I am having this same issue again and unfortunately I'm not sure what cleared it up the last time. I tested voltage and I'm getting 14.2 at idle so I know it's not lack of voltage. I can't visually see anything wrong with the sensors. What I did do was go out to the middle of nowhere and did an emergency stop as someone previously recommended and the abs did not work. Then I tried to accelerate and had my wheels spinning on loose gravel and that still didn't work, abs did not kick in. Problem only occurs once I start driving. I jacked up both rear wheels and put the car in D, and accelerated to about 40 mph, and started to brake and the abs unit did work and the light never came on. When I put the wheels back on the ground and started driving the light came back on. So not sure if this narrows it down for someone, but what could be going on? It's obvious the abs relays and wiring is ok because it works when the rear wheels are off the ground, but it stops working when the entire car moves.

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1990 190E 3.0L