Personally, I'm not a fan of flushing it. I've seen it cause problems ESPECIALLY on trannies with some age on them that were not flushed routinely.

I drop the pan and let as much come out as I can get then do a re-fill. YES I understand you don't get a lot of fluid out but then I do this at 30k miles regularly, so EVERY TIME you do it your ratio of newer fluid to old is stronger.

I've been doing this for over 30 years and to date have NEVER had a transmission problem in any vehicle I've ever owned...

 

Key trick never worked for me either tbh.

 

sounds like the speed sensor, same happened to my 2006. be careful driving it, the truck will shut off if you slam on the brakes hard enough since the truck doesnt think your moving. the reason why your tranny works is a separate computer (this hit me one day thinking why was it still shifting properly). the best part about the speed sensor going out? no added mileage! lol only did a couple hundred miles before