The rear sensor on my '87 560SEL and SEC are in the right (passenger) side of the differential, up front. It reads a ring mounted on the pinion shaft.

I'd clean my front rotor sensing rings (don't remember the right name - the grooved piece the sensors read) before doing anything else. IIRC, a compressed air blast will blow the crap (pun intended) out of the grooves.

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered

I'll give the compressed air blast on that grooved piece a try today and report back on my findings. Thanks.

Well, I blasted all the dirt out of the grooves, but the light still comes on as soon as I go over 3 mph. I checked the resistance on both front sensors, and they read about the same. Therefore, the only other options I see that could be causing the problem is that either the wiring from the front sensor plugs to the unit is not carrying the signal, or it is the rear sensor. Is this a correct assumption, or could it be something else entirely?