Let me start by saying this has been a very frustrating problem for me to try and fix. It's a very intermittent problem, and everytime I have taken it to my trans shop, it has decided to work perfectly normal. It may work fine for an entire week before it acts up again.

Here's a quick description of what it is doing... If the vehicle is stopped and in gear, and I let off the brake, the vehicle will BARELY creep forward. It's like the output from the transmission has reduced to almost nothing. The motor doesn't act like anything is wrong, and will stall up the converter as normal. At about 2k RPM there is a little surge in the acceleration, but nothing drastic. In higher gears, it also feels like the converter isn't locking up (RPM's are much higher than they should be as well. 2800RPM at 50mph or so, with 3.73 gears.). The converter doesn't stall up higher when throttle is applied, as it should if the converter isn't locked, but if I let off the gas then the RPM's drop, almost as if I had put it in neutral.

I'm not really well versed in transmission scan logs/functions so if someone on here who is, could help me out, I would really appreciate it. Here is an autotap log I took last night with some normal driving. I have removed all but the first time stamp in the excel sheet.

FWIW, the car is a 98 with a Yank trans and Yank 3800 converter.

 

Just updating this for anybody else who may search for it in the future...

After scanning it while it was acting up, it seems like the shift solenoid for 1st and 4th gear is going out. The car takes off in second, shifts to third, and what I thought was the 3-4 shift was actually the TC locking up. The PCM is commanding for it to shift into 1st and 4th, but it just isn't happening.

Hopefully there won't be any material in the trans fluid when we drop the pan.

 

oddly this morning i got a p0751 after trying a new tune. I'll see if it happens on the way home, felt like it was locking up at 30mph i'll have to look at the logs i got to see if it was in 3rd or 4th.

 

If your problem is the same as mine, your scanner will show that the car is in 4th because that's what the PCM is commanding the transmission to shift to. You need to look at the RPM differences before it commands 4th, and the couple of frames after it commands 4th to see if the RPM's drop at all.

If your solenoid is going out, it won't let your tranny shift into 1st OR 4th gear. It controls both, so if one goes, they both go.

 

it was a fluke i guess. hard to say. i cleared it and reflashed and it went away. or so i hope.