I recently had a huge electrical problem that was corrected by simply replacing the regulator on my 240's alternator.

The regulator costs less than $40.00 and installs in minutes with a flat-head screwdriver. You can use the original bolts to replace the part, but I recommend changing them with cap-head or hex-head bolts from your local parts store. The standard, round-head mounting screws are a pain to negotiate if you do not have great tools, small hands, or patience. Also, if you are planning on keeping the car for many more years.... You may need to replace them again (after another 180K).

Good Luck,

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William
76 240D - 550K miles
78 300D - 200K+ miles