So now you know it's not the solenoid, common mistake. I presume you are getting code PO755? There are a few possible causes in addition to the shift solenoid; a wiring circuit issue, an internal fluid flow blockage, badly contaminated or low fluid level, an actual mechanical issue with the transmission, a bad pcm/tcm. A complete trans fluid flush could take care of your problem, how many miles are on it?

 

 

We have a term for replacing a computer senor or actuator without any testing, we call it throwing parts at the problem. On newer cars throwing parts at a car has a better chance of success because the wiring is newer and less likely to have problems. As a car ages the chance of a broken wire , bad ground or failed connector goes up. There is no way to short cut the diagnose tree in the manual, (you may find the manual in the manual section on this sight.) Making assumptions about what is and what is not wrong is a mistake. When trouble shooting a problem try not to have preconceived known good or bad just follow the tree. As a GM tech I was called in to help with problem cars and I have seen trained techs make assumption and cause themselves to run in circle. Example, You replaced the solenoid so you assume that is not the problem. WRONG, defective parts off the self is not as uncommon as you think even from GM, aftermarket is 10 time more likely to have a defect. As a tech I use to keep know good parts for testing issues that did not set hard codes. Once I knew by my known good part that the old parts was the issue I would get a new part from the parts department then make sure that the new part also worked.

Let look at your assumption the issue is not the trans because there is no sign of heavy wear. Many transmission failures are instant, this can be cause by a stuck valve or a torn seal, even metal failure may not leave anything in the pan if a weld is broke or something sheared clean but seals would be the most common, it could be as simple a the seal in the accumulator. That is a $4 parts that take 15 minutes to replace or it can be the seal in the clutch pack which means rebuild time.

Open loop, LHM will not cause the PCM to not try to shift. I will effect shift points and how hard it shifts. There is one thing I would do quickly, I would clear the code and see when it resets. If it resets instantly it is most likely a electrical issue, in my opinion an easy one to find because it is instant it would indicate a broken or shorted connection/wire or PCM failure. If it sets the first time it tries to shift into gear then it could be hardware/trans related but you still want to start with the manual trouble tree.

 

 

 

Did you clear code and see if it returned instantly? I hope you are not looking for someone to type what the manual says. You might get someone to post a few pages of the manual if you can not find one.

 

 

GM shop repair manual, the same one the techs use. There are some PDFs located in the manual section at the bottom of this main page. A codes have a tree, Basic its a bunch of if then statements. If you drive it is it working now?