Low-High Voltage

[Aflickted1]

07 GMC Sierra 1500 Classic

My voltage will be like 12.7. Turn off the truck and back on voltage might be up to 15.11. I thought it might be a voltage regulator problem. I had O'Reillys check it and it passed the voltage tests. He said that it's possible that it is running within normal ranges. I just don't understand why it would be such a BIG difference from one start to another. Any suggestions??

Thanks

 

[Bill325]

Both 12.7 and 15.1 are a bit out of the normal range with the engine running, in my experience. More typical are 13 to 14.5 volts. I'd start by removing and cleaning the battery cable terminals, and the ground straps from the battery to the engine, and to the chassis.
When the voltage is 12.7 , are you running something that draws a lot of current, such as a huge sound system ? When the voltage is 15.1, the alternator is trying very hard to charge the battery. Sometimes, such odd behavior can be due to a battery going bad, and nothing will diagnose the problem other than trying another battery. Also, be certain that your voltage readings are confirmed by a second accurate voltmeter.