[86vwTDI]

I can jump my car and it starts but soon the voltage drops. Just replaced the alternator with a 90 amp one. The battery is not new but I was hoping it would be a short other than this battery. The radiator fan stays on sometimes which kills the battery. As soon as I turn on the lights the voltage drops by 2. The new alternator seems to help but doesn't help it run more than a few minutes. The spark plug relay buzzes, I was wondering if that was where the short would be. I don't want to get a new battery yet because in WI the cold will kill it before spring if theres a short in the system. It only gets to be about 12 volts when jumper cables are taken off. I know jump starting a dead battery hurts but I was wondering if you guys could help me out with what this problem is. Thanks

 

[CafeRacer]

It could be a few thigs, and without being there to go through it all its always a shot in the dark. Personally when there is an obvious electrical issue like that I would start at the sorce. The battery. If the battery only has 12v while running its a pretty good indicator that either the battery is dead or the charging system is working. I would inspect the battery cables at the battery and at the starter(hot) and bell housing(ground). Make sure the clamps are good, clean connection, no loose posts etc. Make sure the hot lead to the fuse block from the positive battery terminal is also clean and a good connection. If your using the cheap "clamp to the wire" clamps really inspect them for connection. The last one to check is the lead between the alternator and the starter.

You can do a lot of isolation testing using a jumper cable. You can check the alternator is putting out power by clamping one end of a jumper to the alternator output and the other to battery positive. You can also check the grounding witch is just as crucial by attaching one clamp to the alternator case and the other to battery negative.