Tail light voltage drop

[cragenj3]

All brake, turn and hazard go through the turn cam in the colum. It will cause many issues just like this. The front turn/hazard is totally different circuit from the rear due to the brake circuit. I’d bet money if all the grounds and flashers are good it’s the turn cam.

It wasn't the turn signal lever assembly.

 

[cragenj3]

Cragjen,

Don't obsess about the 6V. You were making that measurement without a load (bulb out) and with a high impedance voltmeter (probably a multimeter on DCV). With the load (bulb in) you got 0V, showing you the electrical system somewhere upstream can't provide enuf power to light the bulb.

You should troubleshoot methodically using a proper wiring diagram for the circuit, testing for 12V power at various points in the tail/turn circuit (from 12V at battery thru multiple switches to the rear socket to ground. You can start at obvious and easy to get to measuring locations. Use a low impedance testing means - like a grounded test probe with a 12V lamp.

If you want to shotgun the problem, replacing the turn signal switch on the column is a good choice. There's a lot going on in there.
Make sure you have good grounds. Pay attention to IDIBOBS, he seems to give good technical advice.

Regards,
Jim x 3

You're right about my weird multimeter; it's actually like a $30 Chinese one that Automatically senses AC vs DC. Idk.

Guess I'll go through and look at everything; just weird that it'll read power at the socket unless there's a bulb in it. Maybe there's literally too much voltage drop, due to grounds or something. Odd that the drivers would work but the passenger wouldn't, if they share that g100 ground.

The trailer passenger turn/brake also doesn't work. So I guess the issue is at or upstream of the trailer harness, since the trailer harness is a splice that feeds the signals; I've probed around in that pile of connectors and still just got 6v unless the bulb was in. Must be even farther up... Trace wires up into my grimey, oiley engine bay..at least the oil keeps the rust away.

 

[jim x 3]

Cragenj3,

Sorry to hear you are still having trouble (and to have muffed your name in my post above).

Not to be pedantic, but your multimeter can't read power, only voltage (or current or resistance and maybe a few other things). This is why a simple test probe is a better tool for your use in fixing this problem. Get a test probe with a sharp tip, 12V bulb, and (ground) wire with a clip. Test it by clipping ground to battery negative and probe battery positive - the lite should lite up. Then use your wiring diagram to figure out what is working and what isn't. Start at the rear pass socket - you thereby confirm the problem - the lite won't blink. But it will on the drivers side. (Don't space out - remember to set the turn signal lever properly for each test).

Then use your wiring diagram. I don't know what truck you have, so I don't know how its wired. My 1988 F-350 shows front and rear blinkers on the same fuse, so (for my truck anyway) the battery connection to fuse for that circuit and thence to the hazard/turn switch is good. Then front and rear have separate outputs at the hazard/turn signal switch. Since you've been in there already, check the switch output to the rear lamps. For me, this would be LG/O: light green/orange for the working drivers rear and O/LB: orange light blue for the problem pass rear. Test both the working driver rear and the problem pass rear - your probe needs a good ground to work. You can demonstrate that by testing the working drivers rear. The probe should blink there. Then test the problem pass rear. (Don't space out - remember to set the turn signal lever properly for each test). If the test probe blinks there, the hazard/turn signal switch is OK. Continue with this process to isolate the non-working portion of the circuit.

When you find where the probe stops blinking you can usually physically see the problem. E.g. a broken wire or bad connection. Then you can fix it.

Good luck.

Jim x 3

 

[cragenj3]

Cragenj3,

Sorry to hear you are still having trouble (and to have muffed your name in my post above).

Not to be pedantic, but your multimeter can't read power, only voltage (or current or resistance and maybe a few other things). This is why a simple test probe is a better tool for your use in fixing this problem. Get a test probe with a sharp tip, 12V bulb, and (ground) wire with a clip. Test it by clipping ground to battery negative and probe battery positive - the lite should lite up. Then use your wiring diagram to figure out what is working and what isn't. Start at the rear pass socket - you thereby confirm the problem - the lite won't blink. But it will on the drivers side. (Don't space out - remember to set the turn signal lever properly for each test).

Then use your wiring diagram. I don't know what truck you have, so I don't know how its wired. My 1988 F-350 shows front and rear blinkers on the same fuse, so (for my truck anyway) the battery connection to fuse for that circuit and thence to the hazard/turn switch is good. Then front and rear have separate outputs at the hazard/turn signal switch. Since you've been in there already, check the switch output to the rear lamps. For me, this would be LG/O: light green/orange for the working drivers rear and O/LB: orange light blue for the problem pass rear. Test both the working driver rear and the problem pass rear - your probe needs a good ground to work. You can demonstrate that by testing the working drivers rear. The probe should blink there. Then test the problem pass rear. (Don't space out - remember to set the turn signal lever properly for each test). If the test probe blinks there, the hazard/turn signal switch is OK. Continue with this process to isolate the non-working portion of the circuit.

When you find where the probe stops blinking you can usually physically see the problem. E.g. a broken wire or bad connection. Then you can fix it.

Good luck.

Jim x 3

Well, as much probing as I've done has my failure at the 24pin or whatever amount of pins that is the connector that goes through the firewall under the column.

I have power inside..and can't get an angle to probe the wire from the bay..but 2ft downstream the corresponding wire has nothing., no visible breaks between it and the 24pin... Must be a wire terminal/pin on the corresponding wire on the firewall side of the 24pin or whatever.. The "cover" on the firewall side is like an HD pigtail connector, with a 10 or 12mm bolt through it instead of plastic clips... Pulling the bolt would just pull the whole connector rather than uncover the terminals for probing..

Maybe tomorrow I'll send 10" of wire through the firewall alone to bypass the pigtail and see if that does it. 250k miles, 33 years old, hopefully all the other electrical connectors aren't next.

 

[ihc1470]

Good chance that if you pull that connector apart and put it together a couple of times that you will restore the connection. I have a Steiger tractor that had issues with the bulk head connector and contact cleaner and working the plugs off and on a few times finally restored the connection. That connector is not under the hood on the engine side and being a tractor it is exposed to lots of dust.