This is on my '48 GMC 1Ton flatbed, 12 Volt negative ground. The aftermarket gauge always read 'Empty' though when the ignition power was off the needle would fall another 1/4 inch below the empty line. The tank had to come out for other repairs, so I took a look at the old sending unit and it had a partially sunk float. I ordered a replacement sending unit and installed it before putting the repaired tank back in the truck. This is a single wire setup.
OK, now I'll make it complicated. I'm almost certain that I checked the function of the gauge by grounding the wire attached to the sending unit and seeing that the gauge moved to the 'Full' reading. Almost certain.
When I turn the key (aftermarket ignition switch) to either the 'accessory' or 'run' position the gauge moves slowly upwards for the first half, then rapidly moves past the full mark, as far as it physically can. If I ground the wire at the sending unit, the gauge slowly sinks to the empty reading though, as before, 1/4" above the very bottom of the gauge. I have about 4 gallons of fuel in the tank. My multi-meter shows about 8.25 Volts at the sending unit.
Should I have 12 Volts at the sending unit? Is this somehow wired incorrectly? Is my gauge shot? I thought I was getting pretty good with electrical things but now I need help again!
Thanks in advance for your thoughts. Jim
