ABS Light always on - 90 20v

[Obli]

Hi all,

My 1991 Audi 90 20v has been sitting around for 9 months, taxed and insured but the MOT expired last Summer. I bought a 2003 VW Golf V5 cheap which has given me some 5-cylinder joy, but it's not quite the same (although surprisingly similar on paper - 2.3 20V 170BHP)

The Audi's ABS light issue started before she was stood around. I don't recall any unusual braking issues prior to winter hibernation...

When ignition is turned on, the orange ABS light appears in the instrument cluster (normal behaviour). I can hear a quick, double-click from the ABS unit in the engine bay - not sure if a normal sound; my gut says no. When the engine is started, the ABS light stays lit and never goes out. The ABS button on the dashboard does not turn off the light. However, every 2nd press of it, I can hear the ABS unit in the engine bay do the double-click that it does when ignition is turned on.

I have had a look at the no. 76 Relay and 10A fuse (this one, hidden behind the panel above driver's foot-well). The fuse is good (has continuity). I believe the no. 76 ABS relay is ok... but not sure how to test.

Is there a relay within the engine-bay ABS unit itself, or the cuboid-shaped box that sits next to it? How to test the no. 76 ABS relay? Does anyone have a spare no. 76 ABS relay I could try out / purchase, please? I'd be most grateful for any help.

Thanks,

James

[Hanuman]

the ABS box in the engine bay has a combination relay, for one description.

The relay is de-energised when the system detects a fault, and thee light is on. It fails safe, supposedly.
Faults can be related to-

poor supply. it monitors the 12v supply rail. In my experience, it requires at something above about 11v to turn the light off, but once energised, it will remain energised down to quite a
low voltage. This seems to vary considerably between vehicles.

No continuity. It needs all valid sensors. So a sensor not giving a valid signal back, even stationary, will turn the light on.

Bad signal. This where the ECU will compare the signals, to get a good waveform signal based on sensing the teeth on the drive shafts. It actually compares these over a short distance (60ft?/so many revolutions) All the wheels at low speed should essentially be giving about the same signal initially. So if one signal is wildly different, the light pops on.

[Obli]

That's great, thanks for the ideas.

The battery is good - it's brand new and charged (72AH 680CCA). So I could rule out the 12v supply, or perhaps it is a bad earth from the unit...?

I was of the understanding that if it was a wheel sensor signal, the ABS light would not be on immediately, but light up once the car travels a limited distance. My light is lit on ignition and stays on after engine is started.

I have a multimeter. I'll find out how to check the sensors.

Is there an easy way to pin-point a rotor with bad / blocked teeth before taking apart wheel bearing housing?

[msh]

Forget about sensors for now, it's just like you're understanding. First question - is the ABS working? I'm not talking about light or no light, just try to brake hard on loose surface.

Double click from ABS hydraulic unit when ABS is being turned on is normal.

[John.]

Originally Posted by Obli

I have a multimeter. I'll find out how to check the sensors.

With meter set to ohms, check resistance of wheel sensors......if there is one that is of a totally different reading then suspect this one.
Worth checking from the multiplug at ABS unit then you're checking the wiring as well.....after checking this way then check just the sensor to confirm it's the sensor and not wiring between sensor and unit.

[Error404]

IME there is a grey transistor inside the No. 76 relay that breaks off, I have seen 3 like that before. So first I would pop the cover off the 76 relay and check all the solder joints are good.
Then I would test the sensor resistances as suggested and make sure the sensors are connected correctly, no damage to the sensosr wires and terminals are good. It is quite normal for them to be seized in the hub carriers and then damaged by people trying to remove them etc
I have never seen a wiring schematic for the early ABS but if you get hold of one that will help to narrow down the culprits as you will be able to isolate and test different parts of the circuit

[Holidave]

I had the same problem with my B3 90 Quattro, after disconnecting the ABS wiring plugs individually and diagnostic with a multimeter it was an internal broken wire on the N/S/F ABS sensor wire.

[Obli]

Thank you all for the suggestions.

I'm not in a position to legally test if the ABS is working or not. The car has not MOT. I'll see what I can do

I will take the cover off the no. 76 relay, and check the terminations / solder joints.

Time to check my wheel sensors. My front disc brake shields are braking up, and last time I was driving her, I could hear scraping, scratching and squealing from the rubbing. I wonder if one of the ABS wheel sensors has had a wire slashed...

I found this wiring diagram, but it just confused the hell out of me

[msh]

Originally Posted by Obli

Time to check my wheel sensors. My front disc brake shields are braking up, and last time I was driving her, I could hear scraping, scratching and squealing from the rubbing. I wonder if one of the ABS wheel sensors has had a wire slashed...

Even if it is, it's not the cause of ABS light not going out.

[Hanuman]

With any system, there will be common faults- types of failures that can affect any machine. Eg. A wheel sensor that fails internally.

There will be some faults which become more common place in certain areas- for instance ABS failure when driving, caused by heavy erosion of the drive shaft teeth resulting in an increasingly poor signal. It was observed by the late Phil Payne in his notes, that while this can affect any vehicle, it was especially noticeable where conditions led to greater rusting/erosion. Eg. A lot of Scottish vehicles.

The ABS does a number of things.

When you turn on the electrical supply, it will determine if the system is functioning.
As far as I am aware, the ECU will need to see viable sensors, a stable power supply, etc. if all is well initially, then the light goes out. If there is a fault, the light stays on.

The ABS obviously cannot check the wheel signals while stationary.

Once you start moving, the ABS is monitoring the wheel sensors. The teeth on the drive shaft generate a high-lo signal (pulses) at a frequency relative to road speed. Spin the wheel faster, the frequency increases, Slow down and it reduces.
The ECU is going to compare the signals from all of the sensors. If one is suddenly very much lower than the others, you've locked a wheel, and it releases the brake on that wheel.

However, if the teeth on the shaft are heavily erode due to rust, or there is a lot of debris between the teeth, after 20 years of driving, then this can degrade the signal from the sensors. The sensors are still working, but they are detecting bad teeth. The system is highly tolerant. The front drive shafts on my ur were so far gone when I finally replaced them, that they were more bumps than teeth.
However, eventually the signal will be too bad to be reliable.
The light comes on.

The system is also disabled by the ABS switch. There are times/conditions where the ABS function is undesirable. So you can turn it off. When you turn it back on, it checks the system again. This is why you can sometimes reset the system where a poor signal from a wheel sensor has caused a fault.
Of course, if the fault is detected again, it is indicated again by disabling the system, and turning on the light.

On the ur, the diff locks are also monitored; turn the diff locks on, it automatically turns the ABS off.

Originally Posted by msh

Even if it is, it's not the cause of ABS light not going out.

Do you refer to the brake shields? Then yes.
But I believe that if a sensor is faulty, open circuit, or short circuit, then this is a detected fault.

I'd be interested to know if that is not the case.