Need help locating wire harness

[Mad gimp]

Can anybody point me in the right direction as to where the wire harness for the fuel gauge/selector valve, comes through the fire wall on a 79. I know its supposed to be 4 wires. Tan, tan/white, green, blue but i do not see them anywhere on the fire wall. I did find those wires under the truck but looks like they was burnt so somebody cut them. Any info is appreciated a picture would be awsome!

 

[chengny]

Can anybody point me in the right direction as to where the wire harness for the fuel gauge/selector valve, comes through the fire wall on a 79. I know its supposed to be 4 wires. Tan, tan/white, green, blue but i do not see them anywhere on the fire wall. I did find those wires under the truck but looks like they was burnt so somebody cut them. Any info is appreciated a picture would be awsome!

Maybe start at the select switch on the dash and trace the harness back towards the firewall. That should give you a good idea where they transit the FW. They have their own 4 pin bulkhead connection block similar to the main bulkhead connector - just much smaller. A round connector plugs into the bulkhead connector on the engine side. After that they probably run rearward in the rear lighting loom. It might be easier to start looking for them there.


The wiring for this setup is kind of involved. But if you're interested, I'll try to help explain the way the system worked - and how/why it was wired the way it is.

The first thing is that all of the following will only apply to the fuel tank level circuitry used in CK 1/2/3 standard pickup trucks with the NL2 RPO option (i.e. dual tanks mounted outside of the frame rails). Also, unlike the later model CK series, the production tank was the one mounted on the RH side - the auxiliary tank was on the LH side.

The solenoid power circuit is simple enough - either on or off - so it isn't discussed.

If the NL2 option was not requested, a single PNK lead started at the instrument panel. Then it penetrated the firewall, ran down the RH frame rail and terminated at the sender connection on the base model (production) tank.

But with the dual tanks, two inputs were required for the gas gauge. The solution was to splice a long TAN lead onto the end of the base PNK lead - and then run that TAN lead all the way forward on the LH side to the tank transfer switch. Additionally, a lead from each tank sender (LT BLU & TAN/WHT) was run up the LH side to the switch. When the operator switched tanks - by supplying power to the transfer solenoid - another set of contacts within the switch shifted. This controlled which sender lead (LT BLU or TAN/WHT) was connected to the common TAN lead. That allowed the online tank to provide the gas gauge with the correct electrical signal. See the attached dwg and maybe it will help explain:

You must be registered for see images attach

 

[Mad gimp]

Maybe start at the select switch on the dash and trace the harness back towards the firewall. That should give you a good idea where they transit the FW. They have their own 4 pin bulkhead connection block similar to the main bulkhead connector - just much smaller. A round connector plugs into the bulkhead connector on the engine side. After that they probably run rearward in the rear lighting loom. It might be easier to start looking for them there.


The wiring for this setup is kind of involved. But if you're interested, I'll try to help explain the way the system worked - and how/why it was wired the way it is.

The first thing is that all of the following will only apply to the fuel tank level circuitry used in CK 1/2/3 standard pickup trucks with the NL2 RPO option (i.e. dual tanks mounted outside of the frame rails). Also, unlike the later model CK series, the production tank was the one mounted on the RH side - the auxiliary tank was on the LH side.

The solenoid power circuit is simple enough - either on or off - so it isn't discussed.

If the NL2 option was not requested, a single PNK lead started at the instrument panel. Then it penetrated the firewall, ran down the RH frame rail and terminated at the sender connection on the base model (production) tank.

But with the dual tanks, two inputs were required for the gas gauge. The solution was to splice a long TAN lead onto the end of the base PNK lead - and then run that TAN lead all the way forward on the LH side to the tank transfer switch. Additionally, a lead from each tank sender (LT BLU & TAN/WHT) was run up the LH side to the switch. When the operator switched tanks - by supplying power to the transfer solenoid - another set of contacts within the switch shifted. This controlled which sender lead (LT BLU or TAN/WHT) was connected to the common TAN lead. That allowed the online tank to provide the gas gauge with the correct electrical signal. See the attached dwg and maybe it will help explain:

You must be registered for see images attach

Thank you very much!!

 

[bucket]

On the firewall, right behind the left valve cover area. That's where it is on my '79 crew cab.

 

[Mad gimp]

On the firewall, right behind the left valve cover area. That's where it is on my '79 crew cab.

Thank for that info. Ill have to check that out when it warms up. I do know there is a bunch of wiring back there that been added here and there. Kinda looks like a rats nest back there lol

 

[bucket]

Mine was the same way when I got the truck. I must have removed 50 lbs of extra wiring. I also found my fuel gauge didn't work because that harness was unplugged. Well, and the gauge was toast as well, but I tried that first.

Btw, where did you find a set of those gold Moroso valve covers? For several years now, they only sell them in red or blue, and they are powder coated rather than anodized.

 

[Mad gimp]

Mine was the same way when I got the truck. I must have removed 50 lbs of extra wiring. I also found my fuel gauge didn't work because that harness was unplugged. Well, and the gauge was toast as well, but I tried that first.

Btw, where did you find a set of those gold Moroso valve covers? For several years now, they only sell them in red or blue, and they are powder coated rather than anodized.

I actually had those valve covers given to me. Once i got them then i had to find the breather lid to match them. Like you said you cant find gold anymore. Ended up finding one under the hood of another buddies race car lol.
As for the wiring under the hood, i dont know what half of that crap you can tell has been added is for. I know there was a bunch of damaged wire between the firewall and the rear of the truck that ive slowly been redoing. Stuff like that REALLY upsets the OCD in me lol

 

[75gmck25]

There is a large rubber plug (1"?) in the firewall below the brake booster and off toward the passenger side. The wiring plug is fairly hard to see if you don't stick your head back in under the brake booster and use a flashlight. The wiring comes through there and down driver's side firewall.

IIRC, on my truck there are four wires - gas gauge (tan), solenoid power (pink/red), left side sender(tan/white) and right side sender (tan). Some newer trucks have more complicated solenoid wiring, so there may be more wires. Also, my truck had the passenger side (right) tank as primary, which is why it has the tan wire on the right side. Newer trucks switched to the driver's side tank as primary.

As the wiring gets even with the left side gas tank, one wire comes out of the harness and goes to that left side tank sending unit. The other wires run across a brace to the other side of the turck, where you connect the right side tank sender and the power for the solenoid. At that point it also connects to the gauge wire, which comes down the passenger side frame rail from the dash.

Bruce