You might be benifited by locating a shop with the bosch LED tester. It was required tools for Bosch Service centers of that era, As such we have one and it sure simplifies testing.

If the light is on all the time then look to powers and grounds on the wiring diagram. Check the relay inside the hydraulic unit by removing the units black plastic cover. The relay looks like a standard "icecube" relay.

The most common issues of ABS light on all the time are: bad OVP, bad pump relay, and bad control unit. The only test for the control unit is another one. They are all the same for years so swap one with a buddy (put yours in his car to keep from scrambling his if there is a weird circuit issue).

The sensors won't cause a continuous ABS light unless they are open circuited. Use an ohm meter to test.

If the light comes on after hitting 10kph then the sensor function (one or more of them) is implausible. It can be checked with a voltmeter but a scope does a better job.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician