Hall sensors and require a less precise installation position, therefore meaning that it can be located further away from the ‘ABS ring’ than the other types of sensors.

Both active sensors are less sensitive to electromagnetic interference, vibration and temperature fluctuations than a passive sensor.

Failure Diagnosis

Generally speaking, ABS sensors are very reliable, however due to their location they have a tough life.
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If any of the warning lights above are illuminated, there is brake pedal judder while braking at low speeds or the wheels lock up while braking, then there is probably a fault somewhere within the ABS system.

Possible causes,

  • Corroded, cracked or swollen ABS ring
  • ABS ring blocked, damaged, missing teeth or windows
  • ABS sensor out of position.
  • Damaged ABS sensor from impact with road debris.

One of the most common calls that we currently receive into our technical department is for the Peugeot 308 rear. This particular brake disc also contains the bearing and ABS ring. The workshop replaces the disc, only to find that the ABS sensor is now in contact with the ABS ring of the new disc. Apec or the motor factor usually get the blame for an incorrect part, however Apec parts are made to the exact dimensions as the original disc so it is not the disc that is at fault. What actually happens is that over time, corrosion forms and builds up under the mounting point of the ABS sensor. This forces the sensor towards the ABS ring and begins to wear the surface of the ABS ring. If this is noticed before the disc is bolted up, then usually the sensor (if not too worn itself) can be unbolted and the corrosion removed. However, if it is not noticed then is can result in the ABS sensor snapping as the vehicle is driven out of the workshop.

Correct Fitment of WIR5243 to PAD1448 on the rear of Jaguar XF & XJ models


To correctly fit the wear sensor, first remove the pad spring fitting from the top of the pad using a suitable tool.

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Insert the wear sensor into the spring.Screen Shot 2017-03-28 at 09.20.19

Insert the spring and wear sensor back it to the pad using a suitable tool.Screen Shot 2017-03-28 at 09.22.56

Pay special attention, not to apply force on top of the plastic sensor housing during this step to avoid damaging the sensor housing.

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What Causes Brake Judder?

Apec Braking take us through the preventative steps you should take to avoid brake judder.

Brake judder is one of the few ailments that can affect all types of vehicles. Brake judder manifests itself in vibration, and it is a sensation that can be felt through the steering wheel, through your pedals and potentially through your seat. Depending on the severity, a car’s chassis has even been known to pulsate.

Customers affected by brake judder often jump to one conclusion: that they’re experiencing a side effect of warped discs or, more technically, distortion. The truth about the real causes of brake judder are, however, far more complicated; in the majority of cases warped discs have nothing to do with disc manufacture or quality and everything to do with fitment or driver behaviour.

In most cases, brake judder develops or worsens over a given timeframe. This is vital to understand from a technician’s standpoint as it highlights that when fitted, a customer’s brake disc was within manufacturer tolerance, and thus aggressive braking or a fitment issue is the root cause of the problem. In fact any disc, regardless of quality or materials, can warp (distort) under the correct circumstances.

Main causes

Let’s begin with the main cause of brake judder: Disc Thickness Variation (DTV). One of the major causes of DTV is lateral run-out caused by improper cleaning of the hub surface. Similar to the butterfly effect, even the most minute particles of rust caught between the disc face and hub (measuring as small as 0.04mm) can lead to lateral run-out (where the turning disc starts to deviate from its axis) in excess of 0.1mm when measured at the centre of the braking surface.

The important point to note is that while this deviation may not immediately induce brake judder, it has provided the foundations for a vibration that will eventually lead to it, causing potentially irreparable damage to the braking disc (although this may not occur for over 2,000 – 5,000 driven miles).

Secondly, it is essential brake discs and pads are “bedded in” properly, and customers need to be aware of the potential damage caused by excessive heat build-up. During the first 100 miles the driver should drive normally but try to avoid excessive braking which would elevate the temperature of the disc and pad.

All Apec brake pads go through a ‘High Pressure Treatment’ process, burning off many of the resins and gases at the factory that would normally find their way into the atmosphere during bedding-in; this gives a good initial braking performance.

During the bedding process a layer of friction material is evenly distributed across the brake discs. Too little heat during bedding keeps the material from transferring to the rotor face. Feathering the brake pedal while overheating the system can therefore generate uneven pad deposits due to the material breaking down and sticking to the disc, causing a stick/slip situation.

