Just want to share a quick way to determine cause of ABS warning light. IF the light comes on as you turn on the ignition ,and stays on, the problem is probably the ABS Control Unit-where the system gets its power. If the light comes on after you begin to drive its probably the sensors,wiring,or relays. Ask me how I know

How do you know?

FYI - On W126 model cars, I think the ABS gets it power from the overvoltage relay. That relay is sort of a "surge protector" for electrical power going to sensitive electronics. The pins can get dirty and it just needs cleaning or sometimes it needs replacement.

I would refine that to be:

1) If light is on after starting without any movement of the car then is probably a module or upstream module issue

2) If the light comes on after movement, then probably sensor issue.

My 201's ABS light will come on as in situation #1. A few key turns and some waiting and it will behave normally.

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On both my E420's, I have experienced the ABS light illuminated after starting, but before any movement of the car, and it turned out to be a bad connection on the ABS pressuring pump (where 18 joins M15 in the picture below). This is the small electric pump under the master brake cylinder. Re-securing this connection solved the problem.

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD2/Program/Chassis/42-0818.pdf

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

The schematic refers to the pump as an ASR pump-not ABS

Regardless of what the schematic refers to it as, a bad connection here will set off your ABS warning light. The ASR and ABS systems are interrelated.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles