Not shifting when hot

Hi Galvy,

Usually, this is fluid related. Not necessarily low but just poor condition. It could be fixed just by changing the fluid but if it is too far gone, then there is damage done to the clutches, pump, and torque converter. If this is the case then it will either need to be rebuilt or replaced.

There is a chance that it is something on the control side of the transmission but there would most likely be DTC's or diagnostic trouble codes in the computer. Can you pull the codes or have someone pull them? Most parts stores will scan your computers for you free of charge.

If you can get those DTC's that would be great. In the meantime, check the fluid level and condition.

If you change your fluid, it takes eleven 1/4 quarts but most likely this is for a rebuild. A drain and fill will probably only take five to six. Just put three back in and then start checking it.

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

Here are the tests for these codes. The P1759 and P1769 appear to be in response to the P0720. So I would start with this one and once that is corrected see if the other two are gone.

The P0720 is a vehicle speed sensor fault. So as I said above this would be considered the control side and it appears your speed sensor has failed or possibly the wiring. Once the TCM is getting a valid speed sensor again, it will begin to allow the transmission to shift. The fact that this happens when it gets hot, point to the the sensor itself.

If you have a scan tool to monitor your speed sensor, just drive the vehicle so it gets hot and begins to fail and then see what the speed sensor/revolution sensor is doing. I would guess that it is acting erratic and either jumping around or dropping to 0. This is causing the TCM to not know what the speed is, so it doesn't allow it to shift because it doesn't know which gear is best.

I also included what appears to be the OEM part number and the location. Looks like this is inside the transmission pan and attached to the bottom of the valve body.

Let me know what you find and we can go from there.