wiring for dual tanks

[gafiredog]

Hey All. does anyone know the color of wiring I need to be looking for on the dual tank wiring harness for my 78 c-30. switch tank to left side, fuel gauge reads fine, and get fuel out of fuel pump. switch to right side, cant hear any sound in the change over valve mounted to the frame, and also, gauge needle goes to 3 oclock, so I have not even put any fuel in that tank. any help appreciated.

[hatzie]

3:00 needle means there's a break between the tank to frame ground and the gauge. In order of probability.

1. Frame ground to sender ground wire is broken.
2. Wire from sender to tank switch AUX terminal inside the cab is cut or badly damaged.
3. The dash switch is shot and not routing the sender signal to the gauge.

Since the valve is inoperable too... the dash switch may be shot.

The dash switch is cheap and easy to replace. Fairly easy to test too.

Check for volts at the tank valve connection stud with the engine off / ignition on and the tank switch set to AUX. If you have volts then the valve is shot. No volts means the fuse is blown or the switch is shot.

This is a graphic I made to explain the internal operation of the switch. It's a Double Pole Double Throw switch that's missing 1 terminal.


The wiring diagrams are in my signature.

[gafiredog]

thanks for the help, Hatzie

[Man With A Gun]

Which tank is the aux? The driver's side?

[hatzie]

Depends on the year.

[Man With A Gun]

1986 in my case.

[hatzie]

There are two different tank valves for the NL2 optioned trucks.

The 1975 - 1980 Solenoid valve, like the OP's 1978, snaps back to a rest position. The rest position is the production tank.
I've owned several single tank C/K trucks with model years between 1973 & 1979 and not all of them had a LH tank... At least one of them was a RH tank truck but I don't remember what year it was... It was Light Blue that's all I remember about it. Service, Wiring, and Parts manuals are not much help either...

1981-1991 trucks don't have Production and Aux tanks it's just LH & RH tanks. Your 1986 is one of these.
The Motorized valve retains its' position after power is removed so there's really no AUX tank.

The short answer is between 1973 & 1980 I don't know which side the production tank was on. Look at the plumbing and go from there. You can pick the tank you want to call AUX when you put the valve in just be sure the sender wiring matches the plumbing. Not very satisfying but...

[ray_mcavoy]

Originally Posted by hatzie

The short answer is between 1973 & 1980 I don't know which side the production tank was on.

According to the listings in the GM Heritage Center Vehicle Information Kits, the production tank was on the RH side for 1975 through 1980. And was moved to the LH side for 1981 - 1991.

Originally Posted by hatzie

1981-1991 trucks don't have Production and Aux tanks it's just LH & RH tanks.

You're right, as far as the switch & valve setup is concerned, it's just LH & RH tanks in those years. So the terms production and auxiliary are really only useful for describing where the tanks were located when making comparisons between trucks with & without the NL2 option.

[hatzie]

Thanks Ray.
My 77, 79 & 85 had single tanks on the LH side... The 77 & 79 were third or fourth owner rigs with stake boxes. Swapping tanks from RH to LH with no bed on is not a big trick.
I think the blue truck was a 1974 or 1975 GMC with one RH tank. It was not the original bed because it had a fuel door. I owned that one in the mid 1980's.

Unfortunately the GM wiring diagrams use Production and Aux on the 70's NL2 rigs not LH & RH. Sounds like the original layout is Production = RH and AUX = LH...