94 E320 check engine light

This light seems to come back (after having been reset) whenever one of the rear wheel slips even a little under power. I noticed it come on when under slight acceleration turning right from a stop.
This time my mechanic hooked up to the computer and indeed tracked the error coming from the rear differential. He did a bunch of other stuff to the car that was unrelated but that's as far as we got with the red check engine light. Of course, within a few days, it's back.
Having spent way too much on engine and much more, I'm reluctant to ignore this light. At the same time, I'm getting sick of putting more $$ into her.
Has anyone seen this before?
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Of course - it is a mercedes isn't it?
mcbrue realisticaly under the bridge in the trailer down by the river
96 S420
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On 124 chassis 94/95 HFM, Check Engine lamp is triggered by the diagnostic module [N-59] and means there is a recognised fault related to emmissions. Your vehicle has Owner access to these fault codes by simply pressing the button on the diagnostic connector box to retrieve the codes. The codes will flash the LED back a number of counts. This is the fault code that has been found and stored. Get these and I will give you the remedy/meaning of that/those code/s.
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Mike, If it's not the speed sensor on the rear axle, and you have changed the plugs and wires, then it's the black box computer behind the carpet to the left of your left leg. P.

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