I had an old 85 300zx a while back. Altanators in nissans are just horrible. They go out like nothing. I had to replace mine 4 times. It's probably the bearings in the Altanator. Does it make a loud schreeching noise when you turn the heat on?or air conditioning? Just take out that puppy and take it to autozone, they can hook it up to a machine and tell you if it's bad. Good luck mate.