fuel gauge issue

[mehoff400]

just bought a 65 with tank in cab. when key is off the gauge floats to about half tank. with key on it will jump down to empty. i have cleaned ground and seems to have a good ground. i have tried to do some research but i am getting foggy answers.

[AcampoDave]

I would pull the sender and check the float. I would also check the resisitance with an ohm meter when I had it out. I just went thru this last year and I think they go from zero in the empty position to 30 or 90 when full. I just can't remember which for sure but im thinkin' a 65 was 0-30..

[mehoff400]

Originally Posted by AcampoDave

I would pull the sender and check the float. I would also check the resisitance with an ohm meter when I had it out. I just went thru this last year and I think they go from zero in the empty position to 30 or 90 when full. I just can't remember which for sure but im thinkin' a 65 was 0-30..

my assumption i need to wait till i have about half tank to pull the float? i just filled it up

[ray_mcavoy]

One of the most likely causes for the gauge to read empty as soon as you turn on the key is a sending unit wire that is shorted to ground. And the most likely spot for that to occur is where the sending unit wire runs underneath the driver side sill plate. It's not uncommon for the wire to get pinched under the plate or pierced with a screw.

A quick check would be to temporarily disconnect the wire from the sending unit. If the gauge still reads empty when you turn the key on, it's most likely a problem with the wire. But if the gauge now reads over-full, the problem is most likely in the sending unit (either a defective unit or a sunken float).

AcampoDave is correct ... the 66 & earlier GM truck sending units should read 0Ω with the float down in the empty position and 30Ω in the full position. And yes, since you just filled the tank, you should wait until the fuel level gets down a bit before pulling the sending unit.

[rudyyloverboiii]

I have a similar problem. When I fill up half a tank my gauge reads full n when it's completely empty it says half. I bought an aftermarket gauge n when hooked up to the tank n grounded it does the same. Bad sending unit?

[mehoff400]

truck has been rewired.. i checked the wire everything looks good.. bad sending unit?

[rudyyloverboiii]

It was my sending unit. When I moved up the float my gauge read full. As I dropped it to empty it wouldn't come down it would only show on the gauge when completly down to empty

[63burban]

Float probably full of gas

[Crazy Rusty]

A bit of an old thread but I just encountered the same issue.

Fuel gauge was working fine. I took the cluster out, got a new cluster harness (for deluxe gauges) and accidently hooked up the battery gauge to the fuel gauge. Turned the truck to accessory and saw the gauge drop under empty (full tank). Turned the truck off, realized my mistake, swore a few times, put the correct wires on and tested again, same issue. Figured I had messed the gauge up. Got my new deluxe gauges in, hooked it up properly the first time, and now I have the same issue, drops down to empty.

I still need to test unhooking the sending unit but I'm still trying to track down what I screwed up.

[1963c-10]

My guess would be the resistor on the back of the fuel gauge was fried. Take a visual look of it and see how it looks.

This one pictured is burnt. Should be white....maybe a little dusty, but if it looks burnt up, it probably is.

[Crazy Rusty]

Both fuel gauge resistors are white. I unplugged the sending unit and it did the same thing. Not sure what it is now... I did notice the sending unit plugs into the fuse box, is that normal?

I've spent three hours undoing 50 years of crazy terrible wiring so I'm questioning everything... including my sanity.

[Crazy Rusty]

Definitely some weird electrical issue. My left turn signal now makes the fuel gauge move rapidly

[AcampoDave]

Rusty, i dont think sending units connect to 12 volt power. I'll look at mine and try to report back. Edit: I checked... no power at sender. I think that wire should go to the guage. Mine goes to the fuse block too, but appears to pass behind it and then enter the harness to the cluster. Plus I'm pretty sure I recall smoking a sender by hooking it to 12 volt while boneheadedly trying to test it when it was out of the tank. I think it works more like a fancy ground that gives feedback to the guage in ohms.

[Crazy Rusty]

Originally Posted by AcampoDave

Rusty, i dont think sending units connect to 12 volt power. I'll look at mine and try to report back. Edit: I checked... no power at sender. I think that wire should go to the guage. Mine goes to the fuse block too, but appears to pass behind it and then enter the harness to the cluster. Plus I'm pretty sure I recall smoking a sender by hooking it to 12 volt while boneheadedly trying to test it when it was out of the tank. I think it works more like a fancy ground that gives feedback to the guage in ohms.

