Wiring help (1 Viewer)

Dizzy

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When I took my harness apart, I realized that a big chunk was for stuff I never intend to run, or you wouldn't miss it if it was omitted (smog). So, I personally wouldn't advise you to take it apart without knowing which ones need to actually get reinstalled, and which ones are worth omitting. The dilemma with deletion is that pulling wires from harness connectors is a bit of an uncertainty until you really know the whole picture, but it also changes the shape of the harness, and is pretty permanent.

If you test for short circuits, without having the battery connected with conductivity testing, then you can at least test for major problems before you see smoke. If you get a functional harness, then if you want, you can take it out, take it apart, replace what is necessary without having to re-engineer it relative to its shape and how it fits in the small space provided with inducing minimal stress on individual wires in large connections, or rubbing them on sharp edges of various parts behind the dash.

My current 40 had quite a bit of mouse damage. It had one obvious short circuit at the rear quarter panel tail light, and no further apparent damage. So I had to repair quite a bit of it. I learned that the '75 Federal Spec wiring diagram was missing some things like the bypass wire to the starter, and the radio interference wire at the coil, and so when I began with disassembly, I didn't know exactly what went thru the mouses digestive, and what actually needed to get hooked up, and where.

I use this in 11x17 but it might not always be clear because it experienced some water damage.
Wiring Diagram.jpg
 

Coolerman

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That is quite a mess!
First, you are correct that wire with the missing insulation is a solid black wire. Depending on whether or not your truck is a California spec truck with emissions, that wire is spliced four to seven times as it travels through out the harness.
It is the 15A engine fuse and powers the following: (Again depending on whether or not you are California spec)
Regulator : Black/Yellow wire
Idle Solenoid : Black wire
VSV1 : Black Wire
VSV2 : Black Wire
VSV : Black/Yellow (Cal Spec)
Emissions Computer : Black/Yellow

Somewhere in one of those branches, there has been a short. Most likely candidate would be the regulator or idle solenoid.
Fuse7.jpg


Get the correct fuse block repair plug and terminals from Matt to replace that mess at the fuse block. The black wire coming out of the fuse panel is 16ga but three of the splices into it are 18ga. It is OK to replace any 18ga wire with 16ga wire.

Pull the harness, and start un-taping it until you find where the black wire is no longer melted. Replace all the melted wire.
Tip: Use zip ties to hold the harness together as you un-tape it.
 

BeerM3

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This is a very convenient breakdown of a '76 era harness courtesy of woytovich if you haven't discovered it already.
 
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I'm just jumping on the pile unnecessarily. I am self-described "NOT an expert" on wiring, and most of what I DO know has come from mud members that have already chimed in to help you.

I thought I'd share my own recent experience to maybe give you some emotional support. My truck came with a Painless wiring harness installed. You can install a Painless harness and have it function, but the knowledge you need to "do it right" is beyond the instructions you would find for it. At that point it is better IMO to just source parts and make your own harness. Following a diagram that calls out the OEM wire colors and trying to keep straight which color the Painless harness says it is, makes their brand name ironic. (example: green wire with black stripe OEM may connect to a purple Painless wire with a white stripe and then back to OEM green with black)

I got my mess of wires working for almost everything. Hazard switch is still not sorted and the cigarette lighter, trouble light plug, and dome light have a short I haven't traced. That said, my plan is to build my own harness at this point. I am on my third version of a wiring diagram specific to my truck ('74 with some added circuits for stuff like phone charger in the center console). I started with everything that needs power in my truck and work backwards from that, rather than starting with a wiring harness that shows every possible circuit it COULD have, my drawing will be exactly what I want.

Either way, start with a diagram (found most often on Coolerman's website). An aftermarket harness will not come taped together which works if you are physically tracing each wire end to end, but is a disaster under the dash and hood.

My second tip would be to not scrimp on the wire connectors or crimpers. The difference between the janky one I had from my Dad's toolbox and the Snap-On one I used on the 40 was much more than I thought it could be when it came to those connections. You DO NOT want to make a fix, test it, and mentally check it off your list only to have the connection fail when you are pulling on an adjacent wire. That alone is worth the cost of a good crimper. Same goes for any plug in connectors you might use. I don't shop for the cheapest, shop by reviews for the same reason as the crimper.

