My Voltmeter gauge is reading @ 13, when I turn on the air(fan switch) ut starts bouncing between 7 and 14 volts. It does also when i turn on the lights. I thought it was my alternator, I changed it out and it hasn't helped a bit. I have 12.5 volts at the battery(battery newer) when car off and 13.77-14.00 when car running. Also I just nticed that the TAch is taching out(8000rpm) when I rev it up, but the engine is only @ 3.5-4 RPM. Thanks for any help, troy.

 

ttt

 

Some people have had their tach guage bounce around when their opti started to go out.