I have a 2003 SLE with a 3.8L engine with 75000 miles. I am having trouble with my voltmeter constantly fluctuating and all the lights in the car are flickering. I have replaced the alternator since it had a ruff sound in the bearing and tested the battery which checked out fine. I took the car to my local Pontiac dealer and they said that the (PCM) Computer was causing the voltage problem. Well today they replaced the PCM and it did not fix the car.
I was wondering if anyone else has had this problem and what it took to fix it. I am a diesel mechanic ( construction equipment) and am not afraid to work on my own stuff but i don't have the test equipment to troubleshoot all the electronics on vehicles today. Any advice will be greatfully appreciated.
Thanks in advance
Tom

Check your battery connections for corrosion.

Here'* a test for the negative battery cable that has proven to be a problem at times https://www.gmforum.com/t279492/

On that negative cable. There was a connection under the seat that Archon had found to be questionable. IIRC GM painted the car then attached the connection. Any loose connections etc could cause the issue.

Thanks for the feedback. I am going to replace the Negative battery cable this weekend and also thought about installing a ground strap from the engine to the frame just to be safe.

My 2001 has a ground going to the engine block near the starter. Check yours. Certainly won't hurt to install redundant grounds.