Fuel gauge problem

[Rochlow]

I did a search and didn't find something that was close to what I'm going through.

My gas gauge was showing below empty with about 5 gallons of gas in the tank. It has a new tank, sending unit, and wiring harness (AAW).

I couldn't figure out what the deal was so I jump from the fuse box to the back of the gauge to see if the needle would move.

Well, it went from empty to way past full.. The only thing I haven't replaced is the gauge itself..

Any suggestions?

[red71cheyenne]

Mine did the same thing when I replaced the sending unit. The fuse in the box had blown. Check to see if yours is good.

[Rochlow]

Checked the fuse and it was good.
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[Bentblazer]

Ground sending unit wire, if gage moves to E it could be sending unit, if it moves to full could be gage. It worked for me, found bad float.

[Rochlow]

I'll check the ground on the sending unit. The whole sending unit is brand new. The wire running from the sending unit to the gauge is brand new..

When I turn the headlight switch on the lights on the gauges are really dim too..

[Rochlow]

There was a blown fuse making it go over a full tank. Now the gauge shows it under empty. So from what I've read I have to look for the tan wire grounding out somewhere correct?
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[red71cheyenne]

When it reads over full, that indicates an open in the circuit. The tan wire should run straight to the fuel connector on the fuse box, no ground in between. On the sending unit there should be a black wire that goes to ground on the frame or somewhere close there to complete the circuit. The sending unit is 90 ohms at one end and zero at the other, can't remember which way. The only other thing in the circuit is the guage, but I've never had one go bad. Doesn't mean it doesn't happen, just never happened to me. Let us know how the troubleshooting is going.

[Rochlow]

I've figured out the needle showing over full problem. It was a blown fuse. Now the needle shows below empty. Which from what I've read indicates that there is a short somewheres or the float has a hole in it.

The float and sending unit are brand new, so I'm ruling that out for now. The wire running from the tank the harness (using an AAW wiring kit) is brand new with no chaffs in it. So all I can think of is that sending unit might be grounding out on the tank..

I'm going to drop the tank tonight and take a look at that.

Thanks for the ideas and help..

[Bentblazer]

If you have it down or pull sending unit have some one watch guage while you operate float, those tanks are a pita

[Rochlow]

Originally Posted by Bentblazer

If you have it down or pull sending unit have some one watch guage while you operate float, those tanks are a pita

Yeah.. No kidding. I thought if I replaced the tank and sending unit along with the wiring that I would avoid this problem.. Cause there is no easy way to get access to that sending unit.

[Rochlow]

Ummmm, I'm a moron! Apparently you need more than 2-3 gallons of gas in the tank for it to register on the gauge.

After pull everything down and checking the sending unit I decide pull the sending unit out of the tank and everything was ok. Then put the unit back in the tank and hook up everything and it shows empty still. Add 5 gallons of gas to the tank and registers 1/4 tank.

So this is my dumbass screw up on this build. So off to put everything together!
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[D.PASSMORE]

I had a sending unit that went straight to "E" after 1/2 tank. Problem: where the float arm moves up and down on the resistor wire on the sending unit. Had a open (burnt thru) half way down the winding. Register at full but would lose the connection once it got past the open. How to check? Use a ohm meter, connect one side to output terminal on top of sending unit and one to ground. Regardless of the OHM reading, you should have a complete circuit (Pos to Neg) all the way through the scale (Full to Empty).

[HardLuck]

I have a 71, my gauge is past full. I have the tan wire ran, but I dont have a black wire? There is no black wire coming off the sending unit. My tank is still in the cab. Am I missing something?

[swyfor5]

Yeah, the black ground wire coming off the top of the sending unit.

[swyfor5]

Oh, sorry, didn't read right. You must be talking about a pickup if tank is still in the "cab". Don't remember how those wire up, but it seems you still would need a ground wire.