The transmission won't go into overdrive if the coolant temperature is too low or too high or if the transmission temperature is too high. A temperature sensor that is reporting an incorrect reading could cause your symptom. An unplugged electrical connector on the transmission will prevent overdrive from turning on too.
Why are you switching overdrive on and off manually? Normally it is left alone. It should only be switched off when pulling a heavy trailer. I heard of someone turning the overdrive off every time she climbed into her van. She misunderstood the instructions from the salesman who sold her the van. What a pleasant surprise she got when her fuel mileage shot way up.
The only thing disconnecting the battery will do is erase the stored fuel trim data that took a long time to develop and memorize. Any diagnostic fault codes in the engine computer's memory will also be lost. Any stored codes should be read and recorded before disconnecting the battery to prevent losing that valuable information.
Your mechanic can connect a hand-held computer to access information from the engine computer. It will show if it is commanding the overdrive to engage. If it is sending the command, but the transmission isn't going into overdrive, there's an internal mechanical problem. If the computer never commands overdrive to turn on, look for a sensor or switch problem.
Tuesday, December 8th, 2009 AT 3:20 AM