Once this has happened, heavy braking will lead to imbalanced heat build-up due to the uneven distribution of friction material across the disc, with high-spots heating excessively in comparison to the rest of the disc. If the temperature at these high-spots exceeds 650 ̊C cast iron changes structurally, transforming into an immensely rigid substance called ‘Cementite’.

As the disc is now composed of varying materials of different strengths, judder will progressively get worse, which explains why customers often return with their vehicles months after installation. This cause may be as simple as poor technique, using the brake pedal to slow the car rather than engine braking on serious inclines and repeated emergency stops without adequate cooling time!

Other factors

Outside of DTV, judder can also be caused by uneven torque on wheel bolts and issues regarding floating, fixed and sliding calipers. Fixed calipers have pistons on both sides of the discs due to the stationary caliper housing.

Excessive run-out will cause piston movement and can result in pedal pulsation and binding pads in the caliper will cause DTV. Sliding or floating calipers that aren’t sliding or that have seized will prevent the caliper housing from moving, so check the slider bolts for movement and corrosion and replace, if necessary. Always replace springs where a floating caliper is fitted.Apec-braking-300x200

Poor quality pads may also be a root cause of brake judder as the friction material can overheat quickly, particularly if the brakes are used often and aggressively. It is also possible to warp a vehicle’s hubs, which will always result in brake judder. Again the root cause of a warped hub is excessive heat build-up caused by excessive friction.

Excessively worn or poorly fitted wheel bearings can also cause run-out at the hub. There are also occasions where manufacturer error can be the root cause of brake judder, but with modern machining and production lines this is increasingly unlikely.

How can technicians minimise brake judder?

Technicians must check disc thickness before refitting a disc, as if it’s outside manufacturer tolerances then it isn’t a serviceable item and requires replacement. Technicians are advised to pick eight equidistant points around the perimeter of the disc and to never base a determination of thickness on a single spot. Any variation in disc thickness will translate into brake judder.

You should always check lateral run-out, even when fitting new discs. With the use of a dial indicator you should target 0.05mm – 0.10mm as your maximum run out limit. If you want to check run-out of the discs independently of the hub, check the disc on a lathe and perform a dial indicator reading. In an effort to remove variables from the equation, make a run-out reading of the hub flange itself, without the disc. If the flange itself is causing the run-out problem, you’ll be able to isolate the cause.

Finally, it is imperative when refitting wheels, that bolts are checked and in serviceable condition. You should check the condition of the bolt threads as well as the integrity of the bolts. Never use fasteners that are suspect and also make sure all threaded locations are clean and free of dirt, grime or other contaminants; poor quality or unclean threads can result in incorrect torque readings. Never use an airgun to fully tighten a wheel nut or bolt as this could distort an aluminium wheel. The final tightening should be done with a torque wrench in the correct sequence.

Good practice prevails

Our final piece of advice is to always service the caliper, ensuring that the pad abutment points are lubricated, sliders (where fitted) are serviceable and working correctly and replacement springs are fitted. Although it can be time consuming, it is vital that the measures we’ve advised are taken and heeded by technicians. It is also essential that the importance of correctly bedding in new discs and pads is emphasised to the customer, because that conversation is nowhere near as expensive as replacing an unhappy customer’s discs and pads.

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Apec EPB Service Tool User Guide

Apec EPB Service Tool User Guide


Electronic Parking Brake – Audi A6

Electronic Parking Brake – Audi A6

ACEA – The importance of oil codes – Comma Technical Bulletin

Are all engine oil manufacturers entitled to make ACEA claims?

According to ACEA (European Automobile Manufacturers’ Association), lubricant marketers or manufacturers making claims according to the ACEA Oil Sequences are required by ACEA to submit a Letter of Conformance to ATIEL (European Lubricants Industry Technical Association). This provides a commitment to develop and manufacture engine lubricants in accordance with the guidelines described in the ATIEL Code of Practice. Comma is a signatory of ATIEL’s code of practice letter of conformance. A list of signatories for both oil marketers and base stock manufacturers can be downloaded from

What are the current ACEA specifications?

ACEA specifications for passenger vehicles are split in two categories, those that are designed for conventional engines (such as ACEA A3/B4) and “catalyst friendly” specifications designed to protect engines fitted with exhaust after-treatment units (such as ACEA C3).

Can specifications for conventional oils be claimed together with “catalyst friendly” specifications?

