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.!!

The larger terminal with the red wire, is also the wire that charges your battery. You should be reading probably around 13.5 volts or better when the engine is running.The purple wire might be for a charging indicator light. I don't think an alternator system will produce power without the battery in the circuit. The old generators would though.

Your red wire is the battery sense, and the purple goes to your ignition +. If the purple isn't getting the signal, the alternator won't charge.

I have an 02 merc 4-stroke that would discharge the battery with the motor running. I had a spare regulator that I put on it that didn't change anything. Got to tracing wires and found a fuse holder with 4 fuses in it that had. 25 amp fuse blown. Replaced the fuse and it charges the battery just fine now.

With the motor running the alternator should be magnetized. If it is not magnetized then it is not working. If it is then as stated above you need to check wires, regulator and fuses.