1986 Chevy Truck Fuel Gauge problem.

tripower428 · 03-23-2009, 02:04 PM

Hello all.
I am new to this forum so I hope I am doing this right.
I have a 1986 Chevy k20 4x4 with a 350/auto and dual tanks.
My fuel gauge shows "way" above full even if I switch to the 2nd tank which is almost empty.
I got another instrument cluster thinking the gauge was bad but still have the same problem.
Most people are saying a "wire" issue but where do I look for this wire issue?
Thanks for any help.

bradrhale · 03-23-2009, 03:49 PM

maybe a sending unit problem. either that or you have a wire that is grounding out being that it is pegged out on full all the time. look to the top of the fuel tank as that is where the connection problem is most of the time.

leolkfrm · 03-23-2009, 05:40 PM

Welcome new person!!!
^^ I agree it sounds like the guage wire is grounded however I would look at the transfer switch since it happens on both tanks

crabtruck · 03-23-2009, 06:08 PM

Welcome to the forum tripower428.

I would take a different view.
Since the gauge reads high for both tanks
the problem is not likely in either (not the senders).
It should have nothing to do with the tank switch on dash.

The problem is
1) the gas valve itself is bad and doesn't connect to either tank.
2) the wire from the valve to the sender is broken (open)
3) the gauge is bad
You replaced cluster so 3) is not it.

That leaves the first two. Most likely 2).
If you follow the wire from cluster to firewall
(believe it's pink), check for pulled wire there
and on firewall engine side (third? bolt down ?).
Believe you will find a two wire connector
as tank wires exit harness to rear that sometimes
is victim to corrosion or pull damage.
Think second wire is for fuel relay circuit or such.
Good luck.

leolkfrm · 03-23-2009, 08:08 PM

hey crab do you have the wiring digram? where does the Gage feed get changed at the solenoid or the dash switch?

tripower428 · 03-23-2009, 09:48 PM

I checked the wires from the gauge to the firewall and everything appears to be connected. Could it be a wire on the tank itself? Crabtruck you mentioned a gas valve, where is the gas valve? Thanks.

hotrodpc · 03-23-2009, 10:56 PM

I am with crab on this one. I see the suggestions as a wire grounding out. It should be the opossite. It is the lack of a ground. The ground wire has likely become rusty or something of that nature. If you see the ground connected, take it off, get some sandpaper, grinder or something and RE ground the wire. If that does not work chase the grounds down and find where it is open. On a buddy's 77 C20 truck, he had the same problem on both tanks, although only 1 tank worked. Thinking it was the switch, he got a new one, and nothing changed on the guage but he was able to get fuel from both tanks now. Then he chased down the grounds, cleaned them and regrounded them and all was great with the guages too. Kinda of wierd I know, when you ground out any other kind of guage it will read full hot or max out. Guess a fuel guage works opposite. That would be my suggestion.

hotrodpc · 03-23-2009, 10:58 PM

Forgive me, when I say he replaced the switch, it was the switch at the tanks, not the dash tank selector switch.

crabtruck · 03-23-2009, 11:48 PM

Quote:

Originally Posted by tripower428

I checked the wires from the gauge to the firewall and everything appears to be connected.
Could it be a wire on the tank itself?
Crabtruck you mentioned a gas valve, where is the gas valve?

Wire from gauge to firewall is only likely if mods done or abusive feet action.
Follow wire from other side of firewall, down loom under cab and to somewhere
midway between tanks where gas valve (for pipes) is located.

At the valve you should have five wires, 3 for gage and two for pumps.
Pumps are operated by the dash switch toggling to reverse hot and ground wires
to valve. This also flips the internal connection for senders.
Since pumps work, I am expecting ground not to be a problem.
2 of 3 gauge wires are to senders, only one is to gauge itself.
Logic says either you see massive physical problems with wiring,
or the sole common wire is bad (valve to gauge).
The run of that wire likely has a connector midway to accomodate
wiring for a single tank set up.

If you were to disconnect that plug and momentarily ground
the side that runs to gauge, gauge should read about empty.
That would put the problem from plug to valve. A voltage check
of that wire (disconnected) would give battery voltage (12v).

If not reading empty or any change, you could ohm in the other
direction towards the tank on sender wire and expect to get
about 90 ohms on a full tank and near zero on an empty,
with a slide in between, ie: 1/2 tank about 45 ohms.

