My mom's 1988 90 quattro has the ABS light on all the time. I STFA'd and the posts suggested to check the fuse, and clean all 4 sensors. I did all this, and still the light remains on. I test drove it in loose gravel, and they definitely are not kicking in.

As for the sensors...is there a simple way to check them to see if they're good? How critical is the spacing between the sensor and the splined hub?

Also, what do the 2 relays on top of the ABS module (under the hood) do? I popped them off and they looked clean/OK. I also checked all the connections (under the rear seat and also in the fender wells) and they looked fine.

Thanks for any help you may provide!

--morgan

Morgan
This recently came up on the 20v list on rennlist.com...

John Emm wrote
"2. My abs is off, always. The orange ABS OFF light is always on, and a quick slam on the brakes in the wet test showed that it is right. What could be the problem? It doesn't matter if I press the ABS off button or not. They
are always off."

Patrick Carlier wrote
"Check the abs relay . I've cured this once , it was no more then a clod solder joint inside the relay."

Worth a look!

they should be between .8K ohms and 1.8K ohms

you just probe the connecters you checked....

i did this and found that i have an open sensor....BINGO!

also, use the same multimeter to test the wiring from the ABS computer to the sensors (again the plugs) and to the hydraulic unit.

Checked the relays and they are fine. Any other suggestions?
Thanks for you reply.

I checked the resistance and all 4 sensors indicate about 20 to 21K ohms. May be my meter is messed up, but they all show continuity. I have a manual but it doesn't have a wiring diagram. Would you have a diagram for the ABS system on a 90Q 1988?
Thanks for any help

i am most familiar with the 80/90 ABS...i also happen to have a 1990 90 Q parts car with ABS components :p

under the dash, position #1 (farthest LEFT if you are looking at the relay panel from sitting in the driver seat) there is ANOTHER relay....this one has a 10 amp fuse in the top of it....check that relay and thatfuse!

if it were a speed sensor, then you could stare the car, and the ABS light would be OFF because all speed sensors read the same because you arent moving (they read zero) and when you move for a while the ABS notices something

there also may be an inertia switch...it senses when you stomp on the brakes, and grounds a terminal on the ABS computer, and i think your car may not have it tho. if it does, its under the seat, with a small wire plug going to it, and has an ARROW pointing to the front of the car.

i do have a manual, but i will have to look later...HTH...

I checked the fuse on the relay and it is fine. I pulled the relay apart and resoldered all the joints on the two cirsuit boards. Not sure what else I can check on the relay itself.
Re-installed it and it didn't make a difference. This vintage 88 90Q soesn't have an inertia switch.
So when you start the car the light never goes out. If you push the anti-lock button on the dash it blinks momentarily and then resumes it's bright glow.
Any suggestions welcome!
Thank you.

mabey thats bad...or the diff lock unit....

i dont think anything is wrong with the wheel speed sensors or the light would go out, and come back on....

the ABS is made up of the following components:

-ABS computer (under rear seat)
-wheel speed sensors (4)
-under-dash relay w/fuse
-2 relays under hood on hyd. unit
-Hydraulic unit
-inertia sensor (you dont have one on an 88-89_
-brake pedal switches (CHECK THESE! TWO on pedal)
-and wiring to connect all.....

start checking continuity of ALL wires, and thoes brake switches....one is vacuum too.

I think both the dash and Antilock switches are ok as when they are depressed they make the light blink once and then it goes back on.
You wouldn't happen to have a wiring diagram of the system?? That would sure help.
If you do or if you don't I will follow your suggestions.
Thanks again,