A different fuel gage grounding problem

[Tucson38]

I know this topic has been done to death but here's a twist. I have the usual
"fuel gage @ the 3 o'clock position" indicating a ground fault but…..it does it with both tanks. So my assumption is the ground fault isn't at the tank as usual so is there a place for a common ground for both tanks? Perhaps at the gage itself?

Thanks for any help.

'85 GMC 3/4 Ton HD 4 WD

[clapage6]

i wouldn't rule out the possibility of it being both ground straps from both senders... they both ground right on top of the frame rail on their respective sides and have a tendency to get quite corroded there. I would definitely check those first.

Did they both start doing the 3 o'clock thing at the same time?

[Tucson38]

They both started the malfunction at the same time. That's why I'm looking at a different solution but I will check out the grounding straps first.

[clapage6]

are you able to verify the switch is actually switching tanks? I know it'd be hard for me to tell if the gauge didn't move when i hit it is why I ask.... Could also help narrow it down though as those ground straps are kind of a pita to get to

[Tucson38]

Good thought but the tanks are definitely switching.

[ray_mcavoy]

An open circuit anywhere in the sending unit wiring (not just on the ground side) will cause the gauge to read over-full.

The tank selector valve should have three pink wires associated with the sending unit wiring. One with a black stripe going to the LH sending unit, one with a white stripe going to the RH sending unit, and a solid pink wire that leads up to the gauge.

Since both sides are equally affected, I would recommend looking at that solid pink wire that leads up to the fuel gauge. Follow it from the tank selector valve along the frame and you should find a single inline connector. Unplug it at that point, and temporarily ground the end of the wire that goes up toward the front of the truck.

Turn the key on and observe the fuel gauge ...

If the gauge now reads empty with that wire temporarily grounded, that means the pink wire is okay from that point forward. Your problem is most likely in the tank selector valve or it's wiring connector. In addition to switching the fuel lines between tanks, the selector valve is also responsible for electrically switching the gauge to the appropriate sending unit.

However, if the gauge continues to read over-full with the pink wire temporarily grounded, that means the problem is most likely in the pink wire itself one of it's connection points. If that's the case, continue to follow the pink wire up along the frame, inspecting it for damage along the way. Check to make sure it has good connections at the firewall bulkhead connector and at the instrument cluster plug.

[Tucson38]

Thanks Ray, I'll make that my next move and see what happens.

And thanks to everyone who took the time to ponder this!

[dieseldawg142]

yup, ray is most likely right. my gauge went to 3 last yr, sender wire was sliced by a clamp

[Tucson38]

Was able to resolve this problem:

I pulled dash to install LED's and found the ground fault at the gauge itself. Here's what happened.

The gauge in this vehicle ('85 GMC) has three threaded studs that slide into 3 receivers. There was a gap in the bottom receiver that I just tightened (crushed) with a pair of pliers. Good contact, good gauge readings.

Thanks to everyone's input on the original post.

Oh yeah, the LED's make a huge difference in brightness. I used 5-LED lamps for the main dash lighting. Waiting for an opportunity to drive at night now.

[tommartin]

Nice, my gauge is doing the same on both tanks too, gauge goes to 3. I'm going use this info and try the same troubleshooting.

Lets see those LEDs.

[Tucson38]

Let me know how it turns out for you.