68 C10 wiring problems!

[cwhitlock]

I have a 68 c10 that I started working on about 2 months ago. I am having some really strange issues with the wiring and wanted to see if anyone had any suggestions short of a new wiring harness. I have checked all bulbs associated with the below issues.

- When you signal left or right, the front markers work normally, but the rear both blink together.

- The floor dimmer controls the headlights but they only work on one setting, I believe it to be the bright. When you dim, no lights at all.

- Neither the running lights nor the brake lights work.

- Gauge lights only come on when you depress the break.

- Dome light does not work.



1968 C10 SWB, with 1970 350 and 1972 power breaks and steering.

[1968aj]

I had a bad dimmer switch once. it affected more than the headlights.

[VetteVet]

Your problems are all related to the orange wire that comes from the fuse block to feed the headlight switch and the brake light switch. It also feeds the dome light switch.I think you have some wires crossed and some may be missing.

I can tell you haow they should be wired and you can check to see if they are wired correctly.

first off, check the brake light switch to see if it has an orange wire coming into it and a white wire going out. The orange wire is the power wire from the fuse panel to the switch and the white wire is power going out of it.

here's a diagram.





The fuse panel is in the lower right and the orange wire is in the middle of the panel. The hot red wire feeds the orange wire which is on the stop and taillight circuit. Follow the orange wire down to the dome light connector. Then up to the brake light switch.At the brake switch you'll see the orange wire coming in and the white wire going out.

Just at the bottom of the fuse panel is another orange wire that goes to the headlight switch where it feeds the front and rear parking lights. It is hot with the key off so the parking lights work with the key off or on.

Here's another diagram.



looking at the diagram the headlight switch is in the lower right and you can see the orange wire going to it from the fuse panel. The orange wire carries power to the two brown wires when you turn on the headlights to light the four parking lights. The headlight switch may be bad if the orange wire at the headlight switch has power.

Looking at the brake light switch in your truck you should see the white wire at the brake light switch. If you follow it, it goes to the steering column connector on the lower part of the column. It should go into one end of the connector and there should be a black wire on the other end of the connector. The black wire is for the horn button under the steering wheel that grounds when the horn button is pressed and it energizes the horn relay and blows the horn. There should be a purple wire in the center of the connector that is coming from the fuse panel and is for the turn signals.

check and be sure that the white and purple wires are not reversed.

Here is a picture.



I think the column side of the connector is in the right hand and the fuse panel side is in the left hand.

Some aftermarket turn signals so not follow the color codes so it is easy to get the wires crossed up. If that's the case we'll have to sort them out to get the wiring correct. Does your horn work correctly? If the front turn signals work correctly then the wires are correct down to the brown wire which is the hazards wire. Then you have the purple TS wire, then the yellow left turn signal wire, then the green right turn signal wire, and finally the brake light wire.


The orange wire powers a green wire at the headlight switch after the power goes through the variable resistor in the switch. The green wire returns back to the fuse panel to the PNL LIGHTS fuse. A small gray wire runs from there To the dash panel plug for the instrument lights. I haven't figured out how they turn on when you step on the brakes unless the head light switch is bad or wired wrong.

Here's a picture of the HL switch.



Your headlight problem is probably a bad dimmer switch or corrosion on the terminal connector on the tan wire.
Pull the connector off and jump from the light blue power wire to the green high beam wire for brights and see if they work. Then jump from the light blue to the tan low beam wire to see if they work. If they do then the dimmer switch is not working. Be sure to turn the headlight switch on. LOL

Well that's about all I can do until you check this stuff out. post back if you have any questions. VV

[1968aj]

I don't know about him but you certainly helped me out with some problems im having. thanks

[michael bustamante]

Dang vette that was a pretty good diagnosis.

[VetteVet]

Originally Posted by 1968aj

I don't know about him but you certainly helped me out with some problems im having. thanks

I'm glad. I try to answer problems so that anyone who has the same kind of problems can benefit.

Originally Posted by michael bustamante

Dang vette that was a pretty good diagnosis.

Well thank you sir. I am pretty curious as to the solution.

[cwhitlock]

Thank you VetteVet!!! I hope to have some time around Christmas to check all of the wiring! With all the info you provided, I should be able to figure it out.

Thanks so much!

[berch 69GMC]

very helpful to me, thanks. I see OP has been dormant for a while. this thread has got me so close to fixing all my electrical issues. My 69 gmc has all powered items working properly except drivers rear blinker and brake light. I have a new rear wire harness, and housing for drivers side. Ground at housing is clean to bare metal. Folowing vettes advice i traced the orang and yellow wire. I discovered the cab light wasnt connected. The wires actually charred some paper behind the seat. After sorting that i am at a loss. Pass side break light works as well as All tail lights. All fuses are conducting, took the steering wheel off and cleaned all contacts, springs look good. Any suggestoons?

