I have posted on this problem a few times. The last time was about 18-20 months ago. Long story short: I am pretty good with electrical systems. When I was done putting my 69 Cuda together, I had the same problem. Using a wiring diagram and doing some experimentation, I determined that power for the brake lights and turn signals all went through the turn signal cancelling switch. Here is a little article I wrote about the problem and how to solve it:
After a two and a half year restoration of my 1969 340 4 speed Barracuda, I was ready for a test drive. But wait; I had no turn signals and no brake lights. They worked before the teardown. After hours of testing and looking at wiring diagrams, I finally figured out why my brake and turn signal lights were not working. As it turned out, my Barracuda’s (and many other older Mopars) brake lights go through the turn signal switch and turn signal canceling cam. I ultimately determined that the problem was the cheap Dorman turn signal canceling cam I put in last winter (column out of the car) when I noticed the original one was broken. The Dorman part looked like an exact replacement, so I thought I was OK. Here is what I found out. I hope this can help you should something like this ever come up. First I tested all fuses and the brake light switch. The fuses were fine, and the brake light switch had current flowing through it when the brake pedal was depressed. So it was time to drag out the wiring diagrams. Please note that even if the brake light switch had been bad, that would not have explained the lack of turn signals. The wiring diagrams indicated that current for the brake lights travelled through the turn signal switch. That was a surprise, I had not realized that. So I removed the steering wheel to reveal the white turn signal cancel cam (picture 1). Please note the small brass rivets (with the red wires attached) on the turn signal cancel cam on either side of the cam retaining screw. The one to the left of the screw is hot when the ignition switch is “On”, and the one to the right of the screw is hot when the brake pedal is depressed. To pull the turn signal cancel cam, simply remove the retaining screw (it screws into the turn signal lever) and gently pry up on the turn signal canceling cam. Flip the turn signal cancel cam over and look at the back of it (picture 2). Notice there are two hard wires about one inch long each that are vertical and connected to the plastic cam with brass rivets. Now look at the four round brass electrodes on the top of the turn signal switch (in between the white turn signal cancel cam and the steering shaft). The hard wires on the back of the turn signal cancel cam making connection with those four electrodes (depending on turn signal lever position) is what send current to the lights when installed. Using a hot jump wire, I determined that the upper right hand (UR) electrode on the turn signal switch sent current to right rear tail light, the lower left hand (LL) electrode sent current to right front parking light, the upper left hand (UL) electrode sent current to left front parking light and the lower right (LR) hand electrode sent current to left rear tail light. So when the turn signal lever (that moves the cancelling cam and the hard wires on the back) is flipped up for a right turn, the turn signal cancel cam will be rotated clockwise a bit, and the hard wire that is hot when the ignition is “On” (the one on the left when the turn signal cam is installed as in picture 1) will contact the UR and LL electrodes thereby activating and flashing the front and rear right hand turn signals. The current goes through the turn signal switch first; then through the flasher. Conversely, when the turn signal lever is flipped down for a left turn, the turn signal cancel cam will be rotated counter-clockwise a bit, and the wire that is hot when the ignition is on will contact the UL and LR electrodes thereby activating and flashing the front and rear left hand turn signals. When the turn signal switch is in the neutral position, the other hard wire on the back side of the canceling cam (the one that is hot when the brake pedal is depressed) is making contact with the UR and LR turn signal switch electrode. So when the brake pedal is depressed, current flows to both rear tail lights. So why weren’t my lights working when running current through the above mentioned electrodes activated all appropriate lights and flashers? I eventually noticed that the hard wires on the back of the turn signal cancel cam were flat down against the plastic, so there was no way they were going to make contact with the electrodes on the top of the turn signal switch then the cam was installed. So I simply bent the tips of the wires on the back side of the canceling cam up a bit so they would contact the turn signal switch electrodes, and everything worked fine (see picture 3). Sorry, I did not think to take a before picture, but imagine the wires laying flush against the back of the turn signal cancel cam so that there would have been a gap between the wires and the turn signal switch electrodes. No contact; no current flow. Had I needed to replace the turn signal cam or switch, those tasks are fairly straight forward. However, the main purpose of this article was to explain how the current flows through the turn signal cam and switch and how to diagnose the, “I have no brake lights or turn signals” problem.
