Fuel gauge issue

[shortbed70]

Ok so my fuel gauge decides it wants to stay full all the time. I am running an okc rear mount 17 gallon tank with their sending unit and it has worked fine for years now decides not to. Goes to empty when key is off but as soon as it sees power straight to full. What can I start checking before blaming the sending unit? It’s tough to get the tank out of the truck!

[Captainfab]

Check to see if the sending unit wire is grounding out somewhere from the gauge back. Also disconnect the sending unit wire from the back of the gauge, Then turn the key on and see what the gauge does.

[luvbowties]

Originally Posted by shortbed70

Ok so my fuel gauge decides it wants to stay full all the time. I am running an okc rear mount 17 gallon tank with their sending unit and it has worked fine for years now decides not to. Goes to empty when key is off but as soon as it sees power straight to full. What can I start checking before blaming the sending unit? It’s tough to get the tank out of the truck!

yOU SAID, "It’s tough to get the tank out of the truck!". MOVE SEAT FORWARD & SEE IF YOU DON'T HAVE ACCES TO THE UNIT'S LOCKING RING. But, 1st, check along top of rocker panel(& under the chrome hold-down for floor covering) for a rusted-thru, fuel gauge wire that is grounded/shorted to floorboard metal. This is a customary problem, altho' it could be shorted inside the tank along the sending unit. HTH,
Sam

[shortbed70]

Lmc rear mount 16 gallon tank so it’s not behind the seat. Cap fab I will check that stuff out soon as it stops raining again

[Ken Cook]

What Captain Fab said.
If you remove the wire from ether end it should go to empty. If you touch the tank side to ground it should go to full.

If it stays at full with the gauge side disconnected the gauge is the problem.

If it stays at full with the tank side disconnected the wire is grounding out somewhere in between.

If it goes to empty with the tank wire disconnected then it’s the sending unit.

[The Rocknrod]

Unless your grounds are bad.

[shortbed70]

How do you check the gauge at the cluster?

[Ken Cook]

Originally Posted by shortbed70

How do you check the gauge at the cluster?

One side of the gauge has power (12vdc) the other side goes to the tank. The float moves a variable resistor that goes to ground. In case you don't know what a variable resistor is, it is like a volume knob on a radio. More or less resistance is created as the float swings up and down.

If you disconnect the non powered side of the gauge and jump it to ground the gauge should move.
I don't remember which way. I think no ground and the gauge will show FULL and complete ground will make the gauge show EMPTY. I MAY HAVE THIS BACKWARDS.

To do this; Disconnect the plug from the gauge and check both wires with a test light or meter to find which one is 12vdc. Then supply only the 12v powered connector of the gauge with 12vdc.
You may need to make a small testing wire with a male blade on one end and female blade on the other end to have the plug off and still supply the gauge with 12v. Then use a jumper wire on the other side of the gauge to jump to ground.

DON'T jump the 12v STRAIGHT to ground or you will blow the fuse.

Also I have a permanent ground wire on my cluster. I'm not sure but I think the gauge itself is grounded through the cluster. So you may need to use a jumper to ground the cluster. I would use a jumper with 2 alligator clips to do this or make a permanent ground for your cluster like mine has. If you do make a permanent ground on your cluster your lights will work better and that's definitely a plus.

Other things to consider is the wire that runs between the gauge and the tank, and the ground from the sending unit on the tank.

The sending unit can have spots that are not consistent and cause the gauge to not read correct. That is what is wrong with mine. It shows full when it is full but when it is 3/4 full the gauge shows 1/2 a tank.

[shortbed70]

Thank you. It worked fine until one of my airride switches broke and grounded out. Now it just stays full but goes to empty when you turn the truck off. I know it’s getting voltage and all the sending unit stuff is fairly new. How did you ground your cluster? Maybe I’ll try that.

[Ken Cook]

Originally Posted by shortbed70

Thank you. It worked fine until one of my airride switches broke and grounded out. Now it just stays full but goes to empty when you turn the truck off. I know it’s getting voltage and all the sending unit stuff is fairly new. How did you ground your cluster? Maybe I’ll try that.

Use a crimp eye and attach it to the cluster. Any of the screws will work. Then I spliced in to the ground wire to my cigarette lighter. I put a male/female in the middle so I can disconnect it when I need to pull the cluster out.

Did your air ride go all the way down?? If so maybe you have a pinched wire or cut wire going to the tank. You could test that by running a wire straight from the fuel gauge to the tank. Disconnect the wire at the gauge and tank first. Run the test wire right out the door and over the bed. Connect it to the tank and gauge. If that makes it work then fine the bad spot in the wire.

If you check the gauge like I described in my last post and find that the gauge shows FULL when grounded then the wire is touching the frame somewhere between the gauge and tank. If it shows Full without jumpering to ground then your wire is cut.

You can call me if you want. I may be more helpful on the phone than with the keyboard. 337-302-2118 Ken

[Captainfab]

I would suggest checking the wire from the sending unit in the tank all the way to where it plugs into the fuse box. That is assuming you have an original fuse box.

[shortbed70]

Originally Posted by Captainfab

I would suggest checking the wire from the sending unit in the tank all the way to where it plugs into the fuse box. That is assuming you have an original fuse box.

It’s a 21 circuit autowire box and local rear mounted tank. Everything looks good with the wiring so need to check the gauge.

[Ken Cook]

You said there was as Air Ride Switch that shorted out.You should check all the fuses with a test light.

I added a radio to my truck recently. After passing the wires back and forth my gauge went to full. The gauge was un-plugged. That would be the same as no power or a blown fuse. I plugged it back in and it was back to normal.

[shortbed70]

Ok so I unplugged the plug at the gauge and it stays empty. Hook it back up and unhook sending wire plug at main harness and gauge goes to full.... Here’s the weird part! I drove it a bit yesterday and it worked for a few seconds then went back to full again. I’ve checked and all my grounds are good, not corroded or broken. What should I look for next? Sending unit or gauge?

[shortbed70]

Originally Posted by Ken Cook

You said there was as Air Ride Switch that shorted out.You should check all the fuses with a test light.

I added a radio to my truck recently. After passing the wires back and forth my gauge went to full. The gauge was un-plugged. That would be the same as no power or a blown fuse. I plugged it back in and it was back to normal.

I’ve already done this after it happened

[Captainfab]

If you disconnected the sending unit from the system, then you have a short/ground somewhere. Grounding the sender wire will make the gauge go to full.

[shortbed70]

Originally Posted by Captainfab

If you disconnected the sending unit from the system, then you have a short/ground somewhere. Grounding the sender wire will make the gauge go to full.

I just unplugged the sending wire that feeds the gauge and it still went full but unplugging the two prong on the gauge it went dead empty. I can’t even get to my tank sending unit without dropping the tank....