99 S4 - ABS wierdness

[Marshy]

So this is one for anyone who knows the Bosch ABS/ESP fitted to the B5 S4 and plenty of other VAG cars. TT perhaps?

A while back, I started getting unexpected ABS activation, usually when slowing down to a stop, but at other times too. When braking, to start with all feels good, but you can then feel the pedal drop in two or three small distinct steps. Then the ABS will kick in fully (lots of ABS pump noise). Finally, it'll cut out, restoring pedal feel, and increasing braking force as I come to a stop. 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.

Also, when coasting with a fraction of, or no, throttle opening (according to my right foot), it feels like the car is tweaking the throttle ever so slightly, with synchronised relay clicks under the dashboard. Wierdly, the headlights, if on, will almost-imperceptibly flicker at the same time.

Most responses I've had so far are along the line of "it's the control module on the ABS pump, send it away for repair". Now that's true of pre-99/00 cars I think: mine has a newer module than everyone I've spoken to who's sent their module away. I sent it to BBA Reman anyway, who said two things: 1) "we've not seen these fail before" and 2) "we've tested it and can't find anything wrong". Actually, they said a third thing: "£50, please"

My local specialist has had a long look at it: they've checked the ABS speed sensors and rings, and say they can't see a fault there. I've confirmed with VAG-COM that they're reading within 1 km/h of each other up to 19km/h, which is in-spec according to the documentation I have. The specialists said they thought it was either the pump itself of the ABS/ESP control module. I'm personally not sure how it can be the pump, as it won't even run unless instructed to do so by the electronics.

Since then, I've replaced the control module (J104), re-coded it to the car, and re-tested. It's made absolutely no difference. I've also replaced the acceleration and yaw sensor which has likewise made no difference. In conjunction with these replacements, I've reset the steering angle sensor and undertaken the ESP driving test as recommended by the docs. These were all cheap second hand parts on eBay sold, I think, by people who don't realise how damn much Audi charge for new ones

All the recent snow and ice revealed an interesting detail: ABS and he traction control part of ESP appear to work just fine when genuinely needed. Lots of fun it was too

Other things I've tried: With the acceleration/yaw sensor unplugged, ESP and ABS both go out of action (leaving the brake force distribution (EBD?) intact). The ABS fault also goes away, perhaps obviously.

I've also tried removing the ESP fuse, which throws up the traction control light on the dash, but the ABS fault remains. That may sound obvious, but how much of ESP overlaps with ABS isn't clear to me. Certainly, they seem to at least share the acceleration/yaw duosensor somehow.

I'm a bit unsure where to go next. I'm certainly very scared of simply tipping it into a main dealer: there are no fault codes stored, and I'm very concerned about ending up in the sort of territory where their only recourse is swapping out expensive components one-by-one. I hear there's another speed sensor in the mix - would that be the "vehicle speed sensor", and does ABS take input from that in addition to the four wheel speed sensors? I'd expect speedometer wierdness if the VSS was on the fritz though, but that's another thing I guess I should check that I haven't yet.

Well; I've written War & Peace here it seems. Sorry about that, just a by-product of trying to remember all the details so far. I'm all ears for ideas on this one, aside from "delete ABS, add 2xK04s, turn it into a track slag".

--Chris

[TooLateForAName]

Cracked reluctor ring?

Are they all on the driveshafts on an S4?

How did your specialist check the rings? Its a while since I had this problem on a 98 A4 quattro and I cant remember if I had to remove the driveshaft to see it or not. Start with the nsf unless you've had a bang on any of the other corners.

Edited by TooLateForAName on Sunday 27th December 15:03

[Tame Technician]

That fault (An ABS event when its not required) only comes when the sensors misread (by a small ammount). Thats why you dont get a fault code, it thinks the ABS is required (one wheel slowing quicker than the others) and it triggers it, so in its mind there isnt a fault.

We have had this on a number of cars, usually older ones like yours. Its really hard to diagnose but it is always wheel speed sensor related, but NOT usually the sensor itself.

It usually a mechanical thing like the rotors being bent or the air gap between the fron sensor and the pickup on the C.V joint being wrong. But can be caused by slight resistance in the link harness' that go to the sensors, sometime there is a visable brake in the wire or you can feel a thiner bit of wire when you wigle then in your hands.

Try and monitor the wheel speed signals at teh speed the fault mostly occurs at and see if you can see one showing less that the others particually at that speed range.

Edited by Tame Technician on Sunday 27th December 18:07

[Marshy]

Tame Technician said:

That fault (An ABS event when its not required) only comes when the sensors misread (by a small ammount). Thats why you dont get a fault code, it thinks the ABS is required (one wheel slowing quicker than the others) and it triggers it, so in its mind there isnt a fault.

Try and monitor the wheel speed signals at teh speed the fault mostly occurs at and see if you can see one showing less that the others particually at that speed range.

Edited by Tame Technician on Sunday 27th December 18:07

ABS/ESP/etc doesn't function when being interrogated by VAG-COM, but that only lasts up to 20km/h, at which point VAG-COM gets disconnected and ABS and all the other bells and whistles function normally. The upshot of that is that I know of no way to monitor the output of the wheel speed sensors when actually travelling at normal road speeds.

Up to 19km/h, the readings are all within 1km/h of each other, which is allegedly within tolerance according to some Audi guide I found via Google.

That said, I take on board everything else you've said: it all makes sense and is one of the things first checked by the garage. How the checked it, I can't say, so I could send it somewhere else for the same thing to be done I guess.

All that said, would you expect a wheel speed sensor issue to cause the ABS pump to occasionally run when not braking, or for the headlights to flicker in time with the pump going on and off, or the strange hunting feeling on a trailing throttle?

Edited by Marshy on Sunday 27th December 18:38

[Tame Technician]

Marshy said:

would you expect a wheel speed sensor issue to cause the ABS pump to occasionally run when not braking, or for the headlights to flicker in time with the pump going on and off, or the strange hunting feeling on a trailing throttle?

hhhmmmm dont know about all that, I would say quite posibly.

An unwanted abs/traction control/ESP event would be caused by a sensor related fault. That could make the pump run even when not braking. (The pump on my car definatly runs when the wheels spin as well as when they lock up.)

If the lights always flicker when the pump runs, then yes could be connected and caused by the same sensor issue.

Can you final control the pump with vag com? If so, do and see if the lights flicker. Could simply be the vibration of the pump runnig making the inside of the light shake.

If the hunting sensation is the result of one of the brakes being applied and released by the ABS module then yes that definatly posible too.

(Again my car does this to apply the brakes to the spining wheel. Although my car is nothing like yours, generally the software in the bocsh/ATE ABS unit is much the same regardless what car its on.)


All that lot at the same time sounds a bit like a bad earth, but if the ABS fault was caused by a bad earth you would expect a fault code, battery voltage to low or somthing like that. So it would be worth checking and cleaning earth pionts etc, but the likely cause for me is still a fault with the control module not getting the correct signal from one of the wheel speed sensors, weather that be cause by wiring, mechnical problem with the sensor rings/pickups etc.

[Marshy]

Alright TT, there's plenty of sense there - thanks (really, it's appreciated).

Since it shouldn't be hard, I'll see if I can find the relevant earth points and check those (I know one is at the base of the driver's side B pillar). Then I'll probably send it somewhere to get the physical side of the wheel speed sensors checked out again. Sigh

I note you're in East Anglia - which garage do you work for? PM me if you'd rather not say on here...

Cheers.