1993 F350 frustrated fuel gauge

[KCMeyer]

Hi everyone!!
I have a problem I just can't sort out. We have a 1993 F350 brush truck with a 7.3L IDI. The rear tank fuel gauge doesn't work, and on a wild land fire truck, you can imagine this just isn't handy. I have checked wiring and grounds thoroughly. So I snatched a sending unit out of an identical truck that is out of service. Thought I would plug it in and see what happened before dropping the tank. Well it worked. Moved the lever up and down and the gauge followed. Problem solved right? Dropped the tank, put the sender in, no gauge. I'm at a loss fellas. Any help is greatly appreciated.

 

[chillman88]

The way you describe it sounds like a pinched wire? I've done that before, have you checked continuity after putting the tank back in?

 

[chillman88]

What is your gauge reading? Full, empty or somewhere in between?

My 91 which I think uses the same sending unit values shows overfull when completely disconnected.

 

[Thewespaul]

Is your rear tank metal or plastic? If its a replacement plastic tank you may need to add an external ground to the sending unit for it to read correctly. Otherwise try plugging in the old sending unit to the front connector and see if you can get a reading that makes sense, if you do you know the sending unit is not the issue and the harness or the selector valve is the issue.

 

[KCMeyer]

Good questions. I should have provided more information. The truck has a factory plastic tank and a two wire plug. One post of the sending unit is signal, the other is riveted right to the top plate. So that wire is my ground. The gauge shows empty when plugged in, but when disconnected it goes past full, as it should.
All be it primitive, as I didn't want to drop the tank again yet, I attached a ground wire and went straight to the frame. Notta. I looked at the harness again, no wire pinched during install.
What gets me is I plugged the sender in with it outside the tank so I didn't drop it just to find out that wasn't it(which happened any way) and it worked fine. I stuck it in the tank, and back to empty. Only thing I can think is something blocking the travel of the arm, although I have no idea what that could be. I checked the float for holes or cracks and it appeared good. And if it did have a hairline crack it could not have filled up that quick to sink before reading something.

 

[KCMeyer]

Forgot this, I have several of these trucks of my own but never had gauge problems. I have heard of the selector switch or valve being a culprit, but have no idea honestly. It does switch tanks and I can use the fuel, just don't know when it's empty till she dies. Bad feeling when your surroundings are on fire.

 

[chillman88]

Only thing I can think is something blocking the travel of the arm, although I have no idea what that could be.

That is specifically why I asked where the gauge actually is reading.

I have been having an issue on my truck where it would read about a quarter tank high, then read completely full then randomly work fine. I think I nailed mine down today. There were two butt splices inline with the gauge wire, no doubt where the "previous idiot" tried to hook up the fuel pump originally.

If you can't figure it out, get a helper to look at the gauge while you wobble the wires around and see if the gauge moves at all from where it is.

I assume you made certain the plug is pushed in all the way and didn't pop out.

 

[KCMeyer]

We got it. I finally have two usable diesel tanks on the fire ground! Multiple wiring issues were uncovered in the frame. But it works, we're fighting fire, and that's what we wanted! Thanks everyone