Voltage at driver brake light drops to 0 when load applied

[ktm]

I've been diagnosing a brake light issue that recently came to light......

I finally managed to get my passenger side brake light working but the driver's side is still having issues. Turn signals, hazards, and tail lights work on both sides. The driver's side brake light will work if I apply power direclty from the battery. To clarify, I removed the rear panel and cut off both ends of the plug for the tail lights. I have spare connectors from Vintage Connections (from when I rewired my car) that I can use to reconstruct that connection. I also ran a ground wire from the light to the battery ground. I can do this since the battery is located in the rear of the car.

 

I tested the voltage drop issue by cutting off the plugs as described above and exposing the bare wire on both (tail light side and harness side) wires (G/R). I attached my DVM to the harness side wire (where it read +12v) and then touched the tail light side wire to the harness side wire and watched the voltage drop to zero.

What is confusing about the issue is that the turn signals work on both sides. The turn signals share the same circuit as the brakes. If both turn signals work and the passenger's side brake light works, why doesn't the driver's side?

The tail lights were working fine for at least 2.5 years (the time since I rewired the car) and its only within the month that they could have gone out.

 

 

[ktm]

Found the issue. First, I was too far down the rabbit hole and admittedly testing the wrong circuit at the combo switch. The problem is that while the wiring at the combo switch for the rear brake lights/turn signals are W/R and W/B, the wiring changes to G/R and G/B at the lights themselves. Sooooooo, I kept testing the G/R and G/B at the switch.

Second, the load issue was real. If I did not connect the harness at the drivers side light, the G/R wire in the harness would read +12v with the brakes depressed but would drop to 0 when connected to the light. After I woke up and realized my error above, I then tested the correct circuit at the combo switch and determined that it was, indeed, in the switch.

I took the switch apart and inspected the contacts. I had previously taken the switch apart 2 years ago and cleaned the turn signal contacts but did not touch the brake contacts. I had forgotten how the brake circuit worked in the switch and taking it apart reminded me. While everything was in intimate contact, it appeared that there was some heavy oxidation on the contacts. I sprayed some Deoxit on both brake contacts and cleaned them up the best I could. After cleaning and reassembling the switch, the driver's side brake light now functions.