Hello....I'm a brand new member and I hope someone can help me with a bad problem. I recently purchased a 92 F350 flat bed 4x4. I had to make some modifications to the frame to accept a dump bed from another truck. In doing so I had to re-wire all of the tail lights and install a trailer plug. There are 4 tail lights...2 are 6" LED lights mounted in the headache rack and the other 2 are LED rear lights. I also installed a trailer brake control box inside the truck. Everything works fine with the lights. When I hooked up my trailer to check the trailer brake box, I immediately got bleed over voltage on the cold side wire of the brake light switch. The voltage is reading 4.50 volts on this wire without activating the switch. The weird thing is that the low voltage is showing up on the 2 6" LED tail lights mounted in the headache rack but is not showing up on the larger tail lights. The very center 4 lights of the 6" LED lights will illuminate when the ignition switch is turned on. I figured that I must have had something wired up wrong so I unwired the trailer control box and still got the 4.50 volts. I disconnect all of the rear lights and the trailer plug from the truck and I still have 4.50 volts on the cold side wire of the switch. I disconnected the wires from the brake light switch and still have the problem. I have worked and tried to locate the problem with this cold side switch wire. I did find that if I remove 2 fuses located under the hood in the fuse box that the bleed over goes away. The fuses are # 10 & #11. #10 goes to the starter relay coil and a couple of instrument panel fuses. #11 goes to the fuel pump shutoff solenoid. I don't understand how this could have happened. Does anyone out there have a clue what I need to do next in finding this problem??? There is no blown fuses...everything on the truck works fine as it should. The problem is the low voltage constantly on the cold side wire is screwing up my trailer brake control box. Any help would be appreciated.

 

That voltage is coming from the PCM (Powertrain Control Module) as designed. The PCM monitors the cold side of the brake light switch and when it sees voltage go high due to the brake light switch closing, it knows that the brakes have been applied.

I have LED taillights on my truck too. There is a few ways to deal with this:

- Install resistors in parallel with the LED lights to cause a higher amperage draw on the brake light circuit. Benefit is that it's cheap and easy, disadvantage is these are big resistors, they get hot, and they defeat one of the advantages of having LED lights which is low current draw. This seems to be a popular choice for people running LED's but I personally don't like it.

- Make a circuit to isolate the existing brake light circuit and switch the power to the lights thru a relay or solid state device while providing a resistive load to the PCM's sense circuit. Advantage is no load resistors hacked onto your truck, disadvantage is this takes more time and some electrical knowledge to do, and the PCM will not be aware of brake application unless you design a provision for that too. Can be done but I didn't want to bother.

- Disconnect the PCM brake sensing circuit. Advantage is it's fast, requires no additional parts, and is free. Disadvantage is the PCM will not be aware of brake application. This is what I did.

Okay so with the last two options the PCM won't know when you apply the brakes. I'm running LED's on my truck as well and have disconnected the brake sense circuit so they'd stop glowing like yours do. Didn't notice any negative effects from doing this, the torque converter still unlocks when stopping (because that should happen when the throttle returns to idle anyway) and the trans shifts up/down as it should, so I don't see any problems with doing so but of course like anything you try it at your own risk. If it doesn't work out for you then just reconnect the wire.

Unfortunately if you want to disconnect that wire you'll have to ask or otherwise find a wiring schematic for your PCM because I don't have anything for your year truck. The circuit in question will run from the brake light wiring to the PCM and, if you look on a connector pin chart for your PCM, will most likely be labeled BOO (which stands for Brake On/Off) or something similar.

 

Thanks for the info.... how do I disconnect the PCM?

 

No you don't want to disconnect the entire PCM, just ONE wire that leads to it. The right way to do that is pull the plug and remove the single wire and pin from it, then reconnect. Insulate the end of the disconnected wire and tape it back to the wiring harness. This way, if you ever want to put it back you can do so easily.

I can't tell you which wire it is, your truck uses a different PCM than mine so you'll have to look up or ask for the schematic or connector pin identification chart.

 

Thanks.... Will this stop the constant low voltage on the cold side wire?..... because the constant voltage is causing my trailer brakes to be activated when the ignition switch is on even when I'm not pressing the brake pedal

 

Thanks for the info....I tried what you suggested but it didn't work. I still had 4.5 volts on the cold side switch wire when I turned on the ignition switch....I just can't figure this out.