1977 C30 Fuel Gauge wiring

[Timtom]

Where does the solid tan wire for the fuel gauge come out of the firewall at?

I just purchased a 1977 C30 Cab and Chassis that I am having small problems trying to fix. The fuel gauge on the truck does not work. It came with dual tanks from the factory but a previous owner changed it to a single tank an unknown period of time ago. The tank is installed on the drivers side, and the primary tank fuel lines were lengthened from the passenger side to the drivers side. The drivers side sender wires are hooked up to the tank but the fuel gauge sits all the way right whether the selector switch is on LH or RH. I got this wiring diagram from this thread https://www.gmsquarebody.com/threads/help-with-fuel-gauge-wiring.9522/

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It shows the solid tan fuel gauge wire making a giant loop from out of the dash, down the back of the engine somewhere, down the passenger side frame rail, crosses over way back around the tanks, then back up the drivers side frame rail to this connector

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Goes into the selector switch, then back out again as a light blue wire for the drivers side and a tan and white wire for the passenger side. On my truck, the driver side of the circuit is completely intact but the passenger side has been cut. This is the light green wire, the tan and white wire, and the wire coming at the camera is the solid tan wire.

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The solid tan wire is long enough to run up the passenger side frame rail to above the starter, the end of the wire has long exposed ends and looks like it was just yanked hard enough and ripped out. I'm fairly certain this is the solid tan wire that (is supposed to) leads up to the fuel gauge and if I'm understanding things correctly the fuel gauge won't work with it ripped out like it is.

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I can't find any good info on where this wire is supposed to come out of the firewall from. Does it come out of the bulkhead connector? I looked at some more diagrams but I couldnt find any that helped me. I'm also reading conflicting info on circuit 30 (fuel gauge) on if the wire coming out of the firewall is actually tan, or is pink and spliced behind the engine into the tan.

Can anyone help me out? For clarification, Im trying to find where this solid tan wire is supposed to connect to specifically where it is supposed to go through the firewall at. I looked but cant find any obvious hanging loose wires, and I'd rather not have to unravel the entire harness behind the engine just to find out I'm looking in the wrong place to begin with. Thank you.

 

[Timtom]

Here's a picture of the truck, and here's a picture of the bulkhead connector. Wires are dusty and lighting sucks so its hard to tell the wire colors in pictures. Truck is stick shift 2wd 454 c&c drw if that makes any difference in wiring.

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[Timtom]

Did more reading. I found this thread https://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/
And the 1977 wiring diagrams mostly match what I see in my truck. Is this the bulkhead connector?

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I think if I'm understanding correctly, the bottom left tan wire on the bulkhead connector is the fuel gauge sender wire?

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Is this the solid tan circuit 30 wire Im looking for? The diagram doesnt match exactly whats on my truck, but its close enough that Ive almost convinced myself.

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I'll get back into it tomorrow and report back with what I find. Thank you.

 

[Timtom]

Okay, that tan wire with the red arrow pointed to it, I'm 90% certain that wire goes to the alternator. That can't be right, can it? If anyone would like to chime in with insight I'd appreciate it.
Is there any reason at this point why I couldn't just run a wire from pin 18 on my instrument cluster to the light blue wire coming in to the selector switch? Just completely disconnect the switch and only use the light blue wire going to the drivers side tank.

 

[Timtom]

Ran a wire from pin 18 to the lt blue on the selector switch, no luck. Ran a wire from pin 7 to the lt blue, still no luck. Ran the wire directly to the sender on the tank, tried on both pin 18 and 7, nothing. Maybe my sender is bad, maybe my gauge is bad, maybe my wires are bad. I don't fn know. I'm done with it. I'll just get used to not having a gauge in this truck. Thank you gmsquarebody.com for all the help you guys have given, I really appreciate it. Bye

 

[Snoots]

WHOA! HOLD YER HORSES!
First off, you appear to do a lot of guessing. Shoot me if I'm wrong.
You need to be using a DMM/multimeter; etc. That's the only way you're going to track this down.
Everybody gets frustrated to the highest degree, EVERYBODY.
The 'tan' wire is likely to be the fuel sensor wire. With the ignition in the 'on' position you should have +12 on one terminal of the gauge and the sensor wire should have some value of resistance on it depending on the level of fuel in the tank.
One thing to check for are the ground connections at a connection block on the left kickpanel just behind the parking brake pedal. Make certain you have a good ground there. That feeds everything in the dash.

Another thing, you may have to do a lot of grunt work by crawling under the hood and follow those wires through the harness.
Yes it's a bitche, but you have to do it. Nobody said it was going to be easy but, DON'T GIVE UP!
We're all rooting for you.

 

[RSMII]

So he gave up??

 

[Mark80K15]

@Timtom if you're still working on this I might have a bit of insight. You're asking about the solid tan fuel gauge wire. Yes, it is Circuit 30 on your diagram and it originates at the dashboard fuel gauge. It proceeds out of the firewall bulkhead connector (more on that later) and joins the harness that goes to the starter, and then proceeds on its own (near the headers!) down the passenger side frame rail. Circuit 30 terminates near the passenger tank in a small plug, connecting to Circuit 30A (also tan). This is illustrated in your diagram. 30A goes over to the drivers tank (but doesn't connect to anything), and then up the drivers frame rail to the firewall plug in your first picture.

See my thread for a picture of that small plug (near the passenger fuel tank) https://www.gmsquarebody.com/thread...e-to-tank-unit-as-they-did.42060/#post-889897

This whole circuit (30/30A) doesn't connect to anything else, and goes directly from the dashboard fuel gauge to the tank switch.

If I understand you correctly, your Circuit 30 is broken between the firewall bulkhead and the small plug. If that's the case then there is no connection between the fuel switch and the dashboard fuel gauge.

On my 1980 K15 I have a tan wire coming out of the very bottom left of the firewall bulkhead connector. I believe that is Circuit 30 that your original question is asking about. You've identified the right circuit in your August 22 posting of the wiring diagram, but I'm not sure which wire you're illustrating in your picture that same day.

 

[Mark80K15]

I'm posting pictures of my firewall bulkhead connector to support my post above. It looks like the bulkhead is different on my 80 K15 than the 77 C30 that @Timtom posted, so this won't really do @Timtom any good, sorry. My Circuit 30 is the wire in the bottom left, which looks to me kinda... cantaloupe color. Go with that. I drew a yellow stripe on it to illustrate.