I can shed some light on your problem after this weekend. I installed a 12 volt fuel gauge from Classic Parts and it is not usable. The needle goes about 180 degrees past full. WAAAY out of bounds. They must have a bad batch of gauges. Send it back and tell them what its doing.
You should be able to put 12 volts on the power side of the gauge and it should read right at FULL, maybe just a tad more. Taking power off the needle should go to EMPTY maybe just a tad less. Until they tell us what we are supposed to do with a needle that rotates all the way around, let alone what it will do with the actual readings, we just have to go with another vendor or wait until they fix the problem.
Also, Zero Ohms is not ground. So to test the gauge by itself, put 12 volts on it.. reads full. Test the sending unit out of the truck.. goes from zero ohms to 30 or so ohms with no erratic behavior as you raise the arm up and down. With both behaving normally, it WILL work great. If it does not, be sure the sending unit ground is at the same ground potential as the gauge.
