1986 c2500 GMC fuel gauge duel tank issues

[Dukester68]

Hello my square body experts! I have a 1986 c2500 GMC. It belonged to my FIL and we took ownership after he passed. Its a California 350 and only has 66,000 original miles. Its in beautiful shape and we are keeping it original. That being said i have a gauge issue. I have watched all the you tube vids and was pretty convinced it was the sender units. I have replaced both sender units and cleaned up the grounding areas on both sides and replaced the fuel lines. Im still getting wrong readings on my gauge when on the drivers side tank. The right side tank reads almost empty as it’s probably correct but when i switch the tank back over it only goes up to a quarter plus, it is full of fuel… I pull the connection on sender separately and both go to full on the gauge. I can hear it switching and it runs on either tank. Im stumped. I have left the bed backed out about a foot just in case i need to get back in there. I have swapped out the new senders but still get same results. Im not thinking its a gauge issue but im not sure. I have had the dash and gauges out when i had the dash redone. Could it be an issue at the tank selector switch under the truck? It switches but thats when the gauge fails…………Thanks on advance for the help…

 

[ali_c20]

If it was a gauge issue it would not work on 1 tank.
The gas gauge pegs out over full when there is no ground.
Does it do the same when you disconnect the ground on the passenger side sending unit?
I would check: The selector valve wiring. The selector switch wiring. The sending unit ohm readings with a multimeter.

What is the black wire end in the picture of the driver side tank?

Nice clean truck.

Good video on GM gas gauges:

xc_hide_links_from_guests_guests_error_hide_media

 

[Dukester68]

Hello Ali.. thank you for the info. The black wire end is the original ground wire bolted underneath. I just haven’t removed it yet. You are very observant!
I am getting normal readings (pegs out at full) at the gauge when i disconnect the ground. I used the multimeter at both senders(both were tested before i installed) and the drivers side is almost full and reads 86 ohms. The passenger is about a quarter full and reads 36 ohms. The tank switch is new and it is switching correctly, however it is sporadic. It hear it vibrate sometimes when I switch it. I have traced the wires from the senders to the tank selector valve on the frame and pulled the plug. There are two pink wires which im sure are the senders and a red wire the im pretty sure is the one to the gauge. Im going to jump one pink at a time to the gauge wire and see if this bypasses what i think could be a bad fuel selector valve. Do you think i can test the switch with this procedure?
Im a bit under the weather right now and don’t feel well enough to be under the truck. PS… this is also the best video explanation of the test procedure which i have watched carefully several times. Thank you fir your input and time spent to reply. Im sure together we will figure this out!

 

[ali_c20]

You are welcome.
I would bypass everything between gauge and sending units and test function. Than bypass the selector valve to check switch. This way you should be able to find the issue.
Are all the wires ok, no shorts or rubbed thru insulation?

 

[mtbadbob]

That truck is a cream puff, wow! Don't count on the port valve switch being fine. More than likely it's CHINESE CRAP, or is it original?. I had to get my auxilary tank in service when I bought my truck, new sending unit, re-wired the correct factory tank selector switch in, and installed new port valve switch. My gauge works correctly when I switch tanks, but if my aux tank is a bit less than 1/4 full, my truck dies most of the time. It'll start right up, but for some reason it momentarily runs out of fuel. I haven't investigated it any further, just happy that I have the extra fuel & the gauge and tank switch work as they should!

 

[Jgonick]

drivers side is almost full and reads 86 ohms. The passenger is about a quarter full and reads 36 ohms.

That is looking correct.

Also- check for loose nuts on fuel gauge terminals. There is also a resistor on the back of the gauge that might be bad. (Newer fuel gauges don't use it)

I know you said you watched a lot of videos- this one is one of the best and explains it very well.
LINK-Video

here are two pink wires which im sure are the senders and a red wire the im pretty sure is the one to the gauge. Im going to jump one pink at a time to the gauge wire and see if this bypasses what i think could be a bad fuel selector valve.

You could- but also you could test the ohms at the location. If the Ohms are correct then you know the problem is the wiring to the fuel gauge or the gauge itself. the wires are usually pnk/wht & pnk/blk to tanks and pnk to gauge (see attachment)

just double checking. You mentioned it read full when you pulled connection at sender.
No ground or no volts at sender the gauge should read above full @ 3 O’clock.