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I have an 85 F150 4x4. The gauge has "bounced" fro 12-15 volts since I got the truck. Checked alternator, it had bad diodes, and was putting out 15.5 volts. Replaced it and voltage regulator for good measure. The gauge still bounces. Sometimes it will hold real steady at 13.5-14 volts, but the majority of the time it is bouncing. Would a ground wire from the ground terminal of the alt. be a good idea, or should I look elsewhere?
I am assuming this gauge is aftermarket. Where is it hooked in at? You might want to try moving where it's hooked up to a different location, especially if you don't notice any flickering in the brightness of your headlights.
I think possibly someone on here before had this problem, and found his output wire from the alternator had a corroded connection up in the harness. There is a splice in there somewhere where it hooks to the wire that goes to the large terminal on the starter relay where the battery positive is hooked.
I actually just had this problem too, in my '80 E150. The short version was "bad connections".
I got a new voltage regulator, and busted out the Dremel to clean every connection under the hood. When I put it back together, the flicker was gone, but so was my charging. The factory alternator meter showed just barely under the even mark.
One jumpstart several days later, and I took a look at the old regulator, found one of the pins to be very rusty. I filed the rust off and shot it with spray carb cleaner (I have an abundance from recent adventures with my carb) and put that one back in. Now it works great.
If that hadn't worked, I was going to cut off the regulator connector and crimp on some female quick disconnects. If you do crimp new ends on, you will want to drill a 5/64" hole as close to the exact middle as you can get it and about 1/8" from the tip so the anti-vibration stud in the terminal can lock it in place.