This is on my sig truck.
Dual tanks.
The front sending unit seems to work properly, but if I switch to the rear tank, the gauge shows 3/4 of a tank, even with the sender disconnected at the tank. If I ground the sender "hot" wire, the gauge pegs out on the full side, so I'm assuming the gauge should go to empty when it's disconnected.
Possible short to ground in the harness somewhere?
BTW, it has a new selector valve in it (bought from partsguyed)

Update: Pulled connector at motorized selector valve, still sits @ 3/4 tank. Grounded Y/W wire from gauge, it still pegs out full, but will not return to E until I turn the key off.

 

Are you sure you have a good ground where you are testing it? It doesn't make sense that it works ok on the front tank, but you are grounding/ungrounding the wire that feeds the gauge directly, and it doesn't work.

I would find a scrap piece of wire, and run it from the battery negative to under the truck and use that for a test. If it still doesn't work, I would make double sure the front tank does really work correctly. If the good ground test doesn't work, it's going to be a connection problem from the yellow/white wire along the frame, up to the gauge. This includes wiring harness connectors and the connector on back of the cluster. You can intercept this yellow/white anywhere along the way to the front, and do the ground/unground test to try and pinpoint where the problem is. It may be easiest to ground and unground it at the cluster as a starting point.

 

I just thought of something else. You may not be really using any of the other gauges? If so, then possibly the cluster gauge regulator is shot. It regulates the voltage to all the gauges, so they all should be out of wack, but I know a lot of people(including me) who don't even pay attention to the factory gauges, only the gas gauge.

 

Hey Dave,
didn't think about the IVR. Like you, I don't really pay attention to the other gauges, even though I do NOT have aftermarkets in the truck. Will include that in my testing.
BTW, it's supposed to be about 7 volts output, isn't it?

Update: Found connector plug under brake booster, that feeds the motorized selector valve. with the valve disconnected, it would still show 3/4. But when I disconnected the plug at the main cab harness, the gauge went to E, as it should. And, to make sure, I grounded the Y/W wire on the cab side, and the gauge went past F.
So, I'm thinking that there is probably a short to ground between this connector, and the connector for the valve. Got too hot out there for me, though, so going further will have to wait. (No garage, direct sun)

 

Sounds like you are on top of it, but just for your info, the voltage from the regulator should be a steady blinking type voltage like a turnsignal. The gauges are so slow, that they average this out to around 5 or 6 volt. They do this because the actual 12v supply in the vehicle is all over the place, depending on what you have turned on at the time.

 

Problem solved. I'll send you a pm, Dave, explaining my boneheaded oversight, so you can have a good chuckle.