Chassis Wiring Harness

[JCampbell]

Anyone know where one could pick up a chassis wiring harness for a '77? This is the harness with the leads for the fuel tank switching solenoid, sending units, and I believe, the taillight plug.

I saved the one off my '77 and have been kicking it around the garage for the past 4 years. Now that I need it, it's nowhere to be found. Naturally.

[ray_mcavoy]

Hi Jason,

What you are describing is actually two separate harneses. The rear light harness is completely separate from the dual tank harness that contains the wiring for the tank switching solenoid and sending units. Also note that the dual tank harness plugs into the regular production sending unit wire (which normally just went to the RH tank and is part of the engine wiring harness).

As for a source, you might want to check with American Autowire and M&H ... they both offer a variety of reproduction wiring harnesses that are exact duplicates of the originals. I think I've seen a dual tank harness listed in one of their catalogs but can't remember if it fit 77 or not. Then of course there's always the junkyard ... I can't remember the exact year range off the top of my head, but I'm thinking along the lines of 76-81 or so will fit.

[JCampbell]

Thanks, Ray. I'll look into that. This whole things got me really hot right now. Like I said, tripping over it for four years...now it's gone. :smh:

[ray_mcavoy]

I've had that happen with misplaced parts too. Seems like you always find them after you buy a replacement!

[JCampbell]

Yeah, really.

I was also thinking about just building my own. Know of any good wiring diagrams?

[ray_mcavoy]

Here's the 77 dual tank diagram:

[JCampbell]

Thanks!

Well, I found a '77 on CL that's being parted out. Made a call and left a message. We'll see how that pans out. Also found an '83, but not sure if that's compatible or not.

[wraprail]

I have the top half of that harness for an 80 (control switch plug to and including bulkhead connector w/fuse panel connector. PM me if you need..

[JCampbell]

Thanks, Man, but I have that part still in the cab.

We’ll see if that parts truck pans out. If not, I’ll hit the local pick ‘n pull next weekend or just make my own. I’m an electrician by trade, so it’s definitely in my wheelhouse, I just don’t want to spend a bunch of time on it if I don’t have to.

[hatzie]

81-91 have the 6 pin motorized valve.

The only problem in building an "authentic" harness are the Molded connectors. If you're not going for a "restoration"... just use a 4 position MetriPak, WeatherPak, or Deutsch DT and several feet of SXL wire with a grommet in the firewall.
The dash switch connector is a Delphi Packard 56 connector and the terminals can be purchased through Waytek Wire, Mouser, Digikey, Newark, etc.
The only molds I'd worry about are the actual sender connections. You can get the sender connection off two different single tank trucks. WeatherPak is the only easily available sealed connector system that has a single pin connector. I'd use those on the tail to each sender.

[JCampbell]

10-4. Thanks.

[JCampbell]

I had a chance to look over the provided schematic and I have a question regarding the "Production Fuel Gauge". At a quick glance, I assumed this was the fuel gauge in the instrument cluster, but on closer examination, it appears the lead is part of the harness that plugs into the firewall and heads down the framerail...outside of the truck.

When I think back, I remember having my stock harness laid out on the frame. I had the leads for both sending units and the solenoid all figured out, but there was one mystery lead I couldn't figure out where it went.

Can someone please clarify what the "Production Fuel Gauge" is in relation to this schematic and where it is located on the truck?

[JCampbell]

Latest in the saga of the missing wiring harness: the local wrecker has a ’77 with dual tanks. Not 100% the harness is still there, but he’s gonna take a look this afternoon.

#KeepingMyFingersCrossed

[wraprail]

Originally Posted by JCampbell

Thanks, Man, but I have that part still in the cab.

We’ll see if that parts truck pans out. If not, I’ll hit the local pick ‘n pull next weekend or just make my own. I’m an electrician by trade, so it’s definitely in my wheelhouse, I just don’t want to spend a bunch of time on it if I don’t have to.

Any work out there?

