Brake Light Issue

[67RS/SSx2]

Took the wife out for a brief cruise last night in the 72 C20. It was after 9pm so it was dark. Got pulled over by a LEO as my left brake light was inoperative. Got off lucky with just a warning.

He did not say anything about my not wearing a seatbelt, my 30 days out of date insurance card (the insurance is current), or my lack of displaying my vintage tag sticker. In Oklahoma you can run a vintage license plate, in my case a 1972 plate from my county, but you are required to place a sticker on the windshield. I do not like stickers on my vehicles so I do not apply them to the windshield.

So I figure it is a burned out bulb or a corroded contact between the bulb socket and the bulb. However, the bulb checked out okay and even after cleaning the contacts in the bulb socket, still no good. Funny thing is the turn signal works just fine. The emergency flasher on that side and the brake light do not work.

I think it is in the wiring before the turn signal and brake light merge, wherever that is. I do not have a wiring diagram but will try to track one down.

Does anyone have a suggestion as to where to begin looking for the issue?

TIA!

[pjmoreland]

I had a similar issue where both turn signals worked, but neither of the brake lights worked. I was getting 12V from the brake switch to the turn signal switch connector. Turned out that the contacts in the turn signal switch were varnished with old grease. A little cleaning of the contacts solved the problem.

[Steeveedee]

Make sure the ground is good. Probably is, if the turn signal operates properly. I would check that the wire coming out of the switch for the left turn signal has power when the brake is applied. The white wire comes of the SLS and supplies power to the turn signal switch. It's possible for the switch to fail to contact when it is in the cancelled position.
Here are some wiring diagrams.

[1970 CST Short Wide]

I had this same problem just 2 weeks ago. Turned out to be the harness from front to back that plugs into rear harness. I replaced rear harness first because of old connections from trailer lighting but still had the same issue. I think the problem was in the plug connecting the two harnesses. Anyway that fixed my problem

[old51sedan]

Originally Posted by pjmoreland

I had a similar issue where both turn signals worked, but neither of the brake lights worked. I was getting 12V from the brake switch to the turn signal switch connector. Turned out that the contacts in the turn signal switch were varnished with old grease. A little cleaning of the contacts solved the problem.

I had a similar problem this week, I was having a problem with both my rear lights flashing with the left turn signal. I installed a new ground wire and thought I had it solved. a few days later my neighbor was over and we were going to back the truck out to work on it, when I got in it I touched the brake pedal and the left front parking light came on and also the turn light on the dash. He asked it I had turned the parking light on because only one came on. I stated that I thought I had that fixed, him being a mechanic all his life said it's your turn signal switch. I said no can't be it's all new inside, he said where's your old column. He put that on the floor of the truck, unplugged the new one and plugged in the old one and problem solved. I never would have believed it.

[pjmoreland]

You're getting 12V on the white wire from your brake switch to the turn signal switch since your right brake light is working. The yellow wire from the turn signal switch connector to your left turn signal/brake light filament is working since you have a left turn signal. You could check the yellow wire coming out of the C shaped turn signal connector for 12V when pressing the brake pedal. I'm guessing you won't find any voltage there.

[67RS/SSx2]

Crap! I wanted to quote the last two posts but cannot figure out how to multi-quote.

I am pretty good with equipment wiring diagrams but have to admit these auto wiring diagrams baffle me. Guess it is because they are more of a pictorial.

Anyway, I found a great wiring diagrams in post #329 of this thread https://www.67-72chevytrucks.com/vbo...=brake&page=14

Clearly, I must have power into the turn signal switch (white wire) but am not getting power out the yellow wire when the turn signal switch is cancelled (like pjmoreland said).

Sounds like the switch to me. This scares me to death. I have never torn into a steering column but looks like I will have to now.

Guess I will have to clear my schedule, take a few drinks to calm my nerves, and dive in.

Thanks so much to all who responded!

Not looking forward to this.

[Steeveedee]

Not as hard as you think. Take pics as you go along, if you need to.

[67RS/SSx2]

Found a Brother's you tube video showing how to replace the turn signal switch. Did not look terribly easy but seems doable. If I can find my puller I will try to pull the steering wheel and give the switch a good cleaning this weekend. If that does not work I will be looking for a good quality replacement switch. Any recommendations for a vendor?

Thanks!

[pjmoreland]

The one GMCPauls sells has worked well for me in the past. I currently have a cleaned up original factory unit in my truck.

https://gmcpauls.com/steering.htm

[67RS/SSx2]

Pulled it apart today and attempted to clean the contacts. No dice. Also, part of the plastic is cracked so I will order a new switch and give it a go.

Thanks for all the information.