All of rhe above is relevant. Have you considered that you may be in the 3-4 shift range and the trans doesn't know what gear to be in? 35-40 is the common range for OD.
I had a bus driver growing up who always drove in the shift range of the Allison trans at 40 mph - up/down.....up/down to the point I said something to her about the shift point. Some people are just dumb and not mechanical (not saying that about you).
Have you tried maybe going a little faster and see what happens? What about driving in 3rd for that speed and evaluate?
I appreciate all the responses... Going to try and answer a few of the questions including the one quoted...
1. I can accelerate out of the problem. It doesn't happen when trying to hold 55 or 65 on the highway for example.
2. Other than a sheet of paper about a warranty on a certified gm remanufactured transmission, I have no other paperwork from the previous owner. So quite possible it was never done. But that being said, outside of this issue, it shifts quite smooth. Goes into all gears easy from park (like Park to Reverse or Park to Drive). When shifting on the go, They're all smooth, so I'm inclined to think it was done, but fully realize it might not have been.
3. The throttle body was swapped out, and I was told it made a good difference with misfires (but not this particular issue), but when the maf sensor was replaced, the mechanic swapped the throttle body back to the original and said it was fine... so I'm assuming the throttle body was fine, but it's also a cheap part to replace, so maybe I'll do that as well.
4. Another interesting piece of info that I left out. I noticed someone mentioned the PCM... The mechanic wanted to get better info from the PCM, so he said he took it to a shop that could access it via hb tuners (the tokens for this truck were only $50)... But when they tried to access the pcm, they couldn't get in. They said it was "locked out"... so my mechanic assumes the previous owner may have either tuned it, but I asked the previous owner and he said "no". Probably lying. I purchased an pcm that was programmed to the vin, but the mechanic couldn't get the ignition key to program with the new ecu installed so he put the old one back in.
So I think I need to get the old pcm out or find someone who can get into it or replace it successfully.
It sounds like from a few responses on here, that a few people think the likely culprits could be the throttle position sensor, throttle body, TCC lockup (can that be replaced by dropping the trans pan or does the entire trans have to come out? Same question for the 3/4 clutch pack or solenoid?), fuel injector wiring harness, and/or PCM issues. I feel if it was the trans, I'd have the problem more often than just that specific scenario?
Thank you again for the responses.
