Don' t know what the problem is, but I have that same problem too sometimes on my '90 vert. I recently purchased an 86-88 alternator without knowing that there was a difference in plug size as compared to my '90 one. The alternator I have is BRAND NEW and I am willing to sell it. If you are interested, e-mail me at themikechu@aol.com

 

Could be any number of things actually. Alternator, battery or connection between either. It's easy to take the alternator off and take it down to Autozone or the like and have it tested. Same with the battery. I noticed on my 86 that the ground and hot wires were in bad shape when I rebuilt it. I replaced both. Good luck.

Chris

 

It's probably the alternator. It's easier to test the alternator than the battery if you can avoid taking the alternator off. All you have to do is have the car running and connect the battery terminal with a volt meter ($15 at Home Depot or Radio Shack). The voltage should be 14v and if it's lower than 13v, it's starting to drain your battery. Most likely, this will just reaffirm what you saw on your dash. If you wait too long, your battery will be dead, cause a break down, and hard to jump start.

Batteries are hard to test. They fail little at a time. When they start to go bad, you might have to jump start it and then it will be fine for wks or even months before it fails again.

Make sure you check for battery terminal connections also.