ABS problem - driving me mad

[Marshy]

2000MY S4, heap big ABS trouble.

The electronic limpet from the ABS pump has been to BBA Reman already, and declared healthy. In fact, my car has a newer part than the one that everyone else seems to have fail. All the wheel speed sensors have been checked, ditto the rings that go with 'em. The relays that feed the ABS system have been replaced. There are no fault codes hanging around.

My local Audi specialist has come to the point where they reckon it'll be either the ABS pump itself, or the control module for the ABS/ESP. So, £800ish list or £1000ish list, without knowing which one it'll be. They've spent a ton of time looking at it, and bless them, only billed me for a fraction of it, but now the car's back in my hands to decide what to do next.

So, in the hope that someone can offer suggestions, here are the symptoms (they're in a previous thread somewhere, but I can't seem to dig it out right now):-

* Unexpected ABS activation
- To start with all feels good, but you can then feel the pedal drop in distinct steps
- Then the ABS will kick in fully (lots of ABS pump noise)
- Then it'll either disengage, at which point you'll get the brake force for the amount of pedal travel you've ended up with, or it'll kick out and in one or two more times
* Commonly, after unexpected activation, I'll hear the pump going under the bonnet for a few seconds... when I'm stationary or back on the gas pulling away again
* When coasting with a fraction of throttle opening (according to my right foot), it feels like the car is tweaking the throttle ever so slightly
* Clicking relays under the dash on the driver's side
- If the headlights are on, the headlights (almost imperceptibly) flicker in time with the clicking relays
* Turning ESP off makes no difference
* Unplugging the electronic module on the pump cures it - but leaves me with the rear brake bias issue: no fun, well, dangerous, with winter on the way

Now, it's possible to drive through all this, but I look like a bizarre sort of learner, kangarooing up to traffic lights rather than away from them.

I'll call MRC in the morning and see if they've any ideas - or a spare ABS/ESP brain that can be swapped in to rule that in or out. I'm just not into the game of buying near £1000 parts that *might* fix it. Not on a car with 183,000 miles!

I'm all ears for other suggestions: what else does ABS/ESP take input from, which can be checked out? It's the time for a cunning fix, or selling the car to someone who wants a track slag with no ABS.

Sigh. It's such a nice car otherwise.

[imolas4uk]

if you drive the car and check the wheel speeds with vag com are they all the same?,do you have matching tyres and tread depths?,check the air gaps between the wheel speed sensors and reluctor rings.

[Marshy]

Sensors & rings were physically eyeballed a few days back, but I haven't had a change to get vag-com on it to compare speeds (my vag-com laptop is in a state of flux right now...) I think my garage did that anyway, but I'll check.

I've disconnected the ESP yaw sensor at the moment, which disables ESP and ABS but, crucially, leaves me with the electronic brake distribution intact, thus meaning the car can safely be driven, unlike with the solenoid module unplugged.

I've also found someone flogging the ABS/ESP controller on ebay for a bargain price. So I'll have that component to try swapping as it turns out. If that makes no difference, it'll be trivial to re-ebay it at no loss.

[Marshy]

Quick update in case anyone has a lightbulb moment that might help out...

I've now swapped in a spare ABS control module (J104 / 8E0 907 389 E), coded it to the car, recalibrated the steering angle sensor (G85, has to be done if the control module is replaced) - all to no avail.

So that's one thing down - much more to try.

I've also independently verified, now I've got VAG-COM working & fully licensed, that the wheel speed sensors seem to be talking sense and in harmony, never more than 1km/h apart, which is in spec.

I seem to recall there's a fifth speed sensor somewhere - any idea which controller and measuring blocks I need to find that one and check it? I also don't know where to find output from G200 (transverse acceleration sensor) and G202 (rotation rate sensor).

Are these things just not exposed? I'd kinda like to get vag-scope or logging on every sensor I can that feeds ABS, and see whether anything measurable goes on the fritz when the problems happen.

[raver100]

there is a speed snsor inthe side of the front diff

[raver100]

sorry for bad spelling my wife was distracting me!

[Marshy]

Cheers Raver. I'd heard of that one before, but I've not been able to figure out how to measure its output to compare with the other speed sensors. Any idea where to find that in vag-com?

[Marshy]

I've also just eliminated the ESP combo sensor (acceleration & yaw - I think), by swapping it out. No change in behaviour.