Headlight switch dimmer question

[squarelyfe]

If the dimmer function or rehostat on a headlight switch fails will you lose ground for the cluster lighting? I believe my headlight switch is bad, I don't feel the notch when you twist to one side for courtesy light. I ordered a new switch but am curious. Anyone have the ground wire schematic for a 75 GMC k25 built in late 74? My headlights work btw.

 

[Ellie Niner]

The rheostat is on the hot side of the circuit, so the dash lights will remain grounded if the rheostat fails; they just wont receive power. The dome/courtesy circuit is the only lighting that uses ground to complete the circuits (dome and courtesy light are both on different circuits. The headlight switch must be grounded to turn on those lights (it grounds through the dash when mounted).

I do have wiring diagrams, but not of them are digitized or scanner friendly. Pretty much any instrument panel wiring diagram for any year of these trucks will work, as the basic wiring stayed the same.

I take it you have head, tail, and dome/courtesy, but no dash lights?

 

[Grit dog]

^What he said.

 

[squarelyfe]

I have a bunch of issues I need help with.

No cluster lights, dome light stays lit, dimmer/headlight switch I don't feel the notch for dome, I don't see the lower instrument lights anymore (stereo washer dim), turn signals up front go into Hazzard but not blinking just both stay lit. I need to see if I have brake lights since all of this happened. I do have headlights.

I have a new turn signal cam & flasher relay.
Under the dash there is no ground bus bar, but is two wires grounded in the same place.
All of this is weird because last year I had it worked out. Installed a new intermediate wire harness to rear and new sockets all around.
Cluster feed buss is getting 12v. I ordered a new headlight switch but won't be here until Monday.

I did install new door pin switches for dome. They fit crappy for the 75. But no spark or fuse blow, the led just stays on when the door are closed. It has to leave ground for dome circuit?

 

[Ellie Niner]

Oof. That's quite a list to tackle. You mention LED for the dome light... do you have LED bulbs installed in other positions? The reason I ask, is that some of them can create issues in lighting circuits that were designed for incandescents, and are hard to troubleshoot... this is especially true with turn signals and front side markers. I would at least do all testing with incandescent bulbs to eliminate the possibility that the LED's aren't playing nice here.

With a correct 1004 bulb in the dome light (1003 for underdash courtesy light), if they're still staying lit, it means that the circuit is being grounded somewhere... usually either at the door switches or the headlight switch. Remove the door switches from the door frame so they have nothing to ground on and see what happens. Same for headlight switch... if it's removed from the dash (but still plugged in), it won't turn on the dome light. If still lit, the white wire is grounding somewhere. If the orange wire gets grounded, the fuse will blow.

 

[squarelyfe]

Oof. That's quite a list to tackle. You mention LED for the dome light... do you have LED bulbs installed in other positions? The reason I ask, is that some of them can create issues in lighting circuits that were designed for incandescents, and are hard to troubleshoot... this is especially true with turn signals and front side markers. I would at least do all testing with incandescent bulbs to eliminate the possibility that the LED's aren't playing nice here.

With a correct 1004 bulb in the dome light (1003 for underdash courtesy light), if they're still staying lit, it means that the circuit is being grounded somewhere... usually either at the door switches or the headlight switch. Remove the door switches from the door frame so they have nothing to ground on and see what happens. Same for headlight switch... if it's removed from the dash (but still plugged in), it won't turn on the dome light. If still lit, the white wire is grounding somewhere. If the orange wire gets grounded, the fuse will blow.

Thank you getting back to me!

I did check and I have no taillights but do have headlights. It's not the factory bulb socket because it a Highliner headliner. From what I've seen with LEDs you just flip it around normally if the led doesn't come on. When I flip it around it doesn't come on with the doors open or closed. I do have the regular bulb for it though. Today my new headlight switch came in but haven't had a chance to get back to it yet. I'd love to put my hand underneath the dash unplug the switch and grab it from under there but because the AC tube that goes across their blocks everything maybe it's better if I just take the cluster out that way I can verify the prongs are all the way in on the harness end to cluster. I noticed on the new rheostat you can feel the springs notches as you rotate it, I don't feel that on the old one that is in there. I can't wait to see if the headlight switch improves anything. Do the early square body's not ground at each light in the rear? I seen a schematic that shows ground on the passenger side as well. My truck was made in late 74 for the first month of 75 it only has it on the driver side.

