I just bought a '95 Suburban and my battery gauge only reads about 12 - 13 with the lights off and about 11.5 with them on. Had the alternator checked, it was fine, and replaced the battery, yet still have the problem. Any suggestions?
Yeah, your gauge may just not be accurate. Since you just bought the 'burb you have no history with it. You should have had the battery tested too before you bought a new one.
Put a voltmeter across the battery terminals while the engine is running and see what the voltage is.
The battery was checked beforehand, it was on it's way out, 7 years old. No, they didn't compare gauge to their tester reading. The gauge fluctuates when the lights get turned on, as well as the other gauges slightly. Looks as if a radar detector was mounted into the dash, and when the brakes are applied while in park, the detector will sound off occasionally. Definitely some electrical issues.
Assuming they properly tightened the battery connections when they installed it, check the other end of both cables, the ground and also probably the starter connection.