Ok, here is what happened to me and what I did to fix it: my voltmeter has been bouncing for about a year. I have tried several things to make it stop, but most did nothing but fix a future problem.
First, I replaced the serpentine belt, since it was totally worn out when I got the vehicle. Didn’t fix the issue.
The voltmeter stopped bouncing one evening when I was driving home, but it wasn’t good news… it was showing 8 volts and holding. Alternator had had enough of the erratic charging commands and burned out.
Changed the alternator. This made the needle stop bouncing until I got about halfway home from work, and it started again.
I decided to buy a new battery, since mine was several years old and winter is only a few months away. Didn’t fix the issue, but it revealed I had an overcharging issue that was severe enough to cause boiling of the acid in the battery. I don’t want to buy another “interstate” battery, so I decided to use my knowledge of Volkswagen repair (I’m a Volkswagen mechanic by trade).
I figured it was some sort of grounding issue and bought a factory replacement negative battery cable. When I went to replace the cable, I decided to reroute the cable to a better location. For the body lug, I bolted it directly to the bottom bolt on the alternator, I didn’t remove the original cable from the body, so I bolted the end of the mew cable to the battery clamp on the old cable using a couple fender washers and a bolt with a nylock nut on the other end. This way, I had more body and frame grounds available and it should be solid. I cut and butt-connected the two small black wires back into the ground plug located a few inches away from the new clamp and cleaned the connector inside with electrical contact cleaner. So far, my voltmeter is solid, dead center while running, my dash lights and running lights no longer pulse, and my battery is no longer boiling the acid out. Hope this helps someone else who has been pulling their hair out.
