[klassickarman]

65 gmc, fuel gauge is stuck on full any suggestions

[klassickarman]

It's a fairly new sending unit. Only way to get fuel gauge to move is putting a ground on the hot wire. Then the gauge jumps almost to half and then bak to full...wiring problem?

[vicrod]

When the sending unit was replaced did the gauge work OK?
Make sure the sending unit is grounded. Sometimes the rust builds up an breaks the ground path to the chassis.

vicrod

[sedanbob]

Try running a dedicated ground from a clean spot on the chassis to one of the bolts on the sending unit flange. See if that helps.

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Originally Posted by klassickarman

It's a fairly new sending unit. Only way to get fuel gauge to move is putting a ground on the hot wire. Then the gauge jumps almost to half and then bak to full...wiring problem?

What hot wire ???

The wire going to the tank( sending unit) SHOULD NOT be hot.. Make sure the tank has a ground...

[sparkchaser]

The wire going to the sending unit has to be hot, it grounds through the sending units variable resistance to operate the gauge. Your trucks system should be 0-90 ohm type. If you ground the sending unit wire, the gauge should read zero and stay there. If you connect a 90 ohm resistor between the wire and ground, it should read full. (you probably won't find a 90 ohm resistor, but you can get a 100 ohm from Radio Shack and it will get you in the ball park) This will tell you if the gauge is working.
As has been previously said, you need to make sure the sending unit is grounded properly. If it still doesn't work, you need to pull the sending unit, connect an ohmeter to it and make sure the resistance changes between 0 and 90 ohms as you move the float arm.

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Originally Posted by sparkchaser

The wire going to the sending unit has to be hot, it grounds through the sending units variable resistance to operate the gauge. Your trucks system should be 0-90 ohm type. If you ground the sending unit wire, the gauge should read zero and stay there. If you connect a 90 ohm resistor between the wire and ground, it should read full. (you probably won't find a 90 ohm resistor, but you can get a 100 ohm from Radio Shack and it will get you in the ball park) This will tell you if the gauge is working.
As has been previously said, you need to make sure the sending unit is grounded properly. If it still doesn't work, you need to pull the sending unit, connect an ohmeter to it and make sure the resistance changes between 0 and 90 ohms as you move the float arm.

How do you ground a hot wire..

Never seen a hot wire going to a sender unit..I have seen a hot feeding the gauge,, But never a hot going to a FUEL sender..

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Originally Posted by klassickarman

65 gmc, fuel gauge is stuck on full any suggestions

What ever you do.. PLEASE DO NOT !!!!!! run a hot wire to the sender unit..

[sparkchaser]

Maybe we have a misunderstanding here, but the power you put into a gauge comes out of the gauge and goes to the sending unit where the variable resistance causes the needle to move. if you connect a voltmeter to the sending unit, you will read voltage. If the wire is not hot, how is the sending unit going to operate the gauge?

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Originally Posted by sparkchaser

Maybe we have a misunderstanding here, but the power you put into a gauge comes out of the gauge and goes to the sending unit where the variable resistance causes the needle to move. if you connect a voltmeter to the sending unit, you will read voltage. If the wire is not hot, how is the sending unit going to operate the gauge?

It work's through a ground, Not a hot.. Why do you think when you ground it, It moves the gauge..... You should be very careful posting to the world to run a hot wire to a FUEL sender unit,,, You might just cause someone to get hurt or even worst blown up...

Think of it like this.. Take a hot wire and ground it... You will short it out... The hot only feed's the gauge,, Not the sender,,, The gauge doesn't work until you make a ground.. The sender makes and breaks the ground,, That's why the tank needs a ground for the sender to work right..

I see now how you came up with the name sparkchaser...Fun intended..

[sparkchaser]

I never said to run a hot wire to the sender. I said the wire coming from the gauge is hot, it has voltage on it. Pull the wire off your sending unit, connect a voltmeter to it and you will be amazed.

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Originally Posted by sparkchaser

I never said to run a hot wire to the sender. I said the wire coming from the gauge is hot, it has voltage on it. Pull the wire off your sending unit, connect a voltmeter to it and you will be amazed.

Look's like you said it here..

That would be post #6

Originally Posted by sparkchaser

The wire going to the sending unit has to be hot,

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Originally Posted by sparkchaser

I never said to run a hot wire to the sender. I said the wire coming from the gauge is hot, it has voltage on it. Pull the wire off your sending unit, connect a voltmeter to it and you will be amazed.

Here's something you may need to read when you have time...Pick one..
Google

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Here's a good write up...The GM Fuel Gauge Mystery 1965 & Newer |

[sedanbob]

When the ignition is on, and your gauge is reading (full, empty, half way, doesn't matter) current is flowing through the sender. Yes, the sender is on the ground side of the gauge, but if there wasn't current flowing through the sender you wouldn't get a reading. As was stated earlier, if you put one lead from a volt meter on the lead going to the sender, and the other lead going to a ground, you will measure voltage. The sender body is in contact with the tank, which is in contact with the frame, thereby grounded - but those connections may not be electrically clean. If you jump a wire from a bolt on the sender flange, to a clean spot on the frame and the gauge then works, the problem was a poor ground.