This is on a 2003 Gmc sierra with a 5.3l. I replaced the battery about a year ago. Two days ago I noticed the voltage gauge would drop when coming to a stop. While driving it would stay around 13.5volts. I checked the alternator with a voltmeter and it was putting out 13.6 and would drop down to 12.6 with all the acessories on. I replaced the alternator with an ac delco unit yesterday. Now I am getting a battery light that will come on with the battery not charging message. It comes on stay on for 5 seconds or so then goes off for 5 seconds. The voltage gauge stays at 14 all the time and I have driven the truck about 30 miles no other problems. The alternator is fine, what could be causing this problem? Thanks for any help, Josh

 

Maybe check the connection between the alt and battery. even do an alternator mod while your at it.
Did you unhook the battery while you were changing the alternator?
I ask cuz some people don't

 

What is the alternator mod? Yes the battery was disconnected. Connections are fine.

 

Alternator mod is where you run an extra 10g wire from the battery + to the alternator. Just to allow a stronger current between the two.

Say you run a big stereo system and your headlights/dashlights dim. The mod will allow the alternator to carry a better current and keep them from dimming.

I don't know if it will help your Bat Low message any. but i thought i'd throw it out there. It can't hurt anything.