My last diagram:

Screen Shot 2019-12-04 at 10.01.36 AM.png



Below is a preview of my current diagram in progress. I'm building it at 100% scale (close enough dimensions for wiring), so I can plan out my purchasing needs. I haven't drawn a single wire on this version yet (have to get all the bits of the truck first). You can see in the .gif how I'm using the layers of Adobe Illustrator so I can view or hide whatever I want see (like having multiple pages of a construction blueprint, except all in one file). Each circuit will be on it's own layer so I can turn off the visibility of all the other wires/ visual clutter and see exactly what the wire connects to. When it's done, I'll have a vendor buddy print it out full size onto some OSB, grab some nails and start rolling wire.
BuildUp.gif



I guess my final word of advice would be, roll up your sleeves and get to know your wiring intimately, whatever that entails for YOU. If there is ever a hiccup of wiring trouble in my truck, diagnosing it and fixing it feels VERY doable now, where before, wiring was like wizard's magic and I would just stare with my mouth hanging open. Once you spend some time and get familiar with the "trick" you find out it's not magic at all, but it's easy to half-ass as many on here have had to fix.
 

Coolerman

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That is awesome work with Adobe my friend!

Have a question about this buddy that can print on OSB? o_O
I am assuming you meant print out on 4' x 8' paper then attach that to the sheet of OSB?
Tip for you: Get a large roll of Velcro and a stapler. Cut the Velcro into strips long enough to wrap around the FINISHED harness then staple it to the board for harness shaping. This allows you to adjust the size of the Velcro "loop". As you add wires you simply expand it.
 
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Thanks for all the great info gericurl and amazing Adobe work. As I have begun slowly removing the wire harness and I am seeing the mess that I am dealing with, I have started wondering if an aftermarket harness would be better. Reading what you have had to say has concreted my decision to fix the one I have. Thanks and I will look into some higher quality crimping tool.
 
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Alright people I am already running into a few questions and in need of everyone's expert advice. Is the rear harness typically spliced in or does it have a plug?

20191204_152846.jpg
 
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I also have another question. Can someone help me out with the dash switches? Maybe post a picture of theirs or just tell me what I have (other than the obvious). Thanks again for everyone's help.

15754965996695618976195906996472.jpg
 

Dizzy

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The rear harness is a 4-pin plug connector on my '75. It is the RH turn, LH turn, fuel sender, and probably, tail lights. The mice ate it where the backup light (either that or tail lights) and brake lights must have connected so, I don't know their stock location, but I think that it had three other smog wires.

From memory, the top row of things from left to right are: delete plug, choke pull cable, hazard switch, cigarette lighter; the bottom row of dash is: brake light warning lamp, wiper, fans, heater water valve pull cable, and fresh air vent pull cable.

Your headlight switch would be on the left of the combination meter.
 

Dizzy

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Your alternator is on the driver's side, correct? That is normally the smog pump side, and could explain part of the hack.
 

4Cruisers

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From SOR's website:
 

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The rear harness is a 4-pin plug connector on my '75. It is the RH turn, LH turn, fuel sender, and probably, tail lights. The mice ate it where the backup light (either that or tail lights) and brake lights must have connected so, I don't know their stock location, but I think that it had three other smog wires.

From memory, the top row of things from left to right are: delete plug, choke pull cable, hazard switch, cigarette lighter; the bottom row of dash is: brake light warning lamp, wiper, fans, heater water valve pull cable, and fresh air vent pull cable.

Your headlight switch would be on the left of the combination meter.
Thanks for the info. I think I am missing a plug for my rear harness. Mine were all twisted together.
 

Coolerman

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Your 1976 should have two connectors for the rear chassis harness. There are variations on what type of connectors were used.
Most common is a six pin and a three pin. See the attached diagram of a typical 1976 rear chassis harness.
1976_Harness_Layout.jpg


Note: There are some unused wires near the reverse switch (Green and Red) that went to a vacuum relay system that to my knowledge was never used in the US.
 

Dizzy

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Thanks for the info. I think I am missing a plug for my rear harness. Mine were all twisted together.
Not a big one in my book. There will be a bit of moisture from the hood vents at that location, my bigger concern is corrosion, but factory configuration is good too.
 

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