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. I have power at the wire for the driver's side brake light, however, if I then connect the wire to the light, the voltage drops to zero. Remove the wire, and the voltage returns. It only took me 6 hours to diagnose that issue.

Does anyone have an idea as to the cause for this perplexing problem?

[SteveJ]

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.

How exactly are you applying power from the battery to the driver's side brake light? Did you disconnect the harness at the back panel?

I have power at the wire for the driver's side brake light, however, if I then connect the wire to the light, the voltage drops to zero. Remove the wire, and the voltage returns. It only took me 6 hours to diagnose that issue.

Remove what wire? They have colors to identify them. Or are you talking about your jumper wire from the battery?

Does anyone have an idea as to the cause for this perplexing problem?

Not yet.

Take a look at this and make sure you trace out your circuits. Look for any place where someone might have hacked into the wiring.

[ktm]

SteveJ, thank you for the prompt response. I should clarify that I am not a wiring neophyte but am not an expert either. I've rewired the car from the dash (including the dash) forward including a Painless fuse block, installed 3 engine management systems, and various other tasks.

To answer your questions:

How exactly are you applying power from the battery to the driver's side brake light? Did you disconnect the harness at the back panel?

Yes, that is exactly what I did. I removed the rear panel and cut off both ends of the plug. 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.

Remove what wire? They have colors to identify them. Or are you talking about your jumper wire from the battery?

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.

[SteveJ]

SteveJ, thank you for the prompt response. I should clarify that I am not a wiring neophyte but am not an expert either. I've rewired the car from the dash (including the dash) forward including a Painless fuse block, installed 3 engine management systems, and various other tasks.

I think I found your problem.

Seriously, though, unless you could provide detailed diagrams of how you re-wired everything, it would be quite a challenge. I'm not sure what you kept from the factory and what you replaced.

Having voltage drop to zero usually indicates bad technique with a meter or a short.

If it's a short, why isn't the fuse blowing?

So, with it not being a short, that suggests it's an open circuit (or you did a bad job of wiring).

Change your technique and use an ohmmeter to figure out where the circuit opened up. That's about as close as I can get you for now.

[LeonV]

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.

When measuring voltage at the power feed to the tail lights, where are you putting the negative terminal of your multimeter to?

It sounds to me like you have an open circuit in your left tail light, i.e. a bad ground. However, you say that the turn signals work on the left tail light, which suggests the ground is OK. Are the bulbs correct?

Strange...

[LeonV]

BTW, what was the fix for the right tail light?

[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.

[LeonV]

Excellent!