In the current ACEA specifications the answer is NO. The ACEA 2010 Sequences introduced chemical limits that make ACEA “A/B” and ACEA “C” unsuitable to be claimed together. ACEA “C” classification products are Low SAPS (Sulphated Ash, Phosphorous and Sulphur) engine oils specifically designed to protect Exhaust After-Treatment Units whilst ACEA “A/B” products are only suitable for conventional engines.


Are there different types of ACEA A3/B4? I see some with numbers at the end like A3/B4-04.

The numbers at the end of the ACEA specifications refer to the year the specifications were introduced. This means that A3/ B4-04 does not refer to the current A3/B4 specification but to one that was introduced in 2004. ACEA specifications are regularly updated and as new specifications are introduced, older ones are made obsolete. Although it’s important for oil blenders to know the technical capability of a formulation, strictly speaking the use of a date suffix like this is prohibited. The claim should be represented on the label as A3/B4 and for this to be valid the product in the bottle must meet the current standard. If it doesn’t then no claim should be made at all.


What are the differences between an obsolete speci cation (ACEA A3/B4-04) and the current equivalent?

There are technical differences that make the current version of ACEA A3/B4 higher performance than A3/B4-04. The best way to demonstrate this is to look at the graphs below:


As you can see the current ACEA A3/B4 specification is a higher performance than the equivalent specification in 2004 particularly in soot handling, wear protection, sludge control and piston deposits. The same is also true for ACEA C3. In simple terms, an oil that meets the current ACEA speci cations will outperform an oil that meets the equivalent but older versions.

OEMs sometimes also update their specifications. For example, the bottom graph above shows the major upgrade in requirements of the 2005 review of VW 501 01 specification.

Is there a risk of damage if I’m not using the current specifications?

If the vehicle requires the current ACEA specification then yes!

We make a recommendation based on ACEA A3/B4 for around 7 million vehicles (20%) on the roads today. A product described as ACEA A3/B4-04 may not be suitable for as many as 1 million of those vehicles (about 15%). As legislation has evolved engine design has adapted to accommodate the demands for cleaner and more efficient engines. Smaller, higher revving engines, smaller sump capacities and the increased use of turbochargers and exhaust after-treatment systems are placing more demands on the engine’s oil. Failing to use the right product in your engine can have some very costly consequences so it’s important you are tting the product the manufacturer intended. The engine and warranty could be at risk if you don’t.


Oil thickening and sludge can cause blocked channels and lters within the engine potentially leading to oil starvation, causing damage to the engine. Also, according to BTN Turbo, oil starvation is one of the most common causes of turbocharger failure.

How to get it right?

To be absolutely sure that you are using a product that meets the latest manufacturer’s specifications and avoid the dangers of using products based on obsolete ACEA claims, always use Comma’s website or Workshop Application Guide.

Changing Brake Fluid Technologies – COMMA Technical Bulletin

Brake fluids are becoming more complicated. Why is this?

Vehicle technology has moved on and nowadays the modern passenger car weighs more, accelerates harder and travels substantially faster than its older counterpart. It therefore requires substantially more braking force to bring modern vehicles to a stop. Combined with complex, computer controlled functions such as ABS and ESP, the higher weight and faster speed of modern vehicles makes modern brake systems considerably more demanding on brake uid than they ever were. Modern vehicles therefore need modern, high performance brake uids in order to keep them braking effectively and safely.


Are there any other types of brake fluid?

As well as synthetic PAG-based products there are other types of brake fluid. Mineral oil based products (Comma’s LHM+) have been used in the combined hydraulic systems of Citreons for years. They are designed specifically for this type of application and are not suitable for use where DOT products are specified. They are also not compatible and should never be mixed.


Which brake fluid do I need for my car?

There are a range of products to meet the demands of modern vehicles. Synthetic fluids, based on polyalkylene glycol (PAG), are the most common type (DOT3, DOT4, DOT 4 ESP and DOT 5.1). They are compatible with one another and therefore can be mixed however, as with oil and coolant, you should always comply with the manufacturers’ specifications for the intended vehicle as using the wrong uid can seriously compromise braking performance.

The final type of brake uid is rather confusingly called DOT 5.0 but it should not be confused with the other DOT specified brake fluids as it is completely different technology. DOT 5.0 is a silicon-based product designed for specialist applications, such as racing cars where the fluid is changed after every race. It is not compatible with any other type of brake uid and is not recommended for conventional applications. (Comma doesn’t make a DOT 5.0 fluid which is why the bottle on the picture is grey).


 I’ve heard that brake fluid is hygroscopic. What does that mean? 

