If it's a CA car, then there should be a LED and push button which will read out the diagnostic trouble code (DTC) from the Diagnostic Module.
If you have it, then check my web page to see some of the TYPICAL DTCs for that module in menu item #5. Note that these codes are for 1992 thru 1995 'S' car (119.x and 120.x engines) and strictly speaking would not be applicable to your car, only coincidently. But they may give you an idea what's wrong.
If you get interested in 'fixing' DTCs yourself, a scanner is a must. AST makes the "POINTER" and it works very nicely for MB's from 1991 thru 2000. The DTCs are read directly from the LCD screen. There's more info on my page under Menu #2.
Regards . . . . JimF
'94 S500 Cpe
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