Fuel gauge reading past full

[bwolff0820]

I’ve got an 83 C-10 and I’ve recently changed both tanks and sending units. Got them all installed and the gauge is reading at the 3:00 position. A lot of post say the sending unit is bad however I changed all parts so that shouldn’t be my issue. Both were grounded to the frame in the same location. Where should I start? Do the tanks need to be filled to “reset” the sending units or is there a wiring issue somewhere?

 

[AZ Highlander]

My fuel gauge is doing the same thing. Watching this thread.

 

[Rumbledawg]

if it looks like this, then your sender wire is grounding out somewhere. pink wire for later trucks and tan i belive for older trucks. quick easy way to check, run a wire from your 4 plug under the hood to the switching unit. if your gauge goes back to normal, start tracing your pink (or tan) wire to see where it's rubbing on something metal. the loom clamp was rubbing my pinkie and wore thru the insulation.

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[Dmack]

I may be wrong, but I believe 3 o'clock is an open circuit, while 9 o'clock is grounded out.

 

[Rumbledawg]

correct, my bad terminolgy
open

 

[SirRobyn0]

Yes 3 o'clock is open circuit. Make sure the sending unit is plugged into the one wire. Make sure that the fuel tank has a good ground, there is a ground wire from the sending unit to ground, if it's broken or was left disconnected it won't work.

New should but doesn't always mean good.

IF the ground connection at the fuel tank is in good shape, and grounding the gauge wire results in a reading of E, then it's most certainly the sending unit.

 

[Jgonick]

This video explains how the fuel gauge and sending unit works and a little troubleshooting.

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[bwolff0820]

Thanks for the replies. I will clean and re-ground the sending units this evening and see if anything changes.

 

[Jgonick]

Someone correct me if I'm wrong.

In a dual tank setup the sender wire runs through the selector valve. Just something else to be aware of once you start troubleshooting where something could be wrong.

 

[SirRobyn0]

Someone correct me if I'm wrong.

In a dual tank setup the sender wire runs through the selector valve. Just something else to be aware of once you start troubleshooting where something could be wrong.

Technically it depends on the year of the truck. By 82' yes they ran though the selector valve, earlier trucks the gauge wire went though the switch in the cab. It's not common for the valves to fail in that way, but if he grounds it out and his gauge reads E it's most certainly a sending unit or grounding issue, if he grounds it out and there is no gauge responce then it's something "up stream" from that.

 

[bwolff0820]

Ok, got home and checked both grounds. Sanded where the grounds connected to make sure it was getting good ground and still reading 3 o’clock position. With the key of it will drop to about the 1:30-2 position. The transfer switch in the cab I believe it out. When I push it I don’t hear anything trying to switch over. Would a fuse or anything contribute to this?

Also trying chasing the pink wire down the frame. It runs from the selector valve on the passenger side over to the drivers and then goes into conduit until the front side of the firewall. I just don’t see how it could be getting shorted out under the frame. I’m about to pull the trigger on new dash switch and under truck selector valve just want to make sure it’s not something I’m missing

 

[bwolff0820]

Also after putting new gas tanks and sending units in both sides truck won’t start. I can pull lines off pump into a gas can and truck fires instantly and runs. Truck sat for about 15 years prior to me getting it. Is there a process needed to prime it or just have to keep cranking and it will eventually pull. I’ve only got about 2/3 gallons in each tank in case I have to drop again. Not sure which tank it’s pulling from since can switch isn’t working so doing the same to both to try and figure it out

 

[Jgonick]

Some questions that will help diagnose it.. Was it running and was the fuel gauge working before you changed tanks and sending units? Did you replace any hoses or lines when you replaced the tanks. Explain what you mean pull lines off pump. I'm assuming your fuel pump is mechanical.

I'm not real familiar with selector valve but others on this forum are.

 

[bwolff0820]

Unfortunately I have no clue if the gauge worked or not. It was running when parked in 2004 and I pulled it from a barn in 2021. Both tanks had holes so I dropped them and replaced with new tanks and sending units. If I pull the fuel lines on the pump and that go from the tanks into the pump and put them into a can it will start and run. I’m not sure on mechanical pump as this is the first time ever doing something like this. I’m pretty mechanically inclined but never messed with anything like this before.

I changed the short rubber hoses that run from the sending unit to the steel lines. Those are the only ones I have changed.

 

[JamesSam]

I'm watching this one now. I had both tanks working when I bought my truck nearly a year ago, then when one tank died on me I swear fuel went from one tank to the other because I had to switch back to the tank I wasn't using and it was full(not before I switched though!). At first I was all o er the place trying to figure out what was up thinking the selector allowed fuel to bleed from one tank in to the other... all kinds of responses and figuring out what to do next calculating the cost and time I might be able to spend on it...I am still driving around with the passenger side tank as my only tank. One day I will get my garage to diagnose maybe?