Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
1) Those gauges are generally pretty robust and reliable.
2) Almost all problems with them can be traced to faulty grounds.
3) The voltage regulation takes place within the gauge cluster, probably within the fuel gauge. Power comes in through a red 18 gauge wire that is spliced to a wire that also supplies power to the oil pressure gauge, so if it's working the problem probably lies in the cluster itself or its grounding.
It gets is ground through the housing to the mounting screws, through the instrument panel, the body tub and eventually the ground straps back to the battery. Before doing anything drastic, scrape a clean spot on the back of the cluster housing and run a good ground wire directly from the battery post. If you see a change in the reading when you scrape the wire to the clean spot, you know it's a ground problem. The most likely place is between the instrument cluster and the panel - rust and corrosion on the mounting studs and the cluster housing, which you might be able to correct simply by loosening and retightening the nuts.
This information comes from experience and my '78 shop manuals, but I doubt there's any significant change between then and '84.