My first MB I purchased the owner didn't know if it was the battery or fuel pump. I picked up a battery, installed it, and it turned the starter for about 3 sec and died. I was thinking crud... Got a volt meter and the battery was only reading 9. something volts. It shorted out internally. Went back got a 2nd New Battery. Car started and off I went.

Check your voltages, not running and running.

Good luck

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92 300D 2.5L OM602 OBK #59