1999 Pontiac Grand am

To add to this..when I first put the Alternator on I got a jump from someone and I found out the battery had a short and the Alternator started smoking. Have not replaced it since then but got a new battery. The car sat for 9 months and I am trying to get it running again. I was wondering if it could be Alternator again. Tensioner or the battery. I did a volt check and when I went plug to plug I got an up and down reading. When I went positive to a bolt I got 13.4 and I got 12.3 off Alternator. Still no clue

13.4 is just a little below the acceptable range. I'd like to see it somewhere between 13.75 and 14.75 volts. Measure right across the battery terminals while the engine is running.

This is a real poor design generator. It is common to go through four to six of them in the life of the vehicle, but what many professionals are finding out is the repeat failures can be reduced by replacing the perfectly good battery at the same time. The old battery will work fine in a 1986 or older car with the good older design generator. The newer style develops a real lot of huge voltage spikes that are normally dampened out by the battery. As batteries age they lose their ability to dampen those spikes. That can lead to running problems from interfering with sensor signals, and those spikes can destroy the diodes and voltage regulator inside the generator.