View Full Version : Electrical gremlins have got me - help?

November 17th, 2013, 02:32 PM
Hey guys - I am not very good working on electrical stuff so I know what is wrong with my truck will likely be something one of you has encountered, so here goes: my 64 2500 series truck operated flawlessly when I first got it a few months back, and all of a sudden for no apparent reason, my pass side stop light and pass side turn signal have ceased to work - no big deal right? So I change the bulbs in both figuring one of them is causing the other to not work, but same thing. I check to see if the lights work when the headlights are on, and the Right taillight comes on but that front rt park light does not still. Only the brake light does not work at the tailights; but nothing is wrong with the wiring that I can see going to the rt park light, and it worked when I got it, so what changed? Could it be the headlight switch or dimmer? Turn Signal switch? Not sure what to test or replace to fix this - I am thinking the turn signal switch is to blame, but cannot imagine what that has to do with the Right rear brake light not working unless that is affected by the outage up front....ideas?

November 17th, 2013, 04:02 PM
I believe all of that is operated through the turn signal switch. You might get lucky and find that the big connector from the column is loose or dirty.
If that isn't the problem you can try using a hot wire to the corresponding terminals at that connector to determine if the problem is at the turn signal switch or a wiring problem in between.

November 17th, 2013, 04:24 PM
Here's the link to Jolly's section on this issue.


November 17th, 2013, 05:01 PM
I have seen this quite a few times. sometimes its the left,sometimes the right. 95% of the time its in the turn signal switch. If you wiggle the switch arm you can sometimes get the stop light to work. Usually a good cleaning of the turn signal switch is all that is needed.

November 17th, 2013, 05:24 PM
GMCNut: I agree with rest of the comments posted so far: check the turn signal switch. While the GMC switches seem to be a better design than the Chevy version, they are prone to getting dirty and if they ever get hot they cause small grooves to be melted in the plastic right near or at the contacts inside the switch. A good cleaning and possibly some light sanding or dremel tool work can smooth out these melted grooves, thus allowing the contacts to do what they are supposed to do. You'll have to remove the steering wheel. You will also have to be careful not to lose any of the upper column bearings when you remove the signal switch from the column. This is also a good time to clean the horn contacts. Test it before you reinstall the wheel. Good luck.

November 18th, 2013, 01:25 AM
Remember...If that doesn't take care of it...
Grounds, grounds and more grounds.

November 18th, 2013, 01:53 AM
Thanks guys - was thinking it was the turn sig switch and thankfully I have a reproduction switch on hand so I will try that and see if it fixes it....thank you!

November 18th, 2013, 09:02 PM
I had bad luck with all of the repro turn signal switches. They do not allow the turn signal switch cup to attach to the steering column housing without interference. Then when you go to tighten the steering wheel down it causes the new turn signal switch to crack. Be careful and please let us know how the new switch fits/performs.

George Bongert
November 19th, 2013, 12:19 AM
Greetings GMC Nut!

I also have to agree that you may have a faulty ground issue. If switch cleaning and/or replacement doesn't fix this, by all means look for a bad ground issue.

November 19th, 2013, 01:13 AM
The after market switches are designed for the Chevy column. You need to remove a lot of the metal cup so the wires will go through in the right spot and cut off some of the turn signal arm. Or you can use a Chevy cup and arm. The chev stuff is really easy to find. I have done this at least 20 times, and while I prefer the better built gmc switch and bearing retainer reality trumps desire. Barry

November 19th, 2013, 01:16 AM
If you all want I can dig up the pieces and post comparison pics so you can see the difference. BLC

November 19th, 2013, 01:29 AM
Barry: I am interested in seeing the difference between the Chev and GMC signal switch cups, etc.

November 19th, 2013, 01:33 AM
In the next couple of days I will pull it all out and put up the pics. Barry

November 19th, 2013, 04:44 AM
Another way is to replace the GMC signal cup with one from a Chevrolet. I know they are fellows that will moan at this suggestion, have you ever tried to find one for a GMC ? it's like slim and none and slim just left town, but good luck anyhow and anywhere you look for one..Vernski:goodluck:

November 19th, 2013, 07:23 PM
here are the 2 side by side. The white one with the plastic switch is Stock chevrolet. The fawn colored one is stock GMC. Notice that the Chevy one uses sheet metal screws and the GMC one uses machine screws. Also the switch arm is shorter on the chevy. These cups and switches are interchangeable between makes. So chevy cup with chevy repop switch is a direct replacement with no mods. This only applies to some 63 and all 64-66. Barry

November 20th, 2013, 01:44 AM
Here is a link for one to fit 64-66 GMC finding the right one for a 63 will drive you nuts I know it's not out there unless you find a oem part for a 63 that someone may have. Well heck fix it anyway you wish only you will know it's not oem was my point I used the Chevrolet cup and turn signal arm was just a suggestion and so on my 65 it works great..Vernski:lolsmack:

November 20th, 2013, 02:32 AM
The bolt pattern is the same but the wires come out in the wrong spot. So those 64-66 gmc repop switches are not drop in. Tighten up the screws and snap goes the Ben Franklin. Penny saved is a penny earned. Barry

November 20th, 2013, 02:37 AM
The 63 shares a switch with international trucks from the 60's. This used to be easy to find. But like most things it's harder now. I have a few and still find them on occasion. Maybe once a year or so. Barry

November 20th, 2013, 03:44 AM
Thanks for the detailed images, Barry. Wish I was aware of this stuff a little earlier in life.

November 20th, 2013, 08:27 PM
Man, I had no idea the cups, signal arms and switches were different - I knew the GMC used a pontiac-style cancel cam, but this is great information to know - I deeply appreciate everyone's responses, and thank Barry for these great detail pictures

May 4th, 2014, 10:01 PM
So what I take from this thread is that if you go with the Chevy switch, better go ahead and go with the Chevy cup and arm as well; otherwise hunt and hunt until you find a true GMC switch?

May 6th, 2014, 01:49 AM
I have a NOS GMC switch right now. If you can, gimme a couple of days before you buy a chevy one. I know how you like to keep it semi-original.

May 6th, 2014, 03:39 AM
I have a NOS GMC switch right now. If you can, gimme a couple of days before you buy a chevy one. I know how you like to keep it semi-original.

That would be awesome! :bananadance: