Transmission won't shift into overdrive anymore.

olderuser53

Had transmission replaced and currently have 100,000 miles on it. Worked fine for about 50,000 miles and now won't go into overdrive. I still get 20 to 23 mpg, but used to get 23 to 28 mpg. Have had some suggest it is transmission, some say computer and some say don't know.

imidazol97

You have 100,000 miles on the current transmission as I read the post? And you are not complaining about rough operation or slipping of the transmission.

How does the fluid look? Has the filter been changed? If the fluid is original, I'd suggest dropping the pan and changing the fluid and refilling to the warm level with the engine hot.


I am not an expert on whatever transmission is in that. I read lots of posts about 4T65 and 4T65E in leSabres starting with 1993.

I believe a car won't go into overdrive if the engine coolant sensor is seeing, or thinks it is seeing, an engine below a certain operating temperature. It may also keep the torque converter from locking up in 3rd and in 4th (overdrive).

Also there may be a transmission code keeping it from completely operating into 4th. I don't know if some of the code readers at the box stores are complete enough to read transmission codes or not.

But I would see if they would read for codes and if their scanner does more than scan codes AND can read the engine temperature from whichever sensor the computer uses. There may be more than one place the engine temperature gets reads the engine temperature--one for a gauge and one for the actual powertrain computer.

The engine may be getting to full temp but the sensor may be reading lower.

I believe in case of overheating the 4th gear and/or torque converter lockup are blocked to make the engine turn faster to promote coolant flow for cooling.

The upper radiator hose should be too hot to hold after a few seconds for most people's hands unless they are very used to handling hot things...