I had my transmission serviced last year, and ever since then I've had an issue that I originally described as "slipping out of gear at 50mph". The original symptom being that if I accelerated hard (like an on-ramp onto the highway), at about 50mph when I'd expect the torque converter to lock up it'd drop out of gear entirely, I'd have to let off the gas, slow back down to 45mph or so, and then slowly and gently accelerate for it to catch again.

Now I'm thinking the issue is actually that it's trying to shift at that point and not grabbing the next gear. In the service I had the bands adjusted, fluid and filter changed, and a few new valves for fluid flow to the cooler that were leaking, and a new shifter cable...

I also replaced the speed sensor a while back. I'm thinking maybe solenoids, or maybe the band adjustment wasn't done right. The issue now occurs at various speeds, it is sluggish from a start but faster than it was when I had low fluid. It just seems to slip from a start and whenever it goes to grab a new gear it has trouble. For example from a start I'll pull away in 1st, but when the RPMs hit 2800-3000 it'll just hold there, not switching to 2nd. When I let off the gas it'll shift and I can put my foot down again, but RPMs stay steady at 2800-3000 throughout the acceleration (slipping?).

The issue is most pronounced when (I think) it's trying to grab 3rd. Either under hard acceleration at 50mph going from 2nd to 3rd, or medium acceleration it'll try to grab 3rd a bit earlier at 35-40ish, or when I'm at highway speeds and I need to pass someone and it goes to drop down to 3rd I loose all forward power.

If it was TC slip I'd expect it to behave like it did before I had it serviced, where it'd lock up at 47-48mph and the whole truck would settle down (RPMs would drop, I'd have more power, and it'd ride nice and smooth). I took it in for the service and ever since I've had this issue... thoughts?

 

I think a band adjustment is where I would start.

 

Once I sort out my brake issue, having AAMCO (the transmission shop) double check their work is the next step. Unfortunately, I've been limping along with the truck for like 3 months, so I've got to cross my fingers and hope they'll stand by their work.