 

[Ellie Niner]

@squarelyfe Okay. You won't have any dash lights unless you have tail lights first... the dash light circuit is fed off the tail light circuit.

Your rear lights should be grounded at a common location in the rear. It should be on the underside of the box on the left side if I'm remembering right. Just follow the black wires from any of the lights to their terminus. You tail lights should illuminate dimly even with a compromised ground... they'll find a false ground through brake lights or front parking lights, etc. Do your front parking lights work?

 

[squarelyfe]

I have the ground behind the driver side taillight. I'm going to check the intermediate harness to rear section plug. Also going to re-scratch the ground at taillight. Install the new headlight switch and see where I get tomorrow. I may replace more fuses as some are old. I gotta check to see if the front parking lights work. Like I said when I signal both side come on and don't flash.

 

[75gmck25]

The wiring diagram for my '75 truck shows an 18 gauge black ground wire at the taillight,. However, IIRC it also has a rear ground wire in the middle of the harness up about behind the license plate area. Its a fairly large gauge white wire. I have the Camper Special package, but I think all that camper wiring connects up in the front, so I think the white ground wire is stock.

 

[squarelyfe]

The wiring diagram for my '75 truck shows an 18 gauge black ground wire at the taillight,. However, IIRC it also has a rear ground wire in the middle of the harness up about behind the license plate area. Its a fairly large gauge white wire. I have the Camper Special package, but I think all that camper wiring connects up in the front, so I think the white ground wire is stock.

That's what I have, the ground on one side of the tail lights circuit. Today I'm going to start at the rear and make my way forward. It's not uncommon to jolt something loose. I did some undercarriage resto and is possible I knocked the plug who know. Hopefully the new switch helps.

 

[squarelyfe]

I got the dome light to function properly, on off when I open /close the doors but I still have no taillights/dash, front markers or side . I cleaned up the ground wire tab to bright copper, used a new washer that I actually checked for continuity to make sure it would work and scuffed the rear part of the bed that it all connects to behind the tail light.
I did find a loose butt connector ground on the core panel that I'm going to clean up but don't think that has anything to do with the rear circuit. Something must not be feeding that rear circuit. Wtf, it really bothers me cuz I had all this stuff working as I said before I replaced sockets and everything so it all be clean work nice and bright. It must be in the panel or between the panel and where the circuits go out off the other side of the firewall or grounding related to it but still as you said before the rear tail lights are what feed the cluster so why is where's my rear feed??

 

[squarelyfe]

Update.. I now have the front parking lights and and side coming on BUT when I signal still come on in Hazzard mode where in the cluster of the two hazard lights flash at the same time and when you signal to one side they both come on and flash. I still have no tail lights no cluster lights but I did gain the washer radio and climate control lighting back.

 

[squarelyfe]

Things kinda leaning towards my feeds so I pull the bulkhead connector and the 46-year-old dried out dielectric grease is caked all over the posts and not in a good way. So I worked on cleaning them. Something is not feeding the rear circuit and something is making a turn signals go into flash mode. Wth man

 

[squarelyfe]

Some before and after.. I know dielectric grease is normally a good thing but in this case it's rock hard and it's covering any clean contact these post I don't think it's a good thing so I got it out of there.

 

[squarelyfe]

Lol it took me to read what I wrote here to follow my steps I realized I forgot to plug the intermediate harness into the rear circuit after I was probing around earlier today. I now have cluster and rear tail lights/side running working lights in front parking lights come on everything works except for when I hit the turn signal it goes into hazard mode where they all blink. Come on fellas someone must have gone thru this or have an idea I'm so close! I'm going to see about testing the flasher module tomorrow. If my horn isn't hooked up could that be a problem with this turn signal flasher mode? Does the turn signal cam get ground from the horn circuit?

I'm Also interested in some led bulbs for my dash since a few are out.