Brake fluid is hygroscopic which means that it absorbs water from the air around it hence why you are always advised to start each new brake fluid change with an unopened container of brake fluid.

However, absorbing water is part of the brake fluid’s function. By keeping water locked in tight it stops from pooling in the lower areas of a braking system. This helps to maintain hydraulic pressure, protects against corrosion and maintains the viscosity of the fluid (i.e. it keeps it doing its job). The downside is that as a brake fluid absorbs water its boiling point drops and eventually brake fluid will absorb enough water such that the boiling point becomes dangerously low. This is more or less how the service life of brake fluid is defined.

The brake fluid test

Comma did some testing of their own with a brake fluid tester and the results were quite astonishing. We tested around 750 vehicles and of those approximately 33% were measured at below the 180°C minimum. One was as low as 100°C which is the boiling point of pure water! The risks associated with driving a vehicle with faulty brake fluid are self-evident and compelling. It’s simply not enough to change brake fluid at the same time as faulty discs or pads. Like engine oil or coolant, brake fluid should be changed at the manufacturer’s recommended intervals and at any time in between if it is found to be faulty with a brake fluid boiling point tester.


When do I need to change the brake fluid? 

As the brakes start to heat up temperature of water-contaminated brake fluid approaches and eventually exceeds its boiling point and this causes vapour bubbles to appear. This vapour is very compressible and absorbs large amounts of the force applied to the brake pedal before it ever reaches the callipers, thus making the brakes feel spongy. As the brake fluid gets older, it will continue to absorb more and more water such that the boiling point becomes dangerously low, and so, given the above information, manufacturers always recommend a change of brake fluid at a particular interval (typically between 1 and 2 years).

How to select the right Brake Fluid

To make this increasingly complicated choice a lot easier you can use the Comma Application Guide or our website where you can select your vehicle using the VRN (Vehicle Registration Number) tool or by using the make and model search. Fluid should be changed at the manufacturer’s recommended, you will then be presented with a printable full vehicle recommended product report including engine oil, antifreeze & coolant, transmission oils, brake fluid and greases if applicable.

Ford Transit Rear Drums & Judder – Apec Bulletin

We would like to bring to your notice the importance of fitting and securing Ford Transit drums correctly. Failure to follow the correct procedure can result in drums being distorted and inevitably cause judder.

  • Follow normal drum fitting procedures (cleanliness of drum to hub etc)
  • Offer wheel to studs and loosely tighten wheel nuts. Do not over tighten.
  • Now gradually tighten each nut in diagonal sequence as shown below and finally to the correct torque settings.


Modified hand brake lever – Apec Bulletin

The hand brake lever on the above part numbers of shoe have been modified to overcome certain fitting issues.

To ensure our customers are fully aware of how the brake cable should be fitted to the new lever design, we show here via the photographs how it should be assembled.

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What is Block Exemption? A guide for motorists and independent repairers.
On 1st June 2010 European Union legislation, ‘Block Exemption Regulations 461/2010 came into force.

This gives motorists more freedom in their decision as to who should repair and service their vehicles – now any garage can be used without invalidating the manufacturer’s warranty (subject to certain conditions).
This was as a result of four specific ‘freedoms’ that were introduced, aimed at safeguarding free competition in the entire market for vehicle spare parts, service and repair. This means:

  • parts distributors can supply parts of the appropriate quality to dealers and authorised repairers without the vehicle manufacturer / assembler using ‘dealer / repairer contracts’ or other means to prevent them.
  • anyone with a legitimate need for technical information has a right of access to it in a usable form and at a reasonable cost.

The BER covers service and maintenance during the warranty period and prohibits vehicle manufacturers’ warranties from including conditions requiring that:

  •  all normal maintenance be provided within the vehicle manufacturer’s network.
  •  all parts used must be the manufacturer’s original spare parts.

The European Commission declared that such clauses in a warranty document would represent ‘an unjustified restriction for the consumer’.

Independent repairers CAN carry out normal maintenance and repair services during a vehicle’s warranty period without invalidating the warranty conditions provided that:

  • the service is carried out in accordance with the manufacturer’s servicing schedules and is recorded as such.
  • any parts used are of ‘appropriate quality’ and are recorded as such.

All products supplied under the APEC brand are spare parts of matching quality as defined by EEC Commission Regulation 461/2010.

All products supplied under the APEC brand comply with current legislation and have full Regulation 90 approval in accordance with European Directives 98/12/EC (BER compliant) where appropriate.