My fuel gauge does not work. When I took the dash apart I found several wiring harnesses not hooked up and one broken. The broken one has three wires two being black and one blue. Could this broken harness hook to the three prongs on the back of the gas gauge?
I have no idea on that question. Generally when people buy these old Chevys the two questions most commonly asked is how wide a wheel and tire can I put on the car and how do I get my gas gauge to work?
The reason old Chevy gas gauges do not work is simple. The gauge needs a good ground to work and forty to fifty years of corrosion have created enough resistance to the gauge so that it will not work until the wires going by the tank (under the trunk floor unless you have a see thought trunk like I have been observing recently) are removed and a freshly wire brushed and sanded electrical connection to ground is made.
If I had to guess I would say some one prior to you had a problem with the gauge and attacked the gauge end of the wiring disconnecting everything. You will need to find an electrical wiring diagram to show you which wire goes to what as I have no idea never having owned or worked on your car year at any time that I can remember.
You can buy large color wiring diagrams on laminated paper to frame and put on your shop wall as man art and they make a handy reference piece. You might also consider buying the original books (or a scanned image of each page on a CD) that GM supplied to their mechanics with instructions on how to repair your car. You can read a sticky I wrote on the subject here:
Sometimes it can be a little more than just a bad ground and like Dave said get all the literature you can. That being said...
The wiring on the back of the gauge looks like this… there are three terminals one at 3o’clock, one at 6 o’clock and one at 9 o’clock. 3 o’clock gets the pink (ignition hot) wire. 6 o’clock gets the black (ground) wire. 9 o’clock gets the fuel sender (tan) wire.
If the gauge pegs to empty, there is an open ground between the gauge and the chassis.
If the gauge pegs to full, the sending unit circuit is broken somewhere between the fuel sender and the back of the gauge. Or, the sending unit ground has been pulled off during tank installation. The fuel sender puts out an ohm signal from 0 (empty) to 90 (full) (After 1965).
If the gauge only reads in the first ¼ of the gauge, the resistor is not providing the proper resistance and should be replaced. All original AC-Delco fuel gauges require an external resistor to function properly. The original resistor is a wire-wound resistor and the later-available resistor is a ceramic shunt type. Repro fuel gauges like OER are internally resisted and don't require these resistors. However, the terminals cannot ground against the back of the gauge or the gauge will not work.
If there is a problem with the pink wire, the gauge will stay at the last position read when the power was cut (much like turning off the ignition switch).
Hope this helps,