Thread: Jow to test alternator?

How to test 1984 MerCruiser altenator has a red and purple wires coming out? I had the alternator rebuild in March 2012 only had it out about 6 times since repaired and was charging first time or two out. Took it back to shop today and they said it was working. I have over 12v on red wire going into alterantor but nothing coming out. My battery runs down when engine is running. Engine will die when remove + plus cable from battery. I'm at loss as what is causing my problem. Thanks for your help.!!

Purple wire is the excite wire and CatFan is right it has to have voltage on it to "turn" on the altenator. To test the output you have to check the power on the larger orange wire and it should read 13 volts or better and climb as you throtle up to say 14-14.5 volts, Sounds to me like the diode tree is burnt out and most repair shops over look these or a bad connection on the battery, or "green" terminal can cause this type of failure. What ever you do please do not go to Auto Zone or some where like that to buy a replacement altenator as they are not marine grade and all it takes is some gas fumes and one spark , and ....well the fire goes out at the water line. A NAPA auto parts can get a marine altenator at a bit of a savings over new but sometimes just putting a new one on saves in the long run.