When it's cold out I noticed the car battery won't charge. Volt gauge will sit about 8 and slowly progress down. The lower the battery gets the worse the car runs almost like the alternator isn't doing anything. Will begin to hesitate and want to stall. When the weather warms up, the car starts up and runs great, voltage on gauge goes to 13 and all is well in the world. I had the alternator checked 3 times, and I have swapped in two different batteries. Grounds seem to be fine. Has anyone had this issue and pin pointed the problem? It's very odd and hard to tell if the car is going to run good or not. It's an 87 Trans Am 5.7.

 

You should check the starter fuseable links . There might be corrosion going on.

 

Make sure the connections at the connector on the side of the alternator case are good.
I had loose connections there drive me crazy for a while.
If they are not tight, replace the pigtail there.

 

You may need to change out and upgrade your alternator wire. Gm thought it was a good idea to put a #8 or #10 wire between the alt. and battery. #6 should be used, because even if a strand or two is broke, that's like trying to push a fire hose full of water wide open through a garden hose. You can also do a voltage check on that wire to check for voltage drop.

 

Alternator wiring looks like new. Car only has 36K miles on it. I will look into it further.

 

New isn't always good if it's undersized. How many amps is your alternator? 105?

 

When the alternator was tested, was it cold such as when it fails? May want to put it into the freezer for a few hours then place it in an ice chest to take it to be tested.

Also, next time it fails on the car measure the alternator voltage at the BAT stud. Should be the normal output voltage. If it is then the alternator is OK and there is a wiring issue.

RBob.