Hello my friends. I'm new to the mustang source and excited to hear from other mustang enthusiasts willing to share their mustang experiences.

I recently replaced the 5r55s transmission in my 2008 V6 Mustang. I ordered it through AutoZone as the price was OK and it has a good warranty. Took it to work and all worked fine going to work. Coming home going down the interstate with the gas petal constant, the transmission down shifted all by itself from 5th to 4th gear for no reason. A few seconds later the O/D light came on flashing. I immediately pulled over and shut down the car. Waited a few minutes, restarted the car and resumed down the road. All was OK then the car down shifts again, this time no O/D light. The transmission shifts fine going up to 5th gear but form some reason it does not want to stay there very long. Got it home, checked the fluid level, it was good.

Hint - The transmission seems to have no issues when its cold. This seems to happen after the fluid gets to temp.

Notes:
- No codes on the computer were registered.
- I flushed out the trams cooler when I replaced the transmission.

Possible self induced issues.
1. Linkage issue ?

Other possibilities
1. Possible torque convertor issue ?

 

Quick update....
Diagnostics showed a bad engine speed sensor. Replaced sensor. All is well!

 

Glad you were able to get this thing fixed.

Also Welcome to TMS!