6066 (1960-1966) GMC Truck Club

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-   -   Ignition switch wiring help (https://6066gmcclub.com/showthread.php?t=51206)

Jethro September 7th, 2021 12:11 AM

Ignition switch wiring help
This morning I set out to do a simple ignition switch replacement on my 64 I1000. Then I found out it ain't so simple. The wiring on my truck has been "fixed" by way too many previous owners.

Here's what I had.


Here's what I have.


The brown and white wire was on the acc. post of the old switch. Is that right? It's a solid tiny wire. Resistor wire? The other end goes to the alternator and the Gen light.

The dark green wire was just hanging there. Where does it go? The other end goes to the instrument panel temp gage and another green comes out and goes to the temp sender.

The wiring diagram I'm looking at doesn't match reality.

Jethro September 7th, 2021 12:23 AM

Re: Ignition switch wiring help
I'm also wondering if my coil is wired right.
As is the pink wire goes (among other places) to the 6 pin connector on the right firewall. From there it changes to a black wire with a cloth insulation and goes to the starter R. From the starter R a yellow wire then runs to the coil +. It runs but, is that right?

Thanks in advance for any help I can get.

AZKen September 7th, 2021 10:05 AM

Re: Ignition switch wiring help
After many years and many owners, you can't trust wire colors sometimes, so be sure they go where indicated.

Ignition switch wiring per 1965 GMC diagram: I do not see a legit 1964 GMC diagram.

1. Red to terminal "BAT. This is an "always hot" battery source.

2. Pink to terminal "IGN"

3. Violet to terminal "SOL"/"START". This wire comes from solenoid terminal "S"/"SOL"

4. Brn/Wht to terminal "ACC". This is part of the charge light (tell tale). It comes from the Voltage reg "L", goes thru the right side 6 pin, comes out as two wires. One BRN/WHT and one BRN. Brn/wht to ignition ACC as described and the Brn goes to cluster "G" which connects to the Gen Light thru the flex print.

5. A brown wire from "ACC" to fuse panel. The terminal it goes to has another brown wire on it that is jumpered to another terminal. Don't see another other way to describe. No number on fuse panel given.

The Dark Green shows on diagram from temp sender thru right side 6 pin, then to cluster "H" which connects to temp gauge thru the flex print.

Yellow from coil pos to "R" terminal on starter solenoid. Resistance wire from "R" to right side 6 pin. Comes out of 6 pin as a pink to Ignition switch as described above AND to cluster "E".

It's up to you to sort out the wires and be sure they go per diagram. If the colors match, all the better.
Report issues/questions before powering up. You are only using 4 out of 5 ignition switch terminals. I believe the 5th is GND, right?

Jethro September 7th, 2021 04:10 PM

Re: Ignition switch wiring help
Thanks AZKen. Looks like I'm all good thru #4 if I connect that brown/white to acc. Which is how it was. Just wasn't sure.

#5, brown fromacc. to the fuse panel doesn't exist on my truck. Maybe the dark green is doing its job? I'll have to trace that one again after work.

I've looked at every diagram I could find on line. Including the wrong year and even a couple cars.
The only one I found showing a dark green to the ignition switch was a 64(?) Chevy II. That showed dark green to the GRD terminal.

And, yes, there is a GRD terminal on my new switch.
Does that ground the switch? Or just give a spot to ground other stuff? The switch should be grounded automatically to the metal dash, right?

I'll retrace the dark green after work and go from there.
Thanks again.

AZKen September 7th, 2021 09:02 PM

Re: Ignition switch wiring help
According to 1965 GMC diagram, the browns are part of the Gen Light. The green is temp. The GND terminal is not used.
Chevy is sometimes the same , sometimes not. Wire per function, not color. That's why I explained the "to-from" as well as the color.

gmcjuniorvan September 8th, 2021 01:45 AM

Re: Ignition switch wiring help
1 Attachment(s)
1964 GMC I1000 I1500 I2500 wiring diagram.

Jethro September 8th, 2021 03:28 AM

Re: Ignition switch wiring help

Originally Posted by gmcjuniorvan (Post 74623)
1964 GMC I1000 I1500 I2500 wiring diagram.

You da MAN!
Thanks. That actually looks like it might be THE diagram I've been searching for. At least it shows the green wire I've been chasing just like it is in my truck. Goes to the ground prong on the ignition switch.

As of now I have pretty much the whole harness unwrapped in the cab. Figgered I'd try and make my own diagram. I was pretty sure that green wire had to go to ground. Now I know for sure.

I'm glad I went this far tho. I found several more electrical gremlins in the dash. I'm gonna chase the wiring all the way from stem to stern. The main objective here is functional reliability.

AZKen September 9th, 2021 02:01 AM

Re: Ignition switch wiring help
Don't think that green wire goes to ground. The ground terminal is if you have a hot light instead of a gauge. Unless you have both. Do you have an actual ACC position on your switch. Like full CCW?
At any rate, that is not important. The green wire that is important is the one from the temp sender to the gauge. But you said you were going to trace the green wire(s). The 64 and 65 diagrams are basically the same. Slight changes.

