There does not appear to be enough wires there for dual tanks. I purchased
an 82 GMC 3/4 ton 4wd new and it did not have dual tanks. I asked the dealer
about adding the second tank and purchased a tank, sending unit, switch, and
changeover valve. When you flip the switch on the dash you are not only
changing the valve for the gas lines you are also changing over the return
line from the engine also. From memory (no comments, this goes back 24
years) there should be one wire to each sending unit for each tank and one
wire to the changeover valve. Up behind the dash these wires should
terminate at the switch. From there the fuel gauge should be hooked up so
that when you flip the switch the gauge is reading the proper tank.
If you have access to an ohmmeter / Voltmeter you should be able to figure
out the wiring. That is the best I can recall at this time. Hope it helps.
Thanks for the comment.
I have the fuel / return / vent lines on each side and they are hooked
When we bought the truck only the passenger side had a gas tank and
that sender wire goes to the valve - pin 1.
The fitting on the end of the wire on the driver's side that
physically can go to the sending unit on that tank runs along the from
to the front of the truck inside a harness.
There is also a wire that goes from the driver's side @ the same area
as the tank sending unit and it runs over to the passenger side to pin
2 on the valve.
The hole in the dash for the switch is factory. The switch is broken
and I have ordered one to pick up after work tomorrow. When the
switch is installed I will connect a battery and take my multimeter
for some tests.
One way or another, I will figure this out.
Thanks again. You have been more help than you probably know :)
On Tue, 23 May 2006 22:36:40 -0400, firstname.lastname@example.org wrote:
Thank you so very much :)
We just got the new switch today. The wiring harness and plug at the
dash is intact. Got busy with a new riding lawnmower after work and
had no time for the truck so will likely get to it this weekend.
Will post my findings.
That wiring diagram is just what the doctor ordered.
On Wed, 24 May 2006 23:14:55 -0400, The Nolalu Barn Owl
OK, today I had some time. Installed the battery and tried at the
- No power with the key in or out, turned on or not at any of the
- The switch won't change the tanks (you can hear the motor) but no
power may well explain THAT.
I undid the covering on the harness from the left tank to the valve on
the right side and all appears well and unmodified.
I also undid some of the cover on the main harness to the front along
the left frame rail and found:
- One wire that has been cut off and I don't yet know what it should
connect to. The meter shows no power and no continuity to ground.
Anyway, I had some animals to feed and the hoards of blackflies are
now thick so I retreated to the indoors for now.
On Thu, 25 May 2006 20:07:57 -0400, The Nolalu Barn Owl
Man, this is bugging me but I am having fun too. Probably because
this is not our primary vehicle?
Anyway, I went out in the dark and retrieved the switch. Spent some
time with the meter and thinking. The switch should likely be
supplied with 12+ and a ground to the two middle leads. The switch
only makes a connection when it is depressed fully in either
direction. This changes the polarity on the other two poles from
straight to reverse and back.
This explains why the wires under the dash are two separate harnesses
for each set of two wires. Two wires for 12+ and ground and the other
two wires service pins 4 & 5 on the valve.
It is the VALVE that switches the gas sending units and NOT the switch
(directly). The gas gauge connects to pin 2 and pin 1 goes to the
left tank sending unit while pin 3 goes to the right.
What I am missing is the connection to the left tank and power to the
From the wiring diagram I found which wire should go to the left tank
sending unit but it is unbroken and unaltered as far forward as I can
follow it along the left frame rail. The best thing to do would be to
cut and splice a connection to this wire and then use a jumper to
power the switch and see what happens.
The cut wire I found may not have anything to do with the gas tanks at
all. Perhaps I'll find out before I finish.
On Thu, 25 May 2006 23:44:47 -0400, The Nolalu Barn Owl
The cut wire has continuity to pin 3 at the valve. I was able to get
a connector at NAPA and can splice that on to that wire. Someone took
out the driver side tank and just cut off the connector, then they
tucked the wire back into the split loam. I had to dig for it.
Color = pink + white just like the diagram.
No power to the switch. No continuity from the switch to the valve.
Somehow I got this old body upside down and under the dash with a
flashlight and followed the power harness from the switch up and tight
to the underside of the dash, over the steering wheel and down the
driver's door pillar to ... Two crimp connectors to nowhere. The wire
stops at the crimp connectors. Have to run new wire to power and
Hopefully, the valve is still in good shape :)
On Fri, 26 May 2006 22:41:36 -0400, The Nolalu Barn Owl
Thanks for the help, gfretwell,
Found the ground bulkhead near the fuse box and made the connection I
needed. Tapped into a live spot on the fuse box and put a 10A fuse in
line to make the power connection.
The HARDEST thing to find was this: The last owner painted the truck
and since he had the engine out at the time, he unplugged everything
and painted. The feed through at the firewall was left unplugged. It
dangled out of sight and nobody was any the wiser. I had to follow
the wires from the switch to the feed through and then figure out what
was going on at the engine side.
This would have been a much harder job without the wiring diagram you
Thanks again :)
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