Abs fault help! Please

[studley07]

MMM this is starting to do my head in! ABS light on and stays on before that use to flicker but then would go out as you drove down the road. had the local garage look into it for me but when they plugged it in couldn't even get a reading. was then told that this would be a pump failure because its not sending out a signal for it to read. managed to get another pump so to test it took out the relays from the pump and put them into the new one wired an earth onto the back and then took out the plug from the abs pump on the car leaving that all plumbed in if this pump is working light should go out right. NO!! APART from selling the bloody thing even scrap the piece of sh*t if it comes to it the way I feel right now. Have spent to much money and no results ANY ONE with any help Please. now due its MOT and this is my daily runner ( at the moment) Good advice please don't ake the p*ss

[twoqu]

Don't think early models (perhaps all!) have any diagnostic capability. You would need the old VAG/Bosch test kit which understandably a local garage is unlikely to have.
Can't exactly understand what you are doing but if I would check the rear diff lock. These sometimes break the arm which operates the diff lock and as its not held off, the switch is activated and therefore the light is illuminated even though the diff is not locked. Might even be a vacuum problem.

[steve briance]

I'd check each abs sensor. Start with continuity test, an open circuit will show up as a fault and that will make the light come on. Abs sensor plugs for rear are under the rear seat on a coupe, and clipped to the suspension turret under the bonnet for front. If there is continuity, do a resistance check to see if they are all more or less tue same. Big difference is likely to cause error. Dirt/rust on the toothed inductor ring in the cv joint may also cause it it I dount it unless the teeth are falling off or full of dust.

[twoqu]

If the light is off until the car moves that might suggest a wheel sensor issue. If its on all the time its probably something else!
However as usual its always best to test what you can to elimate what it might be.
Actually the previous flickering of the light is what happens sometimes when the one waty valve fails on turbo cars and pressure enters the system. Obviously no boost on a NA car but might indicate an issue with the vac side of things.

[twoqu]

Of course we might be overlooking some basics mainly battery voltage of which the ABS system is sensitive to apparently.
So is the battery fully charged (12.6V ) and does the alternator provide 14V into it or close to whatever the regulator rating is?
The ABS relay sems to get a feed from Fuse 31 in the fuse box so is that ok. This is the suppy to the ABS relay plate contact 1 so check its got that far. I believe that the diff lock switch effectively earths the relay through contact 5 on the relay plate so if there is 12V+ between contact 1 and 5 the diff lock switch is indicating it is operative. If its 0V then its not.
So theres a few basic checks to perform assuming you have and are able to use a multimeter.

[studley07]

Brilliant thanks all for your input.

[twoqu]

Some of this might be of use. Lets concentrate on the ABS Combi relay as thats responsible for sending power to the ecu and the gubbins under the bonnet although there are others.
So as far as I can make out the relay plate is assigned as below:
1.Signal from rear diff lock (more about this later)
2. 12V battery feed
3. Unused
4. Earth
5. 12V ign feed from fuse S31 in fuse box
6. 12V out to the ecu and gubbins
7.Unused
8. Signal from ABS switch

So its pin 6 we are primarily interested as that decides whether the ABS operates. The other signals in can disable it.
Personally I think its a good idea to draw a diagram of the relative postions on the plate so that when you are probing behind it you know what pins you are touching. Some tests can be done without the relay in.
The first one would be to test pin 6 and an earth somewhere with the relay connected and ign on. If its 12V then we would have to assume that its happy! If not more tests.
We can remove the relay to test both the battery voltage , pin 2 and 4 which should be permantly live and with ign on pin 5 and 4. If you get no readings on either then test for continuity between pin 4 and a good earth nearby.
The next tests I'm a little unsure off. I understand you should get 12v with ign on between pin 8 and 4 when pressing the ABS switch. As for the diff lock signal I have read two different versions. The first I have already mentioned above and the other the complete opposite. You might have to see what is on it with ign on. If pin 5 is 12v ign then the ony way you would get 12V by measuring between pin 5 and 1 would be if pin 1 was earthed. The wire appears to be brown with a tracer so I think that suggests it is earth related. So you will have to test it under those conditions. You will probably need to move the car to get the diff to disengage.
Perhaps its do do with whether the relay is installed. If I have a euruka moment about this later I'll let you know!
So if you have got this far then basically you should have enough info to do some basic testing. The ABS will be disabled if this relay is not happy about voltage, or has either of the signals present. If it has voltage down pin 6 the the issue is elsewhere.
Last thing, if that relay has the fuse in the top which I believe it should have obviously check that!

Just looked at another schematic so it does look like the signal at pin 1 would be earth when the diff lock is operated. You could check that by measuring between pin 1 and pin 4 as it will either be an open or closed circuit to earth.

[studley07]

Absolutely fantastic. Thank you very much.

[twoqu]

Although ideally we need to test, a quick stab might be to replace the small relay at the pump with another of identical type. This appears to earth the ABS light unless energised which is by the power from pin 6 of the ABS Combi relay. Another thing to check is the small wire from the alteranor as this seems to have a role in the ABS as well.

[M2ki]

Although ideally we need to test, a quick stab might be to replace the small relay at the pump with another of identical type. This appears to earth the ABS light unless energised which is by the power from pin 6 of the ABS Combi relay. Another thing to check is the small wire from the alteranor as this seems to have a role in the ABS as well.

When there is cancel switch, the ABS light is not wired through a relay.

http://s2central.com/S2_Schematics/D/D26.pdf

[twoqu]

When there is cancel switch, the ABS light is not wired through a relay.

http://s2central.com/S2_Schematics/D/D26.pdf

Think this is more do do with the age of the vehicle than the ABS cancel switch. The schematic you have linked to does not show the 87a terminal linked via the diode to the ABS light whilst an earlier schematic for a vehicle a year earlier with the similiar if not the same modulator does show such a link. Its all Bosch ABS 2. So perhaps this feature was omitted on later variants sometime around the date of the schematic linked to. Later they sort of incorporated most of the functionality of the Vag 1710 tester into the controller and deleted the switch.
Hopefully it something relatively easy to find.

[twoqu]

I would have thought that the bean counters at Audi would have used all the same components across the quattro models of the period to save a few DM. That seems to be the case in ETKA which implies the same modulator was used for most of the Coupe Q run until the very last model year. The p/n of 857 gives an idea of its origins!
You would expect that the pin asignments would be the same at least for the hardware which is not suppported by the schematics of different models with the same modulator. Even with the ABS switch delete/ Combi relay delete/ OBD implementation later on, I can't see how that would effect the mechaical end rather than the electrical side. So its not making sense to me.
Might not be an issue as we are suspecting an electrical issue.
Might be worth posting the ABS module part number and the relays.

Arrr.... didn't look hard enough at the ETKA. Guess Audi took the opportunity to phase in the later 4A0 modulator with the introduction of the ABY or B4 models and demise of the 7A/3B. So those schematics must be for the upcoming B4 models rather than existing models at the time which sort of makes sense as what would be the point of drawing up a new set of schematics unless there was some change that needed to be documented.

[M2ki]

Abs

As I understand you got it fixed. What was the problem exactly?