ABS light comes on after cruising for 40'-50'. What's wrong??? on 1990 Mercedes-Benz 420SEL

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Hello all,
Here's the issues. 1. Rough idle, only on warm up cycle, idle is ok when up to normal operating temp. The other issue is with the ABS system, here's how it goes: I start the car, no light. I drive slowly and come to a stop, at about 3-5 mph the brake pedal jumps as though the ABS is operating, then I come to a complete stop, still no ABS light. Once it get going around 25-30 mph, or about 50' the lamp will turn itself on without applying the brakes. Is there any way to test the ABS wheel sensors? Another questions is, how critical is it to use the exact sensor for the car, my parts guy required a VIN to get the correct front left wheel sensor, he told me there were two different options for the 420 sel. I grabbed one at a pick n pull off a 500 sec, front right. Different part number and different mounting point for wire routing, the ohm reading seems to be very close to the new left side unit i purchased (which may be the wrong part anyway). I've swapped out the relays at the top of the ABS pump, no good-still same problem. Any suggestions would be greatly appreciated, I'm tired of chasing my own tail. Oh, one more thing, I did replace the front brake pads and rotors about 6 months ago, the problem occurred a couple months after that service was done, maybe a correlation, maybe not. Just thought I'd mention it.
(1) Answer
(1) Comments
You could check all 4 wheel speed sensors (WSS) with an ohmmeter, and check for a WSS shorted to ground. Check the wiring for integrity and you could also inspect the WSS's tone ring for defects or debris. You could have metal particles stuck to the WSS magnet, that would cause problems. I usually clean those with compressed air.
You could also have a Benz technician with a scanner check and see what diagnostic codes are stored in the system.
A similar sensor, if it mounts the same and has proper clearance, should work but why go micky mouse on a Benz? I think a diagnosis is in order, find a Benz expert.
If you want to find a specialist, here's a directory link for you:
Thanks for the prompt reply, I'll check out the tone rings first. I'm hoping that'll do it and that it's not the main pump itself - bummer. Thanks again.