We need a good service manual. The ABS system uses relays, fuses, an EBCM, and of course, the hydraulic unit itself. I would suspect a fuse, relay, or the EBCM. Have you checked the ABS fuses lately?
Also, check to make sure that you have a hydraulic unit (like your other car) just to check that your car was actually equipped with ABS.
And with that system you can check the codes yourself. Jumper pins A and H. Turn the ignition switch on. You should get a continuous ABS light for 4 seconds. Then it will flash the first code at you. To get the second code, disconnect and reconnect the jumper with the ignition still on.
This won't work on 91 and up because of the newer systems.
1999 540iA Sport
Former 93 SSEi Owner
Former 89 SE Owner