78 C10 Fuel sending unit wire routing

[Speed2acer]

Does anyone know how the fuel sending unit wire is routed? I bought a project truck and have been successful wiring everything except that darn thing, the kid that had the truck before me had Mickey Moused so much of the wiring that I basically started from scratch. Of course the one wire that I didn't even think of was the sending unit which was clipped about two feet from the tank. I'd prefer to repair the lead at the harness rather than run a separate feed. The wire I found is a tan wire and every post I have seen rewiring states a pink wire lead from the sending unit.

Any help would be greatly appreciated. Thanks

 

[RustyPile]

You didn't state the location of the tan wire you found, so I'm not sure what it goes to.. Tan is found in various circuits, one of which is the wire going to the oil pressure sending unit. The closest color to tan in the rear harness is brown.. That's the color of the tail light wire(s).. Pink is the color of the wire going from the tank sending unit to the instrument cluster plug under the dash.. It runs from the main bulkhead (firewall) connector down the firewall and along the left frame rail, located in the bundle with all the other wires.. It exits the harness near the rear of the cab..

Instead of pulling your hair out trying to fix the mess created by the PO, why not just raid one from a junk yard??

 

[Speed2acer]

Thanks for the response. The lead comes from the fuel sending unit. Due to the age and deterioration the best I can describe the color is milky tan.

 

[MarshMobbin907]

You didn't state the location of the tan wire you found, so I'm not sure what it goes to.. Tan is found in various circuits, one of which is the wire going to the oil pressure sending unit. The closest color to tan in the rear harness is brown.. That's the color of the tail light wire(s).. Pink is the color of the wire going from the tank sending unit to the instrument cluster plug under the dash.. It runs from the main bulkhead (firewall) connector down the firewall and along the left frame rail, located in the bundle with all the other wires.. It exits the harness near the rear of the cab..

Instead of pulling your hair out trying to fix the mess created by the PO, why not just raid one from a junk yard??

So would this wire running back mess with how the gauge works? Mines either completly horizontal on the full side or it bounces around. I think there's some loose wires or a poor ground because every now and again my fuel pump will lose power. The needle just recently started bouncing and work for a few days but was completely off.

 

[RustyPile]

So would this wire running back mess with how the gauge works? Mines either completly horizontal on the full side or it bounces around. I think there's some loose wires or a poor ground because every now and again my fuel pump will lose power. The needle just recently started bouncing and work for a few days but was completely off.

The reply I made to the OP of this thread pertains to his 1978 model.. Your '87 is different in some ways.. For one his doesn't have an electric fuel pump..

Which "wire running back" are you referring to?? The pink wire connecting the gauge to the sending unit?? I think the erratic operating gauge and pump you're experiencing is probably a loose ground connection.. Dirt and rust are the main enemies of maintaining good electrical (including grounds) connections.. Re-conditioning and adding additional grounds often times cures intermittent electrical problems..

The fuel tanks in our old trucks don't have baffles in them so the fuel sloshes around quite a bit.. The fuel gauge is sensitive to float movements and bounces around quite a bit..

 

[75gmck25]

I assume a '78's wiring is the same as my '75, and the sending unit wire is definitely tan.

On my truck the sending unit wire comes through the firewall near the fuse block, across the back of the engine compartment, and down the firewall and frame to the passenger side tank, since that side was the primary tank in '75 (in later models the primary tank was on the driver's side). If the gauge reads full right, that usually means the sending unit wire is disconnected.

If you have dual tanks I can also provide a description of how its wired for two tanks and a switch. Its a little more complicated than expected, probably to make it easier at the factory to install and wire the 2nd tank option.

