Converted the 68 cutlass dash to a rally pac and tick tock tach. Worked out all the pin swaps etc and rewired the connectors. When the car came in the original gauges didn't work and had issues. Everything is now working great except the fuel gauge. It's working in reverse. Turn the ignition off and the gauge goes to full. Ignition on and it goes to 3/4 full. The tank is only 1/4 full (I put 4 gallons in myself from empty) and i assume should go to empty when ignition off. Any ideas? From memory there are 2 wires coming out the top of the tank, 1 is fixed to the body with a sheet metal screw the other runs to the gauge. Do these need to be reversed?

 

that does not describe "working in reverse".
the needle does with no power to the circuit is immaterial.

Normal is:
reads E when Empty; reads F when Full

reversed would be:
reads E when Full and F when Empty.

disregard what the gage does when it is not powered.

If it really reads 3/4 full when it has 1/4 tank of gas... well maybe the gage action is reversed. More than one data point would be good to have. If it does indeed run backwards I think the only cure is to rectify the mechanism or resistor attachment inside the tank on the sender.

 

If you used the wiring placement diagram from the 68 Factory Service Manual, it's incorrect.


Let me know. I might have a pic of the correct one around of the correct wire placement somewhere

 

I didn't use the manual, the wiring on the car was a mess of previous attempted repairs. The wrong colour wires are in the harness and lots of extra joins. I've sorted this out correctly now so it's safe and won't cause any further issues. I determined which wire does what using a multi meter and running continuity tests between each wire and all the sender units and switches that operate the various parts of the gauge. The wire that goes onto the fuel level part of the gauge is the same wire that runs down to the back of the car and out onto the tank, I know the pin positioning on the gauge is correct. I just don't know what would make it read in reverse.

I know it's reading in reverse because the tank has 1/4 of fuel in it but the gauge is reading 3/4 of fuel and when turning the ignition on it pulls down from the full position to the 3/4 position rather than from the empty position up to the 1/4 position.

 

Does the gas gauge share a common ground with the rest of the cluster, or is it free standing? If it is free standing, have you tried swapping the two wires going to the gauge?

 

Only 1 wire into the gauge for the fuel gauge so assume it shares it's ground with the rest of the gauge.

 

Check to make sure it is grounded properly by running a known good ground wire to the back of the gauge's metal housing. Then see if that changes the reading.

 

Should be grounded properly, the rest of the gauge works perfect and the ground runs through the gauges to complete the ground circuit on the rear of the lower dash but I will double check this tomorrow using a power probe

 

One wire goes back to the tank. The tank sender then is grounded to the body.
Sounds like the fuel tank sender is bad.

 

Fixed the problem. Were the previous repairs were done and the wiring was a mess the car had no rear courtesy lights, the bulb holders are there, 1 bulb was missing the other someone had smashed the bulb and left the base of the bulb in the holder. Well during my repairs I replaced these bulbs and fixing the wiring to the passenger side one as it was broken. Unknown to me at the time the drivers one had a broken earth wire in the trunk compartment and someone had piggybacked the earth wire into the fuel tank sender wire. So during my original testing without and bulbs of battery on the car, just tracing wires everything was fine, now I had a bulb in the circuit it must have been picking up a live and causing the gauge to go mad. Removed the bulb the gauge works as the circuit is broken again. Repaired the wiring to the bulb holder properly and everything sorted.

Absolute pain! Hopefully that's the last electrical issue with this car.

 

I had a feeling it would end at this. The way others make 'repairs' just blow my mind.
Glad you got that issue fixed for good now.

 

Pretty much every previous "repair" has needed to be re-done because it was wrong or unsafe. The fuel pipes were the worst, rubber line from the pump to engine was barley on the car and about to fall off, when they wired up the starter motor they didn't have a cable of heavy duty as needed so used several smaller cables wound together, the list could go on...........