There would likely be a code if there was a wiring fault. At least, that's what I would think. Believe the PCM just grounds the solenoid to apply hydraulic pressure to the TCC clutch

 

Ehh, I just look for the cheap and easy fixes before the pull it apart for the deep probe fixes. I've gone the other way first a few times, and you can imagine how I felt when I discovered the reat problem. Based on what you say, if it were a wiring harness issue it would have to be one that is open at all times except during reverse, which means there would be no problem detected by the PCM. When whatever it is gets grounded in reverse, it would also ground the TC control line, locking the TC in reverse.

But, if as you say there's no way for that to happen... I don't mind being wrong about troubleshooting a vehicle I've never laid eyes on. Heh heh heh...

 

as I aluded to in another thread, don't assume anything on these trucks...the engineering on some aspects are drunken bar fly quality.

I have my FSM up right now, the transmission control relay provides a constant 12 volt feed to three solenoids: Governor Pressure, 3/4 Shift, and Converter Clutch. The PCM supplies a ground to engage each of the 3 solenoids (C2P8, C2P21, C2P11 respectively).

There is a "Lock Up Switch Valve" in the valve body.

Another potential is that another band or clutch is staying engaged in reverse when it shouldn't.

SWITCH VALVE
When the transmission is in Drive Second before

the TCC application occurs (Fig. 279), the pressure regulator valve is supplying torque converter pres- sure to the switch valve. The switch valve directs this pressure through the transmission input shaft, into the converter, through the converter, back out between the input shaft and the reaction shaft, and back up to the switch valve. From the switch valve, the fluid pressure is directed to the transmission cooler, and lubrication pressure returns from the cooler to lubricate different portions of the transmis- sion.

Once the TCC control valve has moved to the right (Fig. 280), line pressure is directed to the tip of the switch valve, forcing the valve to the right. The switch valve now vents oil from the front of the pis- ton in the torque converter, and supplies line pres- sure to the (rear) apply side of the torque converter piston. This pressure differential causes the piston to apply against the friction material, cutting off any further flow of line pressure oil. After the switch valve is shuttled right allowing line pressure to engage the TCC, torque converter pressure is directed past the switch valve into the transmission cooler and lubrication circuits.

CONVERTER CLUTCH LOCK-UP VALVE
The torque converter clutch (TCC) lock-up valve

controls the back (ON) side of the torque converter clutch. When the PCM energizes the TCC solenoid to engage the converter clutch piston, pressure is applied to the TCC lock-up valve which moves to the right and applies pressure to the torque converter clutch.

CONVERTER CLUTCH LOCK-UP TIMING VALVE
The torque converter clutch (TCC) lock-up timing valve is there to block any 4-3 downshift until the TCC is completely unlocked and the clutch is disen- gaged.

 

Wow! Thanks for all the great information. As if I didn't feel in over my head before I surely do now. I'm a little at loss for the next move.

 

I find it hard to believe that the TCC would be hydraulically applying on it's own in reverse, and electrical would be similarly unlikely. Bands tend to be self-releasing since they are essentially a loop spring.

Thus I turn attention to the rear clutch. The only time it is not applied is in reverse. There is plausible reasoning that it may be having delayed or minimal release, or that the piston is stuck in the clutch bore. That would make the trans try to lock up in reverse only.