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Brake pulsations in cars with ABS

Two common faults can cause your brake pedal to pulsate when braking: dirty sensors and warped disc brake rotors.

Here is a simple way to detect a warped rotor. Go to a deserted open smooth area to perform this test. Accelerate the car up to 10-15 mph, and then let it coast in a straight line. Apply the brakes GENTLY until you feel them pulsate under your foot. If the pulsations are absolutely regular, then they are synchronous with wheel rotation…. which is 99% indicative of a warped rotor. This test does NOT tell you whether it is the fronts or rears at fault.

Disc brake rotor warp occurs when the disc fails to trace a absolutley flat planar path when it rotates. This deviation is often very small, on the order of thousandth’s of an inch, and is normally detected by removing the rotor from the axle, mounting it so that it can rotate freely in a shop, the rotating it by hand and measuring the run-out with a standard dial gauge. Warp occurs from rotor overheating from hard-braking. Aggressive drivers can cause this type brake rotor distortion in a few hundred miles…. others may not see it for many tens of thousands of miles, or ever.

Having a factory published repair manual is invaluable in tracking down and servicing ABS brakes. ABS sensors are cylindrical magnets surrounded by a coil, and mounted inside the disc brake support. They are subject to collect metallic debris from the pads, which will degrade the generated signals to the ABS computer, causing it to actuate ABS inappropiately, evident by the brake pedal pulsating. Most are secured with a single hex bolt. Removal and cleaning the sensors is a straight-forward operation, easily done at home.

Replacing brake rotors are another matter; although possible in a home garage, it may be best performed by a competent brake mechanic. Brakes are a crucial safety measure in a car, and there is no reason to put repair cost before driver safety. It is advisable to replace rotors in pairs: both fronts or both rears. Since the pads need removal anyway, good time to replace them as well, as well as checking the disc brake fluid seals of the pistons.

Infrequent causes of ABS generated brake pusations are factured sensor coils, fractured wiring between sensor and ABS computer, dirty/corroded connections to the ABS computer, intermittent power connections to the ABS computer (ie a fracture in its fuse), or internal component failure inside the computer. Tracking down these faults is time-consuming and can be quite expensive. Each manufacturer has a step-by-step process clearly outlined in their manual.

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Lid sinds: 03-01-15

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PS from author

I am sure that my method outlined above is not original, and has been written up somewere else. What I can vouch for, is that I have performed this test on my 1993 Volvo 240 wagon with over 200K miles.. Its brakes had been pulsating for some time, and after checking the ABS electric components exhaustively; it was only after replacing all four original disc rotors, that the pulsations disappeared. The front pair of rotors had 1.5mm ridges, were replaced first, with most of the pulsations gone. However, there were still minor ones, which disappeared once the rears were replaced. Theoretically, turning the rotors on a lathe could have removed the warp, but then the discs would would be thinner, and more susceptible to thermal distortion. I recommended that this kind of work be performed in a licensed brake shop, simply because not all readers have the proper tools or training to do such critical work in a home garage. Failure to securely remount the calipers could result in severe injury.

wli98122 - Antwoord

I have a 2005 Ford Falcon ute and when I brake the pedal pulsates, is there anything I can do to fix this problem?

Smack That - Antwoord

To the owner of the Ford Falcon:

As I mentioned in my essay, it looks to be warped disc brake rotors. If you are nervous about doing it yourself or lack the proper tools, take it to a licensed brake shop that you can trust, and ask them to perform a “run-out” test on each disc rotor. If any of them are warped, it is safest to replace them rather than have them machine the originals.

A frequent cause of rotor warp is hard braking. These invariably leads to overheating and warpage.

wli98122 - Antwoord

Additional note from author:

It is very important that reassembly of the disc brake components be precise and exact. All surfaces must be clean of grime and dirt, and that final tightening be performed with a torque wrench. DO NOT USE AN AIR-TOOL for reassembly. Use it only for disassembly. Also be extra careful not to kink any hydraulic pipes.

wli98122 - Antwoord

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