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Old Voltage Regulator
The other day my generator light came on and would not go out. I ordered a new voltage regulator and started to read the factory service manual about the generator and voltage regulator. There are some pictures of the new voltage regulator at the bottom of the page. I was lucky because for now the generator is just fine, my problem was temporarily fixed with the new regulator.
Ultimately I had to fix the generator.
There are two generators for 912 engines a 350W and starting in 1968, a 420W. Mine is the former, rated at 14V and 25 amps. They require different voltage regulators because the voltage regulator limits the amount of current from the generator. If you put the later voltage regulator on the earlier generator, it will regulate at 30 amps, and the life of the generator can be expected to be reduced. The rating is stamped on the body of my generator near the fan housing. It probably is on the later ones also.
The generator light came on and would not go out independent of the engine rpm. The problems that could cause this include:
Visual inspection eliminated 1. and 2.
I decided that the after market regulator that came with the car when purchased was the most probable problem followed by a bad generator. Since the generator is expensive, a whole lot more work to replace and the regulator is relatively cheap; I decided to replace the regulator first. If this fixed the problem. I would not have do replace any expensive parts or do any analysis of more interesting possible problems.
A description of changing the generator is here.