ABS Problem, Searched And Found Nothing!

[tref]

In the absence of any other reply...

May I ask if you can here the pump whirring at the 8mph point at which the light comes on? Bowing to the knowledge of others on here, I would question the pump you have just fitted... I've learnt it is never safe to assume something is ok just because it has been changed! However, if the new on is definitely ok, then one has to assume it is something to do with what tells it to whir... and since you now have a spare pump, working or otherwise, I would be trying to see if I could find where it requires voltage and make it whir on the bench... Anyone tried that? Testing the pump other than by just swapping it out and seeing if it works on a car as part of the system?

I will have to go back through my record, and again, I would bow to those with superior knowledge than mine, but I think I had an ABS problem, which turned out to be a sensor, which didn't trigger until the 8mph point... I know that is where I learnt the fact the resistance between the two pins was correct, but in relation to earth it was wrong... now I am wondering if there are two tests run for the sensors - static and dynamic, so a sensor may fail on start up, or also at the 8mph thing...

Oh, one other point - do you have standard sized wheels on the car? I have heard of problems from running wheels of different rolling diameters causing an imbalance such that the ABS throws a fit (because it thinks the wheels aren't travelling at the same speed).

Hope this helps,

Regards,

Tref.

 

[Dave928]

my pump whirrs tref shortly after moving after startup. Apparantly this is normal....

 

[Indi9xx]

Just some pointers..

The best way to test an ABS sensor is with an oscilloscope, spin the wheel, you should get a nice waveform from a good sensor, normally a bad sensor will give no waveform at all or a very weak one.. check all 4 sensors and you should see an obvious bad waveform or no waveform on a bad sensor

In the absense of a oscilloscope, use a multimeter, set it to AC volts, ideally very low volts, and spin the wheel, it will show on a multimeter as an AC voltage if the sensor is good. A bad sensor will show a lower voltage than the other sensors on the car, or no voltage.

Resistance checks on these sensors can be missleading, use an AC test or oscilloscope waveform for diagnosis.

On startup the ABS system checks for presence of 4 sensors and performs a self check, if this fails, it brings the light back on.

A light which never extinguishes after startup, often is a sign of a foobarred ABS ECU.

When the car hits 5mph for the first time in a journey, it tests the pump and valve block, if the pump does not build pressure (shown by the pressure sensor) for any reason (bad pump, relays, wiring, or sensor) it then brings the lamp back on.

Hope this helps.