i installed my new alternator now my battery has died,what could have caused it.

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1987 Trans AM GTA
305 5SPD
Hypertech airfoil
gutted maf
kn filter
High Flow Cat,
3"Walker pipe
Dynomax Super turbo Muffler
TB Coolant bypass
Lubricated by Mobil 1 Tri-Synthetic

 

If it's hooked up right, you could have blown the voltage regulator. That happened once on my turbo T-bird. I'd go back and check all your wiring and grounds. Sometimes it's better to have someone else look, because they'll see something that you've been overlooking.

 

I've heard of a specific characteristic of alternators for years and finally saw it myself a couple months back. The battery in my wife's Olds was dead, had to jump start it. Let it run awhile, but it wouldn't restart. Bought a new battery and installed it. Started it up and checked the voltage - low at 12.5 and dropping as we turned on accessories with the engine running. Figured the alternator was bad. Revved the engine up to 3000 rpm a couple of time and bang!! the voltage jumped up to 13.9 and continued to climb to about 14.5 as we watched. Heard of it, but never saw it till this time. Rev it up a little and see if the alternator "turns on".

 

It really isn't that unusual for the battery to follow the alternator in component failures. Just went through it. Replaced the alternator, then the battery went.

The kicker is, if you don't replace your battery, you could "kill" your new alternator.

 

I'll back up Mike's comment.

Same thing happened to me.

Alternator went dead, replaced it and then the battery died shortly after.