98 5.9L 12v Cummins converter lockup problem
 

I've got a problem where the transmission will only lockup in o/d when the transmission is cold. It shifts into o/d fine even when warm, but the rpm stays pretty much the same as if in D with the converter locked. It first showed up as a tendency to unlock more quickly as you release the throttle. It's also quicker to downshift to D as the speed drops. Once the vehicle is warm (5-10 miles), if the converter unlocks it won't lock again. If I'm on flat road and can maintain highway speeds, it will stayed locked indefinitely.

The fluid is in good condition, but due for a change + filter. I've been holding off in case there's anything simple to be done inside while I've got the pan off.

The o/d switch has gotten flakier in the time I've had the truck (about 4 years). Sometimes will switch right away, others I can poke at the switch until my finger is sore and it won't switch. It's more of an annoyance, so I haven't gotten around to making the time to track down the problem.

I checked the TPS output and it was a bit low (about .75V if I recall correctly). I was able to rotate it without any modification, and adjusted the idle speed to where it should be (had been running quite low) and have 0.87V with about 3.8 at wide open throttle. This is the low end of the acceptable range, but is within spec so I'm thinking shouldn't be the sole cause of the problem. There was no significant difference with lockup after adjusting this.

The engine and transmission are stock to the best of my knowledge and I'm running standard tires.

I use the truck for getting to one job which is about 3 miles each way. I also use it for getting to contract work which is 15-200+ miles each way.

Hope that's enough information to generate some ideas.

Thanks.