2001 Yukon XL K2500 with 135,000 miles bucks into low gears only after running for a while and only when transmission oil temperature exceeds 150 degrees. Runs fine all other times. Any thoughts as to why this is so? Had tranny and gearcase oil changed and filter changed about 10,000 miles ago.
Is that 150 degrees Fahrenheit or Centigrade? The transmission has a thermistor in the oil pan that monitors the temperature of the fluid. If the PCM senses that the transmission oil is too hot, it will revert to full pressure shifts. The problem is likely a bad sensor but it could also be that the fluid is overheating. If you can get the Yukon to a transmission shop when the problem has surfaced, they may be able to pull the code and zero in on the problem. Don’t shut the engine off before they can scan the computer.
Hope that helps.
We have had some issues with the shift solenoids in these transmissions so I would recommend you getting the computer scanned for any codes. Very doubtful you are going to have any issues with the temp sensors because if they are not reading properly the computer will disable lockup and even O/D in some vehicles. From what you describe, it doesnt seem like thats happening. Have your local auto parts store scan the computer for codes and let me know what you find out.
transman