My guage jumps from 13 to 18 frequently and has been for a while but now it is bouncing from below 13 to just above 8 volts, I'm think alternator but both the battery and alternator are brand new. Could it be a loose ground?

 

By your gauge do you mean the in dash gauge?

My in dash gauge always reads below 13-12. Got a voltmeter and it was perfect.

 

My gauge is notoriously bad. I have a radar detector that reads the battery voltage. When my gauge displays 8-13, my detector displays 13.9 steady.

 

Yes the dash gauge, my lights will also get really bright then dim and keep doing it, like the voltage goes up and down

 

In that case, by a good voltmeter (or go to a parts store and many cab lend one free). that way we know what voltage you are working with. No sense in diagnosing with bad readings.

My lights do that as well but always stays within normal range. A note is AC is a huge strain on the system, and ac was not a standard option. And AC and non Ac cars share the same alternator.

 

My AC was taken out