E4OD "Slipping" in 4th

[boostpower]

Just hooked up to a friends trailer and as I was on my way home from his place and my tranny started to slip while in 4th gear. No issues in any other gears or with the shifts, but as soon as it hits 4th and I just cruise, the RPM's will jump up 300-400 and then back down and then over and over again. Doesn't matter if I'm in OD or not. Doesn't seem to do it if I'm on it, just when cruising. Fluid isn't that old and level seems to be fine. Any ideas?

[boostpower]

Turns out to be a short with the trailer wiring that kept causing the brake lights to trigger which would unlock the torque converter.

[mox-ct]

That is interesting because I have a similar problem. I just installed LED rear tail lights on my F350 dually dump. One thing I noticed is when the ignition key is on only the rear tail lights are on (like daytime driving light). No other light in the circuit is on, no running lights, no clearance lights, only the LEDs. I thought this was odd. Everything else worked well. Along with this, the transmission feels like it's hunting in 4th, up, down, up, down, up, down. I'll have a look at the lighting circuit again.

[mox-ct]

I found my problem. I believe the ABS was getting a no brake lights signal which was unlocking overdrive everytime I let off the gas pedal and that made it seem like the tranny was locking up and unlocking etc.

I fixed the brake lights and everything is fine now including the ABS trouble light.

[ipz2222]

LED brake lights are a no no on these trucks. The computer thinks you have the brakes applied all the time, causes different tc apply pattern.

[mox-ct]

I wouldn't necessarily say LEDs are a no no, but they have to be installed correctly. They need resistors to provide the correct resistance to mimic a normal bulb.

I fixed the problem by installed a normal bulb in the third brake light located in the upper cab area. It was burned out. Soon I will install resistors as insurance.