Unusual brake light problem

[Skymarshal]

My '69 has developed an issue with the brake lights, and for the life of me I can't figure out what is wrong!

The turn signals operate normally, and when a turn signal is operating the brake lights operate normally.
However, when the turn signal isn't on, only the right brake light comes on when the brakes are applied! Both bulbs are good and the voltage is fine.

Also, when the four way flashers are turned on, the dash indicator lights just flicker rapidly and none of the turn signal lights flash.

I know the brake light circuits go through the turn signal switch, but would a failure cause just the right brake light to work? I don't want to replace the switch as the first thing to try because it's $100+.

 

[2+2GT]

Might be a ground problem in the housing, socket or bulb. Sounds like a switch problem, though.

 

[hungrymonkey]

sounds like the switch problems I had.

fwiw, the replacement on a 67 looks exactly like the $10 'help' section replacement. Not sure if 69 has a different style switch.

 

[Az Pete]

You say the voltage is fine.....if that means you tested for power at the brake light socket in the rear of the car and it had power when the brake pedal was depressed, then you have an odd problem. Brakes and turn use the same wires but get the signal from different sources.

Now, if there is no power to the rear light when the pedal is depressed, it could be as follows.

If it is like the '66, it is the turn signal switch, it is a bad switch.
The way the system is designed, the brake light circuit within the turn signal is a separate pin setup based on the position of the switch. You could remove the steering wheel and do some checking there and maybe see where the switch is failing. On the '66, it is a small spring type wire that makes contact for the brake lights.

Four ways will flash fast if a bulb is not working in the system, such as the one brake light not functioning properly.

I have also seen a new wiring harness with a wire pulled from a pin, causing one brake light not to function on a '66.

 

[Skymarshal]

It's taken me awhile to get time to look at this problem some more, but I figured it out and thought I'd share.

The cam in the turn signal switch is in fact bad. It's a plastic piece with two springs that wipe across terminals on the main part of the switch. Over the years, electrical arcing eroded the wires, and now one of them doesn't make firm contact. The good news is that the replacement cam in the 'Help' section of the car parts store appears to be a direct replacement- the only difference is that you have to drill a rivet to remove the old cam and then use JB Weld or something similar to hold the new one on.

It was a wierd problem, especially since full system voltage was read from the pin in the brake light socket, but it wouldn't light a bulb- even with using a test lead to rule out the socket. Hope this helps someone else having the same problem!

 

[bacfire]

Glad you figured it out. I just solved a similar problem in a way that surprised me. My rear turn signals worked fine until I turned the lights on. Then, when the left signal was on it was extra bright, but the right tail light went completely out when the left one flashed. I also had intermittent battery drain problems. Searched everywhere for bad grounds, wires, etc., with no luck.

With some advice from a mechanic, I finally pulled all the bulbs out of the car and found an incorrect bulb in the left front parking light. It was some sort of slightly odd-shaped thing that a previous owner had put in that was similar to an 1157, but had a slightly shorter base. Changing it out fixed everything.

 

[Skymarshal]

The mystery deepens...

The replacement cam turned out not to fit the turn signal switch, so I wound up buying a whole new switch for $80.

It didn't fix the problem!!!!

I checked all the grounds I could find, and all appear to be OK. I'm completely stumped here.