when i start the car my voltage gauge reads low 8 to 9 volts after about a minute or two of driving it comes up to 13 volts and is fine. i had another altenator sittin around the garage swapped it out and it does the same exact thing. if altenator is bad wouldn't it not charge all the time? this ones got me stumped. oh and its not the gauge because the lights are dim and windshield wipers are slow but after a minute or two voltage shoots up to 13 volts and everything is fine and back to normal. thanks for any help??

 

I assume we're talking about your 91 Z28?
If you have a voltmeter, you can measure the voltage on the brown wire at the alternator connector. That wire controls charging. Also check the big red wire coming out of the back of the alternator. If you have 12V on both wires, the alternator should be charging. If not, it's likely faulty.

Hope this helps.
Lou

 

If it's not the stock alternator and it's 1 wire you gotta rev it to start the charging(like 1500 or 2000rpm)

 

Ever find anything out?