97 Tahoe 4x4 trans shifts hard

[mrbrent]

The truck has 155,000 miles on it and I commute 30 miles to work. Generally in the morning I have no issues. In the afternoon when it is hotter after stop and go traffic I then have a strech of about 10 miles that is 60-70 MPH. After I leave the freeway and stop at a signal from that point on it is a real hard shift from 1st to 2nd then less harsh from 2nd to third. If I shut of the vehicle at that light right off the freeway the trans shifts normally (no hard shift) what is causing this?? it has been doing this for quite some time. I had the trans powerflushed and that made no difference.
Thanks

[Jeremy-WI]

Take it to a shop and have the computer scanned for pending codes when you have the harsh shift, they should know how to fix it

[mrbrent]

Will the computer store the codes? It is usually late in the day 6-7 pm when I'm seeing the problem so most shops would be closed. Thats why I'm wondering about storing the code.

[Jeremy-WI]

If your problem is what I think, it may store a pending code- I don't know if many scanners other than what is used by GM(possibly SnapOn) would find pending codes. I think you may have a pending P1870 code(trans component slipping) and most likely it is being caused by a worn valve body passage where the TCC valve sets. A good tranny shop would be a good choice and may get the problem fixed cheaper than at a dealer

[mrbrent]

Does this require removing the trans or just removing the pan. Is that a big job? Just trying to guess the $$$ involved.

[gearpump]

I haven't been on this forum for a year. I got on to ask about my 97 Tahoe shifting hard. I couldn't believe the first post was for the same problem. It does it only when the transmission gets hot (warmed up). It's almost like something is restricting the flow of fluid.

[Jeremy-WI]

It happens most often after the vehicles are warmed up and driven at highway speed. The PCM will detect slippage comparing engine rpm and info from the vehicle speed sensor, once it detects slippage it will raise the fluid pressure in the tranny. The high fluid pressure causes the harsh shift. If it is a worn valve body, the passage allows fluid that should be locking the torque converter clutch to slip past allowing the torque converter clutch to slip. You don't have to drop the tranny out to fix, in most cases it involves a new valve and to bore out the valve body passage. Quite a few would recommend higher mileage vehicles to get a total rebuild rather than just fix the valve body because a lot of other parts would have quite a bit of wear

[mrbrent]

Thanks, That is great information. Do you know why turning off the engine and restarting makes it go away. Is it due to the trany going back to a lower pressure after restarting?

[Jeremy-WI]

I think it needs to slip in two consecutive TCC cycles or something in one igntion cycle to set a permanent code and light the SES, when you shut the ignition off, it kind of temporarily resets itself. I don't think you would have the behavior if you drove in drive(3) instead of overdrive either but the problem is still there