[cwhitlock]

Well I have checked all wiring and fuses to the schematic that VetteVet provided. I found a flasher wire that could have been an issue so I replaced it. Also replaced the turn signal cam and the pigtail that runs from the column. Also checked all circuits on the back of the gauge cluster. Finally, I replaced a headlight switch and all gauges, dome light, turn signals, and bright and dim headlights worked correctly. When I had a buddy push the break pedal to check break lights, everything quit. Headlights still work on bright and dim, but no gauges, dome, or break lights. Any other suggestions?

I have tried another new headlight switch but no luck.

[magwakeenercew2jh]

Originally Posted by cwhitlock

Well I have checked all wiring and fuses to the schematic that VetteVet provided. I found a flasher wire that could have been an issue so I replaced it. Also replaced the turn signal cam and the pigtail that runs from the column. Also checked all circuits on the back of the gauge cluster. Finally, I replaced a headlight switch and all gauges, dome light, turn signals, and bright and dim headlights worked correctly. When I had a buddy push the break pedal to check break lights, everything quit. Headlights still work on bright and dim, but no gauges, dome, or break lights. Any other suggestions?

I have tried another new headlight switch but no luck.

Okay...you basically said "no new harness"...I understand. That's real money.

All I can do is chime in as a guy who has had every electrical gremlin you can imagine live inside in my builds.
I think they call it "experience"....And, most of these are re-statements of good advice that's already on the table.

My disclaimer is, that I would have to put, "In my experience" before every suggestion that follows.
Instead, I'm saving my typing fingers:

-Usually, the headlight switch is not faulty. Seems like a good place to fubar...but *usually* not.

-The foot dimmer switch is a typical culprit.

-The wire(s) to the dome and...along the floor under carpet or mat ARE MAJOR culprits.

-The grounding of the tail/brake lights (and even lic plate light) are issues that a lot of us have.

But as for yours in particular:

Can you get it back operating fully by finding a fuse (or more than one) in the fuse block that blew
immediately after the brake pedal press caused a Fourth of July?

You say EVERYTHING stopped, right? Except the headlights. Did the turn signals stop?
Side markers?

I suspect that at least one hot wire is crossed, all right. As VV suggested, I'd be heading for the brake light switch.
If all looks okay there, I'd follow those wires and make sure you don't have a fray or an incorrect connection with them.

That is, after I looked at the fuse block to see what was blown. If it's the 3 amp fuse that is the cluster feed,
you're getting a little closer to the problem.

I guess I'm Mr, Obvious when I suggest you get it up and operating again. And that you DON'T
press on the the brakes when you do have it cooking again properly.... until you see where it blew.

-Your dash is going to be okay, I think.

-But, again, do the turn signals or wipers work? What about the side markers? Especially in the rear.

Just throwing out some thoughts.

[VetteVet]

Originally Posted by berch 69GMC

very helpful to me, thanks. I see OP has been dormant for a while. this thread has got me so close to fixing all my electrical issues. My 69 gmc has all powered items working properly except drivers rear blinker and brake light. I have a new rear wire harness, and housing for drivers side. Ground at housing is clean to bare metal. Folowing vettes advice i traced the orang and yellow wire. I discovered the cab light wasnt connected. The wires actually charred some paper behind the seat. After sorting that i am at a loss. Pass side break light works as well as All tail lights. All fuses are conducting, took the steering wheel off and cleaned all contacts, springs look good. Any suggestoons?

The blinker and brake light wires are the same wire from the turn signal column. If you aren't blowing fuses you can do a simple check to determine if the problem is in the rear or if it's in the cab in the turn signal switch.

Disconnect the column connector and jumper between the stop /tail fuse holder and the yellow wire in the connector. Make sure it's the yellow wire that goes to the rear of the truck and not the one to the turn signal switch.

If the left brake light doesn't come on then your problem is in the rear and if it comes on then the problem is in the turn signal switch. There is a connector mid frame for the rear lights as well as the one on the firewall and then there is one nearer to the rear lights. You could have a problem on the yellow wire on one of those if the rear light doesn't come on with the jumper.

You can also jump between the white brake switch wire in the connector and the yellow wire but you'd have to step on the brake pedal to power the yellow wire. Since we know the right brake light works then the brake switch is good.

If the rear light does come on then your problem is in the turn signal switch or the wiring to it. That includes the main column connector.

