Looking for help from the auto trans guys (99 silverado)

[GMC Truck]

My daily driver started something weird last night... coming home from work noticed a strange vibration in the drivetrain, seemed to be mid to rear. When rolling up to stop sign truck "lunged" like it was having trouble coming out of a gear? When I went to take off from stop sign it shifted so hard from 1st to 2nd I though it would snap a u-joint! It did that through 3 more stop signs until I stopped to check it out. Well after stopping (shutting it off) and then going again all seems to be well again and the problem has yet to return. Trans was serviced several years ago and truck has 144-thousand on it. I'm not getting any "check engine" light.
Doing a brief goggle search it seems that there is a trans kit for this very problem. Looking for input of suggestions from a trans guy's perspective on what this could be and possible fix??

 

[76zimmer]

PM Greg Ducato!

 

[Blue85]

That harsh shift is probably the tranny in limp mode. When certain faults pop up it will just go to full line pressure, which is often described as "brutal 1-2 shift". Even if you don't have a lamp, it probably logged a transmission code. If it doesn't happen again for a while the code will eventually go away, so it would be good to find an OBDII scanner. I think Autozone will do that for free.

The check engine light is mostly about emissions issues, not every code will light it.

 

[Greg Ducato]

I'm guessing you are going to come up with a code 1870, trans component slippage when you have the computer scanned. In most instances this is caused by the TCC regulator valve wearing the valve body bore in such a way that it allows fluid los to the point of converter clutch slippage. It doesn't take much slippage for the computer to detect and when it does, it's response is to icrease line pressure in the trans. The result of the line pressure increase is the harsher shifts and changed rpm points. Shutting the engine of and cycling the key will clear the code but if it comes back enough times it will be stored and may throw a check engine light.
There are some instances where a dirty mass air flow screeen will cause the same harsh shifts due to the computer not comprehending, it may be a good idea to spray clean that first since it is easy to do and gauge results.
If it is in the trans it can be fixed by removing the valve body, reaming the TCC bore oversized and replacing the TCC regulator valve assembly. This is usually a less than $500 repair depending on the shop and will also include pan service of the trans. Left unrepaired, the harsh shift will take out the splines in the reaction sun shell, leaving you without reverse, second or fourth gears. Get the repairs done as soon as you can afford to in order to save yourself from a much larger bill.