ABS Light always on - 90 20v

[Error404]

I believe the light will not go out of the ECU cannot detect all 4 sensors.

Yup those style of wiring schematics are crap.

[Roger Galvin]

The ABS system is very sensitive to low voltage, if the alternator/battery are not producing a solid voltage the ABS system will call fault.

The commonest faults in my experience should be simply checked as follows:

Check the ABS relay (No 76) in the auxiliary relay panel that the fuse (10 amps) has no blown, if it has then replace it if it blows again try replacing the relay. If the fuse is good then try changing the relay anyway, this is the most common area of failure in the electrics.

If with a known good relay in place the light still remains on then substitute the ABS computer with a known good unit. IME 95% of cases are resolved by this sequence of test and substitution.

If despite good relay and computer the light remains on you need to check the wheel sensors. The diagram shows solid coloured wires white/red/green and yellow respectively travelling to the four wheel sensors. Using a multimeter set to continuity, test the wires at the multiplug for continuity, all four should confirm as being unbroken. If any are found broken further test between the wiring harness to the plugs, and then from the plugs to sensor and back. When a break is found IME its usually in the sensor rather than the wiring.

If all four circuits to the sensors and back are found good test the resistance. Again using a multimeter (set to 2 kilohms) check what readings are found. They should ideally be just under 1 kilohom, a result of 0.95 would be typical. All four should read just about the same, if any is substantively different then suspicion falls on the sensor relative to that wire. Swapping the sensors on the harness will asist in identifying whether the cause is inherent in the sensor to the plug, or due to incorrect resistance because there is a damaged wire in the harness (possible but unlikely).

Where the cause of ABS issues is related to 'faulty' sensors or dirty or coroded pickup rings/teeth commonly the light will extinguish but due to the computer being dissatisfied with the amplitude or consistency of signal it will call fault and turn off again. This will only occur when the car is moved until then the computer has no comparative data. I am therefore doubtful that this is the cause of the light failing to extinguish. Other components of the system, accelerometer switch, ABS pump, on/off switch etc. are possible causes but I've never known these to fail. If the above doesn't find the solution then you'll probably need to test the wiring input supplies/earth returns etc to identify the cause.

Hope this helps

[msh]

Originally Posted by The Real MSH!

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.

It is not the case. You and Error404 are believing in something wrong This old ABS isn't that sophisticated - it can't detect missing sensors unless remaining sensors are providing signals. As an example - I have garage, around 300 metres from my home - when I had half completed car, which was also missing one ABS sensor and two were too far out to provide signals, it took almost all of that distance for it to detect it and turn itself off - basically when I drove in one way, I drove with ABS on, and had to brake a couple of times hard, because ABS was interfering, "thinking" that a couple of wheels are blocked, and shortly before arriving it switched itself off.

[Obli]

8 years later and I've fixed the issue of the ABS light staying on permanently from ignition. Sorry to revive this old thread but I wanted to post the solution.

I replaced the ABS controller unit under the rear seat. ABS light now going off when car started and ABS switch in dash working.