I have a '91 Z28 that has a alternator issue. The alternator will only put out 11.30-11.99 volts. The battery, starter, and alternator are all new. I have two new alternators and the same thing happens with both. I only drive the car once in awhile and it always fires up after sitting. Once running it just won't charge what it should be charging. I unhooked the wire that goes to the battery and had no voltage off the terminal on the back off the alternator. Shouldn't I have about 14 volts here if the alternator is putting out? I have checked all grounds and don't see any problems. Any ideas?

 

how many wires is the plug that goes to the alternator?
one of those is an "excite wire" that needs to be hooked to 12v ignition to kick the alternator on when the car is running.
if all the wiring is in tact, it sounds like you have a bad diode in the alternator.
it is very common with a cs130 alt.

 

I have two wires that go into the plug on the alternator, both have power with the ignition on.

 

take it off and have it checked. its dead.

 

Make sure the the brown wire hasn't been hooked directly to keyed 12V. It should go to the volt gauge. It needs to have some resistance or it will fry the alternator. @ 400 ohms or a light bulb is enough.

 

I found my problem today. I had my fans wired up to run off a switch and was getting power from the fuse box where the fan fuse should have been. I guess the fan and alternator charge wire are on the same circuit. So I put the fuse back in and the alternator charges what it should charge.

 

good. I had a problem with my alternator a wile back, previous owner wired it wrong and it fried.