6l80 transmission - Manually shift will not down shift.

Hi All,

Hopefully this is a fairly general question that some more experienced persons on this platform can answer. I have a friend who recently got their 2009 c6 corvette tuned. Prior to this the transmission was rebuilt however the car was not run at all between rebuild and startup and tune. It's been tuned with HP tuners.

Since getting it back, the car will manually shift up gears, but when commanded to go down gears the dash will display an X and the trans will not shift down. They have checked wiring to the connector to the trans and it appears fine, the up shifts and "X" on the heads-up display would also confirm this. When in auto mode (Drive) it drives ok, except it changes from 3rd to 4th quickly (too early) and it doesn't kick down when you plant it. When a scan tool is plugged in the tool displays the commanded gear but will not actually change down mechanically.

For those more experienced, and possibly US based. What are the basic things to check in this instance? My assumption is it's a speed reference issue where a speed sensor is either not working or has been calibrated incorrectly during a tuning phase? Or perhaps the TCM inside the trans has gone bad?

Any help appreciated. If this is not in the right section, or not applicable then please move as required.

It doesn't seem to be a common issue from a quick GOOGLE search - does it kick down under hard throttle?

Hi Gord,

Thanks for the reply, google didn't seem to help me either. Although I did learn more about the platform.

In Drive/Auto the car will change down when the car rolls to a stop. It doesn't kick down like you would expect an auto to usually. It will only go as far as second, even when the vehicle is stationary.

Update: The car has since been scanned again and the shift solenoids appear to be holding pressure when they are being released. This might not be the problem, but the valve body was not reconditioned at the time the trans was. the scan suggested that the TCM was fine as the wiring and commands. The TCM and solenoids are now out of the car and the owner is determining to rebuild/replace currently.

In paddle shift, it will only go up. It won't go down until the car is put into drive/auto mode.

Thanks for the reply.