replaced bad alternator was missing voltage regulator.used old on 1990 Mercedes-Benz 560SEL

one, now car wont start.

I recommend never using "used" electrical parts on any vehicle,alway get one from a par ts store,you never know what condition the used one is in,or how long it will work,or in your case, what might be missing from the used part.
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Replace voltage regulator. About $15-$25 part. 5-10 minutes to replace. A simple DIY job actually. Always use a new voltage regulator. It is strange the new one didnt come with one as they usually do, at least the OEM ones.