92 f350 7.3 idi. I replaced glow plugs now my volt gauge jumps around and shows low on gauge. at idle it showing 12.1 and when I rev up engine volts drop below 12 in the 11's. I put a used alt on and it's still doing the same thing. Any ideas?

 

bad alternators...or the glow plug relay is stuck on...

 

Two good places to start troubleshooting. Does your GPR "click" when you turn the key? Or "click, click, click, click, click, etc.," when you turn the key?

If it's the former, test the alternator. If it's the latter, the GPR is bad.