Quick ABS question.

[Rinse 0]

My ABS light is constantly on and its failed the MOT thanks to this.

 

When I start the car with the brake pedal pushed down it drops down about 1cm after its ticked over, but doesn't really pulse.

Is my ABS pump knackered??

[gsbellew 0]

Have you tested the sensors?

 

Mine did exactly what you describe, but it was only faulty sensors and a broken wire that had the light on.

[Critical_Mass 10]

Best thing to do is get it connected up to VAG-COM. It could just be one of the wheel sensors that has packed up, this would disable the ABS.

[Rinse 0]

Ah safe. I was thinking since there was no pulse it was the Pump.

I'll have to track done someone/somewhere with VAGCOM up here to find out what sensors causing it.

Cheers for the quick reply lads!

[PROVR6 0]

The dropping down by about 1cm is probably the pulse as some people describe. New ABS pumps are a fortune but there are plenty of used ones around for £30-40.

[mr.ots 10]

If the fault is due to a wheel sensor it shouldn't light up the warning light until the car is moving as the system detects there is speed present with no pulse signal from the sensor.

If you want to check the resistance of the sensors they should read somewhere between 1100 - 1300 Ohms.

[gsbellew 0]

If the fault is due to a wheel sensor it shouldn't light up the warning light until the car is moving as the system detects there is speed present with no pulse signal from the sensor.

If you want to check the resistance of the sensors they should read somewhere between 1100 - 1300 Ohms.

 

If a speed sensor has gone open circuit would it not put the light on constantly? As in when the ABS does its self test it can not 'see' the sensor?

 

A quick test with a multimeter is the easiest in the absence of VAGCom alright, should show up a dud sensor or broken wire pretty quickly.

[Supercharged 2]

^ Yeah that's correct Gearoid, if the light comes on spuriously when moving at speed and flags a sensor error then it will usually be to do with a distorted or bent ABS ring / cage.

[coullstar 0]

^ Yeah that's correct Gearoid, if the light comes on spuriously when moving at speed and flags a sensor error then it will usually be to do with a distorted or bent ABS ring / cage.

 

Same thing happened to me last week. Changed rear discs a few weeks ago and one of the abs rings was a bit loose now the light has come on.

[mr.ots 10]

If the fault is due to a wheel sensor it shouldn't light up the warning light until the car is moving as the system detects there is speed present with no pulse signal from the sensor.

If you want to check the resistance of the sensors they should read somewhere between 1100 - 1300 Ohms.

 

If a speed sensor has gone open circuit would it not put the light on constantly? As in when the ABS does its self test it can not 'see' the sensor?

 

I'm not sure about an open circuit ABS sensor, but certainly a faulty sensor (high or low resistance) may well only put the ABS warning light on when the car is moving. I had this problem last year, I'd switch the ignition on and the ABS light would come on then go off but once i was on the move the ABS light would come on again. I checked the resistance of the sensors and the NSF was reading low (around a 1000 Ohms). I replaced it and the fault was cured.

 

I don't know if this also applies to an open circuit sensor but one way to find out would be to unplug a good sensor on a working system and see if the ABS light stays on with the car at standstill.