Mk2 ABS help.

[sam j]

Thats my plan. Going to drag the oscilloscope out some point this week and see what kind of signals I'm getting off the sensors. I assumed that if a wheel sensor was giving a crap signal the warning light would come on again?

[Gti_Jamo]

Thats my plan. Going to drag the oscilloscope out some point this week and see what kind of signals I'm getting off the sensors. I assumed that if a wheel sensor was giving a crap signal the warning light would come on again?

The light will only come on again if the abs system shuts down and reverts back to non-abs mode. But since you were getting juddering through the pedal that suggests all is well electrically and its a dodgy ring causing the pump to compensate for an innacurate reading.

[sam j]

Not had time to look yet, but one thought... on the end of the ABS unit there is a 2-pin connector on what looks to be some kind of sensor / switch. Any idea what resistance this should read and when it should change?

(the 2-pin connector on the far-left end of part #15 here)

[sam j]

Just to update quickly...

Went through all for wheel sensors with the oscilloscope, and all gave pretty good signals. Just to make sure I went through each sensor in turn and bypassed it, connecting the sensor on the other side of the axle to both ECU inputs (ie, connect front left wheel sensor to both front left and front right connectors on the abs wiring loom, bypassing the front right, etc etc), and still the pedal jumped around of its own accord when driving slowly...

So, each sensor has good resistance, seems to give a stable signal on the oscilloscope, and makes no odds if its bypassed... I am swaying towards it not being a wheel sensor. And if it is, its got to be at least 2 of them!

So, question now is what is left:
ECU,
Pump / valve unit / master cylinder
Is there a speed sensor anywhere?

Guess a lot more cable tracing needs to be done, check all earths and make sure there are no damaged / shorting wires anywhere. Don't know why this would only affect the system at very low speeds though...

[sam j]

Just to update this again.

I double checked myself, and bypassed the front right sensor again, using a bit better wiring this time, and it all worked fine... so, I've now changed both front wheel sensors, and all is good. All working as it should

My theory on this, front left sensor was dead, which was keeping the light on. Replaced that and light goes out Front right sensor the insulation around the connector in the engine bay was all brittle and falling apart / held together with tape, so whilst the sensor measured okay from a resistance point of view, the signal coming from it was poor with a lot of crosstalk etc. Both changed, all working, job done

Now on to the rest of the little jobs... starting with the rear wiper motor being crap...