Pressure tests will determine why it's slipping, but unless the transmission expert says otherwise, I doubt you have an electrical or computer problem. The solenoids are switched on or off to open and close valves. There's no in between state or partial opening. If a solenoid fails to operate or sticks, the transmission fluid will flow to the wrong clutch pack and the transmission won't shift to the proper gear but it won't slip. Slippage is almost always due to worn clutch plates or leaking seals that prevent sufficient pressure to build up to apply the pistons and squeeze the clutch plates together. With all the transmissions I'm familiar with, computer-controlled or the older hydraulically-controlled, there's no valve switching combination that can leave multiple clutch packs released when it's in reverse or a forward gear.
Your original description about not going into 3rd or 4th gear when warm COULD be due to a solenoid problem, but that is not slipping. Maybe just the terminology is inaccurate. That can also be caused by a sensor problem. Chrysler transmissions, for example, default to second gear, called "limp mode" that allows the vehicle to be driven slowly to a repair shop without needing a tow truck. I haven't heard whether GM uses the same strategy, but if they do, it should shift properly again immediately after stopping and restarting the engine. That will reset the computer.
Besides the pressure tests, your mechanic should be able to watch what's happening with a scanner that displays live data during a test drive. If the proper combination of solenoids aren't being activated at the right times, that rules out a transmission internal problem. If the computer is requesting 3rd gear by switching on a combination of solenoids but the transmission doesn't respond appropriately, that would point to a solenoid problem. Wiring shouldn't be the cause because the computer will detect that and set a fault code.
You also noticed the problem occurs when it's warm. That DOES agree with how a computer problem commonly acts, but you might want to observe how this happens. If you have a computer problem, it would be typical for it to inappropriately downshift out of 3rd or 4th gear while driving at a steady speed on the highway. If there's a valve problem, it will typically fail to up-shift out of 2nd gear as you accelerate. That is not "slipping", and will be electrical in nature. The transmission specialist should be able to narrow down the cause by using the scanner to command the various solenoids on and off through the computer.
Sunday, March 25th, 2012 AT 7:29 AM