[hatzie]

Originally Posted by JCampbell

I had a chance to look over the provided schematic and I have a question regarding the "Production Fuel Gauge". At a quick glance, I assumed this was the fuel gauge in the instrument cluster, but on closer examination, it appears the lead is part of the harness that plugs into the firewall and heads down the framerail...outside of the truck.

When I think back, I remember having my stock harness laid out on the frame. I had the leads for both sending units and the solenoid all figured out, but there was one mystery lead I couldn't figure out where it went.

Can someone please clarify what the "Production Fuel Gauge" is in relation to this schematic and where it is located on the truck?

The Production Fuel Meter is the GM gauge in the dash.

GM put a disconnect in the single-tank sender-to-bulkhead plug. Dual tanks or not were a matter of plugging into that connection with the appropriate sender wire or dual tank harness. I'm not sure where the disconnect is located as I haven't unplugged mine.

[JCampbell]

Wait...

[ray_mcavoy]

GM used the term "production" to denote the regular standard equipment parts & wiring that remained unchanged when an option was added. So like Hatzie said, that "production fuel gauge" is just your regular gauge in the dash.

The physical routing of that tan (circuit #30) wire runs from the gauge/cluster plug, out through the bulkhead connector behind the fuse box, through the engine wiring harness, and comes out of the engine wiring harness down near the starter. From there it runs along the passenger side frame rail. On a single tank truck it plugged directly to into the sending unit on the RH tank.

With dual tanks, that ckt #30 tan wire still follows the same physical routing except that it plugs into another tan wire (ckt #30A) on the dual tank harness instead. The location of that connection where they plug together is shown right below the solenoid in this pic:

[JCampbell]

Ah! Now I get it. Thanks for the clarification. It’s been torn down for so long, I tend to forget some of the detail stuff like that. Now that you mention it, I do remember a lone circuit traveling down the pass side frame rail. Thanks, guys!

The local Wrecker found the harness. He wants $50 for it; which I thought was a little steep. I called around to a few other places, but came up skunk. Looks like he gets to name his price this time.

I think I’m going to hit up Pick ‘n Pull on Saturday before I buy it off him. I might get lucky.

[JCampbell]

Yep, got home, pulled out the engine wiring harness and sure enough...one long, lone circuit that stretches all the way to the fuel tanks.

Thanks again, guys!

[JCampbell]

Minor setback in the harness dept. The local wrecker said that after he got the donor truck up on the slab, he discovered the harness I needed was junk. It had been burned and hacked apart just under the firewall. The good news, however, it that the connector that plugs into the firewall is still intact. Meaning, I can still build my own harness using the factory connector. He wants $20 for the connector, so I’m still gonna hit the Pick ‘n Pull on Saturday real quick before I commit.

[watahyahknow]

if you can find one that sells it you prolly can find a new connector for that price you can use the plastic but to do a nice job you need all new copper pins to solder/crimp the wiring too

[hatzie]

Originally Posted by watahyahknow

if you can find one that sells it you prolly can find a new connector for that price you can use the plastic but to do a nice job you need all new copper pins to solder/crimp the wiring too

The whole dual tank harness with the exception of the switch plug are molded over the terminals and wires. I checked into getting them re-produced. The connector molds can cost around $4000-$10,0000 each

[watahyahknow]

i just finished looking at painless and eazy wiring harnesses they just dont make them

maibe when the new 3D printing stuff gets more common you can have the plugs made cheaper

[JCampbell]

The latest, and hopefully final, installment of the fuel tank harness saga:

Went to Pick 'n Pull this morning. Dual tank equipped trucks...check. Dual fuel tank equipped trucks with harnesses...check. Crew cab dual fuel tank equipped trucks with harnesses...zero.

I ended up swiping a harness from a standard cab longbed. I'll just splice in an extension and make it work. I got all the important parts of it now anyway. The best part...the harness cost me a whoppin' $3.50.

[JCampbell]

Now I'll go out into the garage and find the original laying on the floor.