Originally Posted by cwhitlock

Well I have checked all wiring and fuses to the schematic that VetteVet provided. I found a flasher wire that could have been an issue so I replaced it. Also replaced the turn signal cam and the pigtail that runs from the column. Also checked all circuits on the back of the gauge cluster. Finally, I replaced a headlight switch and all gauges, dome light, turn signals, and bright and dim headlights worked correctly. When I had a buddy push the break pedal to check break lights, everything quit. Headlights still work on bright and dim, but no gauges, dome, or break lights. Any other suggestions?

I have tried another new headlight switch but no luck.

Mr Mag is so right on all the usual suspects when you're blowing fuses.
Man that's a scary picture.

As you can see from the diagrams in the posts above, the orange wire does supply power to all the things you say quit working. Your first place to go is the stop/tail fuse. If it's not blown then check the fuse holder connections. With the fuse installed check for 12 volts on each side of the fuse holder.

When your buddy pushed the brake pedal, he sent power to the brake light circuit. It starts with the orange wire to the brake light switch and then through the white wire off the switch to the turn signal switch connector, up to the turn signal switch where it makes contact with the yellow and dark green wires and back out of the TS switch back to the column connector and then to the firewall connector. Then to the harness to the rear of the truck.

If the stop/tail fuse is blown then you need to trouble shoot the brake circuit to find the ground.

If you want to sacrifice a couple of fuses you can isolate the ground to the turn signal switch or to the harness to the rear lights.

1. disconnect the steering column connector.
2. jump between the white wires in the connector halves. and step on the brake pedal. Does the fuse blow? If it does then the short is in the white wire that goes to the turn switch.or in the steering column harness. If it doesn't then. Place the signal lever in neutral.
3. jump between the yellow wires in the connector halves.
4. step on the brake pedal- does the fuse blow? if yes there's your short.
5. If not then jump between the dark green wires in the connector halves. If the fuse blows then the short is in the right turn/brake wire to the rear of the truck.

If you have a multi meter it is very easy.
1. disconnect the negative battery cable.
2. disconnect the steering column connector.
3. connect the multimeter red lead to the white wire from the brake switch.
4. connect the black lead to a known good ground.
5. Place the meter switch to the resistance (ohms) setting
6. Step on the brake pedal and read the meter looking for a high resistance.
7. If the meter reads near zero the white wire is grounded between the brake switch and the column connector.
8. Place the red lead on the white wire on the steering column side of the connector, leaving the black lead on the ground. Read the meter. A low, near zero reading, indicates that the white wire in the column or either the yellow or dark green wire in the column is shorted to ground. A high reading indicates that the short is in the harness to the rear of the truck or in one of the rear brake light bulbs.
9. IF you get a zero reading then you can isolate the left or right brake wire by placing the turn signal lever in the left or right turn position. If the white wire is shorted or if both yellow and dark green wires are shorted the zero reading will remain on the meter. If you get a high resistance on all points then.
10 place the red lead on either of the yellow or dark green wires to the rear harness of the truck. Leave the black lead on ground.
11. One of the two wires should show a near zero reading on the meter and that will be the shorted wire. If both show a near zero reading then the harness is probably shorted(pinched) somewhere in the path to the rear.

It is very common to get a short inside the steering column by pinching one of the wires.

[berch 69GMC]

So jumping from fuse to yellow wire at firewall plug illuminated the brake light. Should I replace the whole steering wiring harness or is there a quicker fix. Thank you VV, my next Wounded Warrior Project donation is in your name, thanks for serving as well.

[VetteVet]

Originally Posted by berch 69GMC

So jumping from fuse to yellow wire at firewall plug illuminated the brake light. Should I replace the whole steering wiring harness or is there a quicker fix. Thank you VV, my next Wounded Warrior Project donation is in your name, thanks for serving as well.

Sure sounds like a break in the yellow wire in the steering column or a non connection in the turn signal switch. Those little contacts are easy to mess up. You should jump from the fuse to the yellow wire in the half moon harness at the column to avoid having a break in the yellow wire from the column plug to the firewall plug.

I'm very honored by your gesture but I would be happier if you would do that in the name of the truck forum.

[berch 69GMC]

Took the steering wheel apart everything looked ok. I separated the half moon connector and found two of the "teeth" inside bent. Straightened those and now everything works. May be coincidental with wrestling with the turn signal switch. I ordered a replacement switch just in case as I am rebuilding the whole column. Thanks for the knowledge and inspiration VV.

[magwakeenercew2jh]

Originally Posted by berch 69GMC

Took the steering wheel apart everything looked ok. I separated the half moon connector and found two of the "teeth" inside bent. Straightened those and now everything works.

Sweet!!!