I'm working on my friends truck. It's a 92 or 93 f350 with a 97 cab and powerstroke the previous owner swapped in. Gas to diesel, auto to stick, some wiring looks questionable so perhaps all bets are off.

A few months ago he brought it by because it had no working lights on the flatbed. I broke out my multimeter, identified the lights, wired them up to new Led stop tail turn lights, tested them, and all was good.

He called me a few weeks later and said he had no brake lights but that the led lights had a few leds dimly lit all the time. He brought it by and I checked it out. I found 4 volts constantly hot on the brake wire, whether the brake pedal was depressed or not, whether key is on or not. Hazards and turn signals and parking lights all worked appropriately. I unplugged the lights to double check that they didn't have some kind of internal short. Still the same. I didn't have much time to do further trouble shooting.

Fast forward to today, I've been trouble shooting for a while. Checked for correct operation of the switch on the brake pedal. 0 ohms when depressed, open when you let off the pedal. I don't know what voltage that switch operates on, appeared to be reading less than 1 volt though. Normal? I expected 12.

Checked third brake light on cab. Bulb was burned out. Constant 4.something volts there on the terminals too.

He had mentioned that the random leds that the 4 volts is enough to illuminate went dark when he plugged his trailer in. I didn't believe him so we plugged in the trailer. Sure enough, the taillights go from 4 volts to zero the second the plug is wiggled into the 7 way connector (still no brake lights) . I immediately disassembled the plug, expecting a small short leaking voltage into the brakes. No. The orange wire (supposed to be key switched 12 volts Iirc) was bent back and taped off. Found the ground was loose but tightened up the connectors. Can't find anything that would be causing his problem.


It's been pissing down rain all day and I drove 75 mins out here and I'm really upset I can't figure this out.

I have no idea what else to troubleshoot.

Fuses all look fine.

Can anyone help?

 

Id try unplugging things until you lose the 4v.
BrKe light switch
turn and hazard flashers
multi function switch

 

Checked under hood fuse box. Someone jumpered fuse 22 or 23. Wiggled it and brake lights work, but it's still got 4 volts on the brake lights with the key on.

That's gotta be related to the trailer plug somehow.

I hate working on hack job bs.

 

that is your problem,

hack job,

haha,

If you had time, make your own wiring loom......

not sure if this video will help, loose connection testing.....

 

When I installed LED tail lights, before adding a load resistor, they were dimly lit too. My truck is far from hacked together. The torque converter won't lock in this condition. Load resistors are needed, wired in parallel to the LEDs. I don't recall my lights being lit with the key off though, that sounds incorrect. But by plugging in the trailer, you essentially added in the needed resistors.

 

Just look at this mess.

 

chexk for breaks, chewed up wiring in the harness to the taillights.

 

makes you wonder what all of those jumper wires go to.

I would be careful disconnecting anything unless you can find out what the wires are powering, might cause more problems than just brake lights.

I would say if the wiring is pieced together to look for loose connections, sometimes the connectors that fit in between wires can create a bad connection.

 

I'm guessing at least one bypasses the safety switch that requires the auto be in Park. Not sure about the others. Can't figure out why plugging in a trailer drops the mystery 4 volts to zero.

 

Finding 4 volts is NORMAL. That circuit is wired to the PCM which is constantly monitoring to see if the brakes are applied. When you install LED taillights they will glow dimly because of that low voltage which is always present with the ignition on.

There's a few ways to fix that. The cheap way is to install resistors at each LED. These are quick and easy but they do get hot and your aging wiring and switches don't benefit from the decreased current that LED's normally provide.

A better way would be to devise a simple circuit to switch the PCM sense circuit high when the brakes are applied, without actually using the power on the brake light circuit. When the brakes are released this circuit would apply a resistance equivalent to two unlit brake light bulbs.

As for me I just disconnected the brake sense wire at the PCM. No more glow and no problems with transmission operation either.

 

I have never heard of that. What the heck is brake sense?

 

More commonly known as the BOO (Brake On/Off) circuit, this is an input to the PCM that tells it whether you are applying the brakes. It's wired in parallel right off the brake light circuit if I remember right.

When the PCM sees you applying the brakes it will unlock the torque converter clutch on an auto transmission. Not sure if that circuit is used on the manual transmission trucks.

 

Not that I am aware of Dixie.

 

It was an auto originally. I guess that explains the 4 volts.

 

Just to add to this, the PCM, RABS and trailer brake controller all emit a pull up current of less then a mA(they are trying to bring the brake wire to 12v but don't have enough current to do so hence why get you 4v), this is how they know if the brake switch is switching, now the 3rd brake light is very important in this circuit and should stay as a regular bulb or a LED with load resistor to keep your LEDS off. As for no brake lights, check fuses and make sure your getting 12v to the brake switch and 12v on the other wire with brakes applied.