Hi all. I'm new to this forum and have a long-standing issue I'd like some advice on please. I have a '72 GMC 1/2-ton pickup with a cab mounted fuel tank. Ten years or so ago, my dad had this truck before passing on. I got it from him. He had replaced the fuel tank sending unit at a local dealer because the gauge quit working. It then worked for a short time before no longer working again. After I inherited it, I removed the sending unit and discovered that the float was totally shot. I replaced the float and the tank registered 1/2-tank. I ordered a new sending unit from (at that time) Chevy Duty and replaced it. The gauge registered 1/2-tank when full for awhile and now registers nothing even when full. I have performed the usual checks. With the wire disconnected, the gauge goes to full. The wiring appears to be fine. I've been told on other forums that it has to be a grounding issue. I've tried grounding the tank directly to the frame and nothing changes. I see no ground wire from the tank to a ground. The only wire is to the fuse block and on to the gauge. I think the sending unit grounds to the tank by virtue of contact with the locking ring. I could try removing the sending unit again and moving the arm in its full range of motion to see if the gauge goes to full, but I can't help but think the problem is in the fuel gauge. Before ordering a replacement, I'd like to hear whether or not you think that is the problem or if I should be looking elsewhere. When my dad had the truck, someone installed an after-market bed tank and there is a lever inside the cab that switched the tank from the main to the auxilliary tank and the gauge worked for that. When that auv tank was removed, someone cut the wires, but I'm not seeing any issue there and I would think if that was the source of the problem, the gauge would not have registered at all when it went to 1/2 tank on full and when removing the sending unit wire from the unit, it would not move the gauge to full. I'd appreciate any advice. This is a constant source of frustration not being able to use the gauge. Thanks in advance for your input. Rick
I Have A 69 C10 When I Bought The Truck The Man Said That He Had The Fuel Gauge Sending Unit Replaced At Chevorlet Garage And It Worked For A While And Then It Quit Working. I Checked Everything I Could And Could Not Find Anything Wrong. I Then Took The Sending Unit Out Of The Tank And Found Out That The Float Had A Pin Hole In It And The Float Was Full Of Gas. I Replaced It And Everything Works Fine.
Been there, done that. I had to replace the float the first time around as well. This last time, the float is fine, so that's not the issue.
Unplug wire off the sender and ground it while watching the gauge (with key on, of course) gauge should swing one way or the other. If it does wiring etc should be ok. I'd guess you may have a problem at the dash gauge, somewhere in the printed circuit. The only other thought would be the tank that was removed at the frame. Did it have a switch that switched both the sending units and the tanks? If it had a switch for the sending units and it's still there you might be chasing that problem. Good luck...Mike
Thanks Mike. I will see what I can do to follow the wiring. I have already done the first thing ytou mentioned and with the sending unit wire disconnected and grounded, the gauge goes all the way to the right. I have thought all along that there must be a wire that connects somehow to the gauge from the old bed tank because my mom told me the fuel gauge registered for the aux tank when dad switched it over. I haven't found a switch anywhere, but will try tracing wires. If that doesn't work, it sounds like the next option is to buy a new gauge and replace it or the circuit board.
I'm starting to think there are Gremlins in my truck. I tried removing the sending unit wire and grounding it. The gauge went to full when I pulled the wire and then to empty when I grounded the wire. Now when I reattached it to the sending unit, it swings to past Full and stays there which is just the opposite of what it was doing before the test. So instead of the gauge moving, it now stays on full with the sending unit wire on and also when I remove it. I even tried running a ground wire jumper from the tank directly to the negative battery post. Nothing changed. Would this still indicate a bad sending unit or is it the gauge? Or did I fry the gauge running the test? Sure wish I knew a little more about automotive electricity! I bought the sending unit from Classic Parts just a few months ago and it has never changed anything. This is the 3rd sending unit in about 10 years and the gauge still refuses to function. It really sounds like a ground, but you'd think a direct connect to the battery would resolve that. Is it possible that the original gauge doesn't match the ohms of the new sending unit? The unit I removed last time is a 90 ohm and I didn't check the last one I installed. I'm trying to find a solution short of pulling the seat out again and removing the sending unit. Last of all, what would cause the gauge to read Full all the time now since the test of grounding the sending unit wire? Is this a typical thing to have happen after that test? Thanks for the help folks.
The wire to the sending unit must not be making a good connection because it is the same indication if the wire is off on the sending unit. Also, make sure the sending unit is centered properly in the opening of the tank. If not centered properly, it may be grounding out on the tank. I noticed on my 72 Chevy that it was easy for the sending unit to be off to one side or the other and not centered in the hole.
