Diagnosing a Pulsating Brake Pedal Fault

Mike Noc · 18th April 2020, 20:32

Whilst sophisticated diagnostic equipment can be invaluable for diagnosing some ABS faults, if you have a pulsating pedal at slow speeds and no ABS alarm, then French Mike's method of using a voltmeter to test the sensor output works a treat.

Had this fault on my car, and this was the T4 screen:

So thought I'd start by checking the front offside wheel. A test of the sensor outputs showed 48 high signals, which is exactly as it should be:

Next up was the offside rear wheel (the most common to fail) and this told a very different story:

The outputs were a bit hit and miss and going round a couple of times improved things, which is why a roadtest in real time didn't show up any speed variations between all four wheels.

Didn't have the chance to try a good tip from Mark (Sworks) of taking a high speed video and playing it back in slow motion of the sensor speed readouts, which should have shown up the culprit wheel.

Whipping the hub bearing off revealed the cause, rust build up had damaged the magnetic reluctor strip:

Rubbed down the offending area and painted it (note to self; new back plates to be ordered). Fitted a new bearing (luckily had one in stock) and all was well.

Interestingly T4 did highlight an incorrect tooth gear fitted to one of the wheels. It must do a sum calculation over time on all the inputs from the ABS sensors and compare them. The difficulty it has is that the ABS system activates when it detects longer time gaps between the signals as that is exactly what happens when a wheel starts to lock up. The very same thing also happens when the reluctor segments are damaged or missing and not picked up by the sensor, and maybe that will cause the incorrect tooth gear fault to flag up.

Thanks again French Mike for posting up the test.

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18th April 2020, 20:32	#1
Mike Noc This is my second home Rover 75 CDT Manual Connoisseur SE, Rover 75 CDT Automatic Connoisseur SE & a Freelander Td4. Join Date: Jul 2009 Location: Hampshire Posts: 11,550 Thanks: 3,470 Thanked 3,119 Times in 2,247 Posts	Diagnosing a Pulsating Brake Pedal Fault Whilst sophisticated diagnostic equipment can be invaluable for diagnosing some ABS faults, if you have a pulsating pedal at slow speeds and no ABS alarm, then French Mike's method of using a voltmeter to test the sensor output works a treat. Had this fault on my car, and this was the T4 screen: So thought I'd start by checking the front offside wheel. A test of the sensor outputs showed 48 high signals, which is exactly as it should be: Next up was the offside rear wheel (the most common to fail) and this told a very different story: The outputs were a bit hit and miss and going round a couple of times improved things, which is why a roadtest in real time didn't show up any speed variations between all four wheels. Didn't have the chance to try a good tip from Mark (Sworks) of taking a high speed video and playing it back in slow motion of the sensor speed readouts, which should have shown up the culprit wheel. Whipping the hub bearing off revealed the cause, rust build up had damaged the magnetic reluctor strip: Rubbed down the offending area and painted it (note to self; new back plates to be ordered). Fitted a new bearing (luckily had one in stock) and all was well. Interestingly T4 did highlight an incorrect tooth gear fitted to one of the wheels. It must do a sum calculation over time on all the inputs from the ABS sensors and compare them. The difficulty it has is that the ABS system activates when it detects longer time gaps between the signals as that is exactly what happens when a wheel starts to lock up. The very same thing also happens when the reluctor segments are damaged or missing and not picked up by the sensor, and maybe that will cause the incorrect tooth gear fault to flag up. Thanks again French Mike for posting up the test. . Last edited by Mike Noc; 18th April 2020 at 20:51..