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Working with a '97 F350 4x4 cc/lb. Used to have a 460 and e4od, now it has a '97 Cummins and NV4500 5spd. Got this truck on the road after my swap about a month ago, after a couple weeks of driving the cruise control stopped working and I noticed my brakes lights would be partially on. Turns out that I'm getting around 7.5v dc back-feeding into the brake light circuit as measured from the plug for the switch on the brake pedal.
I will get this back-feeding voltage (+/- a couple volts) with all chassis wiring unplugged including the cab wiring to rule out the 3rd brake light but only if I plug in any one of these three: PCM, cruise control servo/module, or abs module. If all three are unplugged I don't have anything back-feeding into the brake light circuit.
Aside from trying to find "new" modules for each of those three from the junk yard, what else could I check? Again, this issue wasn't present when I first started driving my truck after the engine swap, it only came up after a couple weeks and around a hundred miles of driving around town.
Working with a '97 F350 4x4 cc/lb. Used to have a 460 and e4od, now it has a '97 Cummins and NV4500 5spd. Got this truck on the road after my swap about a month ago, after a couple weeks of driving the cruise control stopped working and I noticed my brakes lights would be partially on. Turns out that I'm getting around 7.5v dc back-feeding into the brake light circuit as measured from the plug for the switch on the brake pedal.
I will get this back-feeding voltage (+/- a couple volts) with all chassis wiring unplugged including the cab wiring to rule out the 3rd brake light but only if I plug in any one of these three: PCM, cruise control servo/module, or abs module. If all three are unplugged I don't have anything back-feeding into the brake light circuit.
Aside from trying to find "new" modules for each of those three from the junk yard, what else could I check? Again, this issue wasn't present when I first started driving my truck after the engine swap, it only came up after a couple weeks and around a hundred miles of driving around town.
Have you replaced the brake switch? It could be sticking.
The PCM monitors the BOO switch status. On E4OD equipped vehicles it unlocks the torque converter when you depress the brake pedal. That's your ~7.5 VDC phantom voltage.
Regardless if you have a manual or automatic that BOO signal goes to the PCM. I do not think that is the cause of your brake light issue, just stating where I think the phantom 7.5 VDC is coming from. Many folks who try to use LED lamps find issues with transmission shifting and other brake light related functions. Using those devices requires a special bulb or resistors to pull enough current to make the circuit happy.
I use the term phantom voltage because it may not really be there. A high impedance DVM does not load an open circuit enough so it will sometimes display a voltage when it is not really there. An older analog meter or simple test light with an incandescent bulb comes in handy for these circuits with long wires surrounded by other energized circuits. Even in DC applications I have "measured" voltage on an open wire using a DVM.
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