The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Sending Unit problem -- still

There has to be a solution to this problem somewhere. I'm back to my '72 GMC 1/2-ton sending unit problem. I've replaced the sending unit (twice now) and still have no reading on the fuel gauge. Does anyone out there have a good diagram showing how a sending unit for a '72 should be wired? This thing doesn't have a ground wire attached. There is one post on the unit for the sending unit wire and it runs to a terminal on the fuse block. When I pull the wire from the sending unit with the key on, the gauge goes to full. So then I ran the second part of this test with that wire disconnected and jumpered to a good ground with the key on. The gauge went back to empty. But as soon as I reconnected the sending unit wire, it now goes past the full mark and stays there, although I know have have about 1/2-tank of gas now. I've been told on this site that the sending unit doesn't need a ground wire as it grounds itself on the tank. I'm about ready to pull the gauge and replace it, but it doesn't seem this would be the problem. I just don't know what else to look for. I've traced the sending unit wire all the way from the sending unit to the fuse block terminal and it is in good shape. No splices, nicks, etc. Does this sound like a faulty gauge and why is it now over past full with the wire connected? Sure has me stumped that's for sure.

Rick,

The sending unit doesn't usually require a ground as it 'can' get that ground through the screws attaching the sending unit to the tank, then through the tank to the body, then through the body hopefully back through the frame to battery. If any of these routes are insulated, its a No-Go deal.

The quick check is to run a wire from one of the screws holding the sending unit to the tank right back to battery NEG.

Looking at another truck site I see this problem being mentioned in the early part of '06.

Stuart

If I remember correctly and sometimes I don't the 67-72 in cab tank does not screw in to the tank it has a round rubber gromet and a locking ring. The sending unit has a single wire to the fuel guage connection on the fuse panel. At least that is the way my 68 1/2 ton is wired. Could be a problem with the resistor on the guage. They are availabe from several of the vendors here. Make sure that the guage and the sender have the same resistance value, some are 60 and some are 90 ohms.

Yep, you are correct. This one doesn't screw in. It has the special retaining ring and the rubber gasket/grommet). It has the single wire from the sending unit to the fuse block. I'll have to check into the resistance value. How do you check that? Pull the gauge and check it and also the sending unit?

Rick,

Regardless of the type of retention, the important issue is that the sending unit can find a good ground. If you have a meter, you are looking for ZERO ohms of resistance...none! If you have no meter, clamp a test wire to the retaining ring and stick it on the NEG of the battery, this would remove all doubt very quickly and you could move on to other ideas.

Stuart

It was suggested on another site that I disconnect the sending unit wire and ground it to a good ground to test the gauge. I did that. Now with the sending unit wire reconnected to the sending unit, the gauge is stuck past the Full mark on the gauge. It doesn't change when I remove the wire again and I tried also running a ground jumper from the tank directly to the battery. Still stuck past Full. I don't know if I have fried the gauge somehow or the sending unit or both. What would this indicate? The tank is about half full. Prior to running this test grounding the sending unit wire, the gauge read empty. Removing the sending unit wire moved the gauge to Full.

Have you tried to just test the system by replacing the suspect parts with say a 100 ohm potentiometer, some would call it a reostat, and an ohmeter. You could hook the pot directly to your gauge and see what what ohm value drives it to full and empty. Then hook the pot in at the gas tank and repeat the test. Then measure the resistance of your sender with the ohmeter at the tank. This should answer some of the questions you have. Good luck.

Okay, here are the results of the resistance test on the sending unit. I ran an ohm test at the battery to engine ground. Reading was 0.00 ohms. I ran it from the tank to a ground, same reading. From the surface of the sending unit to ground, same reading. I then disconencted the sending unit wire and ran the test again from the tank to ground and from the sending unit surface to ground, both with the same readings. So it appears the tank is well grounded. I think I mentioned previously that earlier I had disconnected the sending unit wire and grounded it with the key on. Now the gauge only reads past full whether the sending unit wire is attached or not and with the key on. The gauge no longer moves with or without the sending unit wire attached and the key on. Previously the gauge read empty with the key on and the sending unit wire attached and swung to Full when the sending unit wire was removed. There is no motion in the gauge at all at this point. Suggestions? Thanks guys. I hope to get to the bottom of this sometime soon. I'm stumped now as to where to go next.

Rick,

I think the fuel gauge needle is "stuck"...physically stuck inside the gauge. This seems to be a result of your previous test, but not part of the ongoing problem you have been chasing.

I would try tapping on the gauge itself to see if I could dislodge the needle. Does this sound like a possibility.

Stuart

I thought that might be it, too, Stuart. However, if I disconnect the sending unit wire and ground it again, the needle starts back toward empty. As soon as it is reconnected, it goes back to past Full and then when disconnected again, it stays at Full. It only moves when grounding the sending unit wire, but it isn't stuck. Just haywire.

Sunday I took the '72 to NAPA to pick up some parts. When I started it, the fuel gauge went back to empty like it was doing before the darned thing seemed to freeze past full after the ground test. Don't ask me why it went back to the way it was again. I had started it and turned the key off and on numerous times and it stayed frozen until Sunday morning. The only thing different was it wasn't raining. Guess the next step is to either replace the gauge or pull the sending unit for the 5th time.