Bruce

 

[Charlie]

 

[foot]

I too have purchased a project truck 1979 K10 shortbox with dual tanks. tanks and wiring were all clipped and tanks gone.. so I have 4 wires coming down frame rail blue stops by left tank location. then their is a green to 1 wire switching valve then I have 2 pink wires one is solid and the other seems to have white spots in places? which goes to the passenger side tank and what is the other pink wire for? both pink wires are long to reach the pass. side. any help would be much appreciated

 

[chengny]

I too have purchased a project truck 1979 K10 shortbox with dual tanks. tanks and wiring were all clipped and tanks gone.. so I have 4 wires coming down frame rail blue stops by left tank location. then their is a green to 1 wire switching valve then I have 2 pink wires one is solid and the other seems to have white spots in places? which goes to the passenger side tank and what is the other pink wire for? both pink wires are long to reach the pass. side. any help would be much appreciated

I'll try to help explain the way the system worked - and how/why it was wired the way it is. The first thing to understand is that all of the following will only apply to CK 03 trucks with the NL2 RPO option (i.e. dual tanks mounted outside of the frame rails). Also, unlike the later model CK (03) series, the production tank was the one mounted on the RH side - the auxiliary tank was on the LH side.

If the NL2 option was not requested, a single PNK lead penetrated the firewall, ran down the RH frame rail and was connected to 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 up 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 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. I'm not very articulate tonight. See the attached dwg and maybe it will help me explain. I didn't get into the wiring to the solenoid - mostly so as not to confuse the issue. Plus it seems like you have that circuit already figured out:

You must be registered for see images attach

 

[millertyme]

I'll try to help explain the way the system worked - and how/why it was wired the way it is. The first thing to understand is that all of the following will only apply to CK 03 trucks with the NL2 RPO option (i.e. dual tanks mounted outside of the frame rails). Also, unlike the later model CK (03) series, the production tank was the one mounted on the RH side - the auxiliary tank was on the LH side.

If the NL2 option was not requested, a single PNK lead penetrated the firewall, ran down the RH frame rail and was connected to 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 up 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 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. I'm not very articulate tonight. See the attached dwg and maybe it will help me explain. I didn't get into the wiring to the solenoid - mostly so as not to confuse the issue. Plus it seems like you have that circuit already figured out:

You must be registered for see images attach

This is helpful, sort of. I hope this thread isn’t too old and I hope some who knows something can help me out with a similar issue. I have a 1978 K20 with dual tanks from the factory. At the firewall I have 4 wires from the plug: light blue, brown with white stripe, green, and brown. Light blue is the driver side tank, brown with white stripe is the passenger side tank, green is for the tank selector valve.

What is the solid brown one for? I traced it down and it crosses the frame with a single pin connector that was spliced by a previous owner, ran back to the front on the passenger side (inside the frame, outside the frame, through the suspension, etc), runs up through the ignition routing tube, across the firewall, and ties into the fuse block. Seems to me it could have just gone from the plug to the fuse block, but the factory wiring takes it all the way back to the selector valve.

 

[1978Chevysilverad]

@Chegny Thank you. that diagram helps. It was the pink wire that I had no idea where it went. I had four wires coming out of the fire wall green, blue and two brown. then they were spliced into yellow, brown, blue and green. Every wire had a place to go except that one pink wire that goes to the cluster. now i have an idea where to look. Yes!!!
Thanks alot

 

[1978Chevysilverad]

i hooked everything up and I have a reading on the fuel guage aux tank but no reading on the main tank. The guage goes to the 3oclock position. I removed the ground wire to the tank sending unit cleaned it and the frame. I checked the resistance with an ohmeter from the ground to the wire going to the firewall and there was a reading. but needle on guage still reads three oclock

 

[ryan80square83]

On my 80 K20 nl2 dual tank truck, same thing missing everything. The harnesses, the tanks, sending units, all the wiring to the rear and half the dash wiring. I was able to buy a underdash harness with fuse block, rear wiring and the harness going back to the tanks. I have all the exact colors, but don't know where to exactly plug in that tan wire to the pink wire. Where is that connection supposed to be? Under the truck, under hood, I don't know.