There are a number of things that will give the light sysmptoms you are seeing. Voltage source (OVP) is a posibility but a bad alternator, a bad pump relay (in the actuator), a bad brake light circuit, a bad ABS controller, any open circuited sensor, etc. will turn the light on (or more precisely "leave the light on").

On systems before self diagnostics we use the Bosch ABS tester. Unplug the controller and plug it in. There were about 8 or so leds that were on when the proper circuits were OK.

When, the light is on the ABS is off and the brakes work as they would if there were no ABS. Dual circuit front and REAR hydraulic brakes.

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