Full gauge issue

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Erik Anderson

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GMC K25
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I have a 75 gmc k20. I've posted before and this page has helped me many times I'm hoping for another miracle. Fuel gauge reads way past full when the ignition is on and about 3/4 with the ignition off. The truck used to be dual tanks but the passenger side tank and valve was removed before my time. The switch for it is still in the cab I just leave it on the left hand tank. I've read other posts and the ground at the sending unit is good and I have 12 volts on the pink wire at the switch. I'm just not quite sure where to go from here. Hopefully someone can help thanks.
 

QBuff02

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check your signal voltage at the sending unit at the tank. If you have a good ground and good supply power it's either the sending unit is bad or the signal wire/connection on top of tank has an issue. I know on my truck every once in awhile the gauge will go past full and when it does I can stick my hand up in there on top of the tank and wiggle the sending unit wire and it will take off working. I figure there's just enough corrosion on the probably original 39 year old tank and sending unit that it loses contact from time to time. It's something I figure I'll address when I get to the point of replacing the fuel system. But all my wiring checked out so it was either the connection was bad or the sender itself. turned out to be the connection. Sometimes you can take a pair of pliers and "pinch" the connector closed for the signal wire and make a better connection. might work for you.
 

75gmck25

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Could you first answer a couple of ques.
- Was this a factory dual tank truck (fillers on both fenders, and black rocker switch on the dash)?
- When they went to only one tank, does it look like they left all the factory wiring in place?

The reason I ask is that I know how the factory wiring was routed for dual tanks in ‘75, so I may be able to help you figure out how they modified it.

The wiring route seems really weird until you realize they set it up based on a single passenger side tank, and then tacked on the wiring for the driver’s side tank. I’ll look for the wiring diagram I made several years ago and maybe it will help.
 

Erik Anderson

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I tried sticking my multimeter prob in that connector on top of the tank and I'm getting no voltage at all so I believe my problem is elsewhere unless I'm just not getting a good connection with the prob
 

Erik Anderson

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The wiring looks to all be in place as well as the switch on the dash. As far as I can tell it looks like the tank wiring comes down the frame on the driver's side in loom has a tan (I think) that comes out of the loom before the rest of the wiring crosses over to the passenger side and is coiled up and ziptied
 

Erik Anderson

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So little update with more information, I have 12 volts to the pink wire on the transfer switch, I have no power at the transfer switch. I pulled the actual gauge out of the cluster and I have 12 volts at the gauge, the ground at the tank ohms good and so does the ground at the gauge. I cleaned the clips behind the gauge and still reads over full. I also tested the gauge on the bench with the sending unit and they are both good. I'm at a loss of what to try next.
 

Erik Anderson

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Connecticut
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Erik
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1975
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GMC K25
Engine Size
350
So little update with more information, I have 12 volts to the pink wire on the transfer switch, I have no power at the transfer switch. I pulled the actual gauge out of the cluster and I have 12 volts at the gauge, the ground at the tank ohms good and so does the ground at the gauge. I cleaned the clips behind the gauge and still reads over full. I also tested the gauge on the bench with the sending unit and they are both good. I'm at a loss of what to try next.
Sorry no power is at the sending unit
 

Raider L

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That tan wire should be the factory sending wire from the passenger side tank on a single tank truck. And since now you're saying "No power at the sending unit." There's either a short somewhere in the birds nest of wiring, or the sending unit has some kind of intermittent problem? Unless the previous owner was real good at removing the other tank and labeling the wires, you'd have to take each wire by itself and see where it goes and what kind of shape it's in.
 

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