Ya, after I posted that I did an ohm test on the sender (which was fine) and then a continuity test on that fuse block spade to the fuel gauge connector and that was correct. So it definitely plugs in there but gets its power from the pink wire.

I don't get any fuel gauge movement until the right turn signal comes on so I'm betting there is a bad ground or switch somewhere. Kind of strange though as I would assume that I would get default movement (the fuel gauge would read correctly) until the turn signal came on. So far I haven't been able to narrow it down though.

[ray_mcavoy]

Originally Posted by Crazy Rusty

I did notice the sending unit plugs into the fuse box, is that normal?

Yes, that is normal. However, in your photo it appears to be plugged into the cigarette lighter power receptacle ... the next receptacle up should be the one for the fuel sending unit.

Originally Posted by Crazy Rusty

I don't get any fuel gauge movement until the right turn signal comes on so I'm betting there is a bad ground or switch somewhere. Kind of strange though as I would assume that I would get default movement (the fuel gauge would read correctly) until the turn signal came on. So far I haven't been able to narrow it down though.

Yes, a bad ground at the fuel gauge will cause it to read empty even if everything else in the system is okay. The fuel gauge is grounded through the cluster's metal case.

Poor instrument cluster grounds can cause some lights/gauges to find an alternative ground path through another circuit (such as the turn signal indicator bulb's filament). So that can explain why the fuel gauge is moving in response to the right turn signal.

[bhc41051]

I have two weird fuel gauge issues, I ran a separate ground wire to the frame. The gauge now works like a tach the higher the rpm's the higher the reading on the gauge (pegged when cruising) I unhooked the ground wire to the frame and rocked the truck to move the float and the generator light cycles on and off.

[Crazy Rusty]

Originally Posted by ray_mcavoy

Yes, that is normal. However, in your photo it appears to be plugged into the cigarette lighter power receptacle ... the next receptacle up should be the one for the fuel sending unit.

Whats strange is that I get no tone from any of those spades to the fuel gauge plug. On the other hand I do get a tone from the brown wire on that plug to the frame, which to me means it's grounding out somewhere (and that's with the sending unit unplugged at the fuse panel). That makes me think maybe something got fused somewhere that I can't see. But that also rules out a ground issue, the line is definitely grounded, just grounded to the wrong place.

Originally Posted by ray_mcavoy

Yes, a bad ground at the fuel gauge will cause it to read empty even if everything else in the system is okay. The fuel gauge is grounded through the cluster's metal case.

Poor instrument cluster grounds can cause some lights/gauges to find an alternative ground path through another circuit (such as the turn signal indicator bulb's filament). So that can explain why the fuel gauge is moving in response to the right turn signal.

I added a ground from the case to the frame and that didn't solve it. Just for fun I put a voltage meter between the pink and the spade for the gauge and it read a normal 12.2 but then started bouncing when I turned the turn signal on. Since the brown portion wasn't connected, I though that was more odd.

At this point I'm going to trouble shoot it more tomorrow and if I can't figure it out I may break down and buy a new dash harness. This one has been hacked up and it's made it very difficult to troubleshoot.
Posted via Mobile Device

[Ronw435]

One of the most valuable tools I have encountered is a Power Probe III. You can provide power as well as ground to any situation. It would point out if its a ground you can just toggle the tool to ground and apply it with the probe. I sometimes wonder how I ever got along without one. And they come with a very long cord that will allow you to check tail lights while hooked to the battery.

[Crazy Rusty]

Originally Posted by Ronw435

One of the most valuable tools I have encountered is a Power Probe III. You can provide power as well as ground to any situation. It would point out if its a ground you can just toggle the tool to ground and apply it with the probe. I sometimes wonder how I ever got along without one. And they come with a very long cord that will allow you to check tail lights while hooked to the battery.

Thanks for the tip! I'll have to pick one of those up. I ended up finding the issue. Someone had switched the left turn signal and gas gauge wires on the dash harness connector. I also added an extra ground to the dash backing which actually did take care of a little wiggle that was left after I fixed the first issue. All that time.... thanks a lot previous owner.

[Ronw435]

You can never have too many grounds on these fifty year old trucks!!