There are several issues that crop up in these clusters
First, make sure your switched ignition (dark blue) or "ignition run" power to the cluster is solid
Second, make sure your sender wire and sender are not the problem
Then....
1 The cluster harness connector can be loose/ corroded
The PC board connector pins can be loose/ broken corroded
So solder the pins if possible, in my case SEVERAL of the pins were broken/ loose, corroded, bad shape. So I elected to go with a pair of "Molex" style connectors which I bought at RadShak. I just soldered leads to the board traces and brought them out to the Molex connectors. If you search the site, there are several posts about guys fixing these connectors
2 The PC board / cluster can have a poor ground
Loosen/ tighten all board mount screws, clean the board if necessary, and either solder a wire to the board ground, or attach a lead wire to a board ground screw. Bring this out and screw to the column support
3 The VR can be faulty, even though it is somewhat functional, may not be the correct output
The VR SOCKET in the PC board may not be making good contact with the board
Replace the VR, and check resistance and examine the brass fingers that make the contacts. On my 67, I soldered short wires from the brass clips to the board traces. They were NOT making contact on mine
4 The gauge nuts ("fake" sheet metal) may be loose/ corroded, as can be the PC board surface
Replace the nuts with real nuts, clean the board if necessary, and don't overtighten them.
5 And of course the gauge can be faulty
After you've checked and fixed everything else, this is a possibility. Both fuel and temp use the same sender resistance, this is where the test resistances come into play. You can either get resistors from (limited selection) at Radio Shack, or places like Mouser Electronics or Digikey
the resistance to obtain MT, 1/2, and full (or "hot") are
L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)
Another way, if you have your fuel sender out, is to lay it out, move it until you get the resistance listed above, and then hook to the sender end of the gauge with test leads.
The gauge needs a properly working VR, or you can rig a supply (battery) of 5--5 1/2V
You can buy battery holders from RadShack, and if you can rob some AA cells out of some of your remotes, etc, and they are "down" just a little from new, they should be very close
And, for example, if you think the temp gauge is OK, you can compare readings between the two gauges. With the same voltage to the gauges, and the same sender resistances, they should read very close to the same. That should give you and idea if one is way off from the other.
