Dad's 550 w/ torqueshift slowly started occasionally delaying, then hammering shifts. (Went from barely noticeable to occasionally obnoxious over the course of 3 yrs) The truck only has 34k mi. Recently started hammering in and out of gear at random (not necessarily when shifting) badly enough to park it.

Did some reading on here, scanned with Autoenginuity, codes thrown for D solenoid (main drive clutch) which is apparently not uncommon.

I checked the connector pins going to the transmission, which all looked spotless. (No scortched or corroded pins.)

The old solenoid tested good with a multimeter, but changed it out anyway because we already had it apart just in case that was the problem. There was some sludge on the magnet, but seemed normal for an auto tranny. (No flakes or shavings)

Picked up a new solenoid from Ford, swapped it out, changed the fluid that came out with the pan (a full flush probably wouldn't hurt...) and it ran perfect for 2 days.

Just got a call saying it started again (tow/haul light flashing) - so headed back to the farm to see what new codes might have popped up.
This is my first time diagnosing a 5r110. Any pointers are appreciated. I'll repost in an hour or so when I pull the new codes.

Update: Same code for D solenoid - have a copy of the ford service manual for the 6.0, so I'll spend some time with a multi-meter to see if I can find a problem with the wiring hardness.

 

Does anyone have the wiring diagram for the external wiring harness for the 5r110?