Somewhere in another thread I believe I posted a diagram for this
already. Look around for it and I will also see if I can find it.

hotrodpc · 03-23-2009, 11:53 PM

hmmmmm, Crab? Are we on the right truck or am I missing something? an 86 model should have a manual pump at the engine block, no pumps in the tanks.?.? If so, my info above is wrong for this issue. I was with the understanding there is the dash switch, gate valve for the fuel at the tanks, and then guage. Not sure if that makes a difference on the wiring colors, codes, etc.

03-23-2009, 02:04 PM	#1
tripower428 Full Member Join Date: Mar 2009 Posts: 37	1986 Chevy Truck Fuel Gauge problem. Hello all. I am new to this forum so I hope I am doing this right. I have a 1986 Chevy k20 4x4 with a 350/auto and dual tanks. My fuel gauge shows "way" above full even if I switch to the 2nd tank which is almost empty. I got another instrument cluster thinking the gauge was bad but still have the same problem. Most people are saying a "wire" issue but where do I look for this wire issue? Thanks for any help.

03-23-2009, 05:40 PM	#3
leolkfrm Senior Member Join Date: Mar 2008 Location: N.Y. Posts: 4,300	Welcome new person!!! ^^ I agree it sounds like the guage wire is grounded however I would look at the transfer switch since it happens on both tanks __________________ 4 wheels move the body 2 wheels move the soul B.W.A.I

03-23-2009, 08:08 PM	#5
leolkfrm Senior Member Join Date: Mar 2008 Location: N.Y. Posts: 4,300	hey crab do you have the wiring digram? where does the Gage feed get changed at the solenoid or the dash switch? __________________ 4 wheels move the body 2 wheels move the soul B.W.A.I

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03-23-2009, 03:49 PM	#2
bradrhale Senior Moderator Join Date: Apr 2006 Location: Texas Posts: 8,026	maybe a sending unit problem. either that or you have a wire that is grounding out being that it is pegged out on full all the time. look to the top of the fuel tank as that is where the connection problem is most of the time.

03-23-2009, 06:08 PM	#4
crabtruck Senior Member Join Date: Jul 2007 Location: on your 9 Posts: 3,366	Welcome to the forum tripower428. I would take a different view. Since the gauge reads high for both tanks the problem is not likely in either (not the senders). It should have nothing to do with the tank switch on dash. The problem is 1) the gas valve itself is bad and doesn't connect to either tank. 2) the wire from the valve to the sender is broken (open) 3) the gauge is bad You replaced cluster so 3) is not it. That leaves the first two. Most likely 2). If you follow the wire from cluster to firewall (believe it's pink), check for pulled wire there and on firewall engine side (third? bolt down ?). Believe you will find a two wire connector as tank wires exit harness to rear that sometimes is victim to corrosion or pull damage. Think second wire is for fuel relay circuit or such. Good luck.

03-23-2009, 09:48 PM	#6
tripower428 Full Member Join Date: Mar 2009 Posts: 37	I checked the wires from the gauge to the firewall and everything appears to be connected. Could it be a wire on the tank itself? Crabtruck you mentioned a gas valve, where is the gas valve? Thanks.

03-23-2009, 10:56 PM	#7
hotrodpc Senior Member Join Date: Jul 2008 Location: OKC, OK Posts: 3,155	I am with crab on this one. I see the suggestions as a wire grounding out. It should be the opossite. It is the lack of a ground. The ground wire has likely become rusty or something of that nature. If you see the ground connected, take it off, get some sandpaper, grinder or something and RE ground the wire. If that does not work chase the grounds down and find where it is open. On a buddy's 77 C20 truck, he had the same problem on both tanks, although only 1 tank worked. Thinking it was the switch, he got a new one, and nothing changed on the guage but he was able to get fuel from both tanks now. Then he chased down the grounds, cleaned them and regrounded them and all was great with the guages too. Kinda of wierd I know, when you ground out any other kind of guage it will read full hot or max out. Guess a fuel guage works opposite. That would be my suggestion.

03-23-2009, 10:58 PM	#8
hotrodpc Senior Member Join Date: Jul 2008 Location: OKC, OK Posts: 3,155	Forgive me, when I say he replaced the switch, it was the switch at the tanks, not the dash tank selector switch.

03-23-2009, 11:53 PM	#10
hotrodpc Senior Member Join Date: Jul 2008 Location: OKC, OK Posts: 3,155	hmmmmm, Crab? Are we on the right truck or am I missing something? an 86 model should have a manual pump at the engine block, no pumps in the tanks.?.? If so, my info above is wrong for this issue. I was with the understanding there is the dash switch, gate valve for the fuel at the tanks, and then guage. Not sure if that makes a difference on the wiring colors, codes, etc.