Jethro September 9th, 2021 01:15 PM

Re: Ignition switch wiring help
I do not have a hot light. I do have ccw acc. on the switch.
Sure looks like green goes to the ground terminal on that diagram.
I did trace the whole wire. And all the rest.
Here's my rendition of the green circuit.


gmcjuniorvan September 9th, 2021 05:37 PM

Re: Ignition switch wiring help
I think the dark green on the ignition sw is for when you turn the key to ACC the temp gauge would go to H. On a chevy the hot light will come on. This is to check the chevy light. If you want to check the gauge use it. I would not use it. If you left the ACC on for some time you could burn out the gauge. Just tape it up so it will not ground out.
If you look at the 4 pin panel connector on the wiring diagram you will see 20 L.GR 36 your truck could have this wire itís for the chevy cold light.

One more thing, when you rewrap the wiring harness donít use electrical tape use OEM factory wiring tape it will not stick to the wires. When you have a metal body clamp around the harness wrap Teas 51036 tape around the harness, itís a cloth tape that has a high abrasion protection

AZKen September 9th, 2021 09:09 PM

Re: Ignition switch wiring help
Your drawing of green is what the 1964 diagram says. So you can hook the dangle up to GND or cap it off. Your choice. In 1963, 1964, 1964, GMC could not make up it's mind about idiot light or gauges. Bulb check and various circuits. It's all the same result. I think you have the info now. It makes no difference if you hook it up or not if you wiring is per factory diagram. If you don't hook it up, it will only take a few miles to see if temp gauge works. I think the idea was to see it spike to hot while cranking, for confirmation. Try it with and without the hookup to see.

Jethro September 9th, 2021 09:57 PM

Re: Ignition switch wiring help
You guys are great! I have the answers now to just exactly what I need to know. Thanks!

My truck does have the light green wire to the 4-pin panel connector but it ends right there. No wire in the cluster harness and an empty L pin.
Earlier in my adventures I found the temp sensor wire connected to the coil +. Maybe what fried the printed circuit? Pretty sure everything under the hood is right now. Went thru the park/turn lights too.
I just fixed the panel and don't want to fry this one.
I think I'll try it first just to check the gage and then run it unhooked. It seems redundant.

I rewrapped some of my harness with the original. The rest I used the plastic loom stuff and just taped the ends and a couple other wraps just to keep it closed. The fuse block is really rough. I'm looking for some replacement terminals. Or a used block. Either that or I'll have to install a more modern chip fuse block.

Jethro September 9th, 2021 11:51 PM

Re: Ignition switch wiring help


AZKen September 10th, 2021 02:55 AM

Re: Ignition switch wiring help
Yes, that 20 L.GR 36 is shown dead ended on both ends. Not used.

Your fuse panel terminals are pretty rusty. I have actually sand blasted those terminals carefully and with the appropriate pressure to not destroy other areas of it. It would have to be removed to do that. If you are going to remove it to replace it, try that first. There are possible chemical rust removers that may not attack the plastic during a soak.

This is a good idea that you may find a way to use. By Edd China. Starts at around 9:45 https://www.youtube.com/watch?v=LzxeK6yp4hQ

Jethro September 10th, 2021 06:48 AM

Re: Ignition switch wiring help
Some of the terminals are missing and some rusted completely away. I think I might be able to use one from a 66 Chevy I have with a bit of work. I'll be out of town for the weekend but I will be back to working on it as soon as possible.
That vinegar trick looks handy.
Got any tips on easy ways to get grommets in and out of firewalls?

AZKen September 10th, 2021 09:02 PM

Re: Ignition switch wiring help
Soap and push. Soap and pinch inward. Assume you mean rubber grommets. If old and cracked, Nuclear option.

Jethro September 15th, 2021 09:54 PM

Re: Ignition switch wiring help
Yep. Rubber grommets. Every time I have to do one I think there must be an easier way.

Update; There is a 66 Chevy project in my garage. When I saw the condition of the fuse panel in the GMC and didn't have any luck finding rebuild parts I decided to crawl in the Chevy and compare. The panels look alike.

Since I was out of town for a long weekend I didn't have a chance right away to go further. Monday I pulled the Chevy harness and sat down to pick thru and compare it to the 64 GMC diagram. At the core the 2 harnesses match. The 66 Chevy just had some extra wires added in.

Last night I pulled both harnesses apart and used the best stuff to build one for the GMC. I still have a few wires to repair before I can put her all back together but I am confident it's going to work out well. I'll let you know.

Thanks again for the advice.
And a special thanks for the diagram. I looked all over and couldn't find the right one.

Jethro September 18th, 2021 02:57 AM

Re: Ignition switch wiring help
WooHoo!!! She's back alive!

Thanks, AZKen, for that vinegar trick. I used it on all of my connector terminals. Worked great!
Not so much on the fuse clips. Those I just scraped with a needle file.

This "ignition switch replacement" sure did snowball. I'm glad tho because now I know the whole cab is wired right if nothing else. I plan to finish cleaning up and fixing the wiring to the lights too. Already did the engine harness.

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