No matter how I try to explain this, it's going to be long and drawn out!

Last week I went to help a friend try to figure out a non-working fuel gauge on his 64 GTO (clone) that I sold him about 8-10 years ago. I supplied him with a 65 GTO tach and gauge cluster which he used. I modified the original 64 idiot light harness in the car so he could use the 65 cluster. He recently finished the car and has not been able to get the fuel gauge to work. It always showed full. All the rest of the cluster was fine. I figured "simple, when they always show full it's a poor ground" so showed up at his house real cocky that we'd have it working in no time..... You know where this is going.

I took along a GM special tool for testing gauges. It has a rheostat in it and you can hook it up to a gauge, simulating the fuel tank sending unit in the car to test the gauge function. Also for test purposes, he had a spare fuel gauge.

-Hooked the battery up in the car, tested for power at the gauge to figure out that it had power and to figure out which side of the gauge had power to it. Yup, all seemed good.

-Hooked up his spare gauge to a battery on the bench and tested it with the GM tool. Same issue, always full no matter what I set the tester at.

-We tested things back and forth, trying a spare tank sending unit he had, etc. We knew from reading online that the sending unit for 64 does not interchange with 65 but I knew that I had purchased the correct NOS 65 sending unit to go with that cluster, and it was the one installed in the car. (64 is 0-30 ohms, 65 is 0-90 ohms)

The more we tested, the more stumped I got until it finally hit me, this thing was working exactly backwards. For fun, we switched the wires around on the back of the spare gauge. And sure enough, the gauge tested perfectly on the GM tool, the needle moving to the correct spot on the gauge as we dialed in the corresponding ohms resistance. I picked the wires out of the gauge connector in the harness, swapped them side to side, and bingo, his guess that he had around a 1/4 tank was pretty close!

I posted up on a GTO tech forum asking about this and sure enough, the gauge guru Peter Serio replied right away that the wires had switched sides on the gauge plug between 64 and 65!

Soooo, if you have a non working gauge in a Beaumont, check to make sure you have the correct wire to the correct side of the gauge (in case someone has swapped it for the other year gauge) and make sure you have the right tank sender as well. The connector on the gauge is the same for both years, the wires just swap sides. At least I am assuming it's the same for Beaumont as GTO. The Beaumont parts books show different gauges as well as different tank senders between 64 and 65.