I am NOT even close to any kind of an expert- huge disclaimer!!!

But I just replaced my voltage regulator. Not sure of what tests you can do to determine if it is or not. Fastlane has some off-brand for $18 that they have never had trouble with. I had been having trouble with my lights dimming when rpm's dropped, and it wasn't cranking as hard or starting as quick as before, and the battery light had been coming on.

It wasn't too hard of a job, did it with the alternator still in. Had to custom fabricate a supershort screwdriver by ductaping a slotted screwdriever bit into a 1/4" ratchet socket to get to the bottom screw, which I accessed from underneath. Overall a 2 hour job, for an advanced beginner. Running great now...

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1983 300D- silver blue, tan leather, 205,000.
1975 Datsun 280Z. Stock, ready to restore.
1983 San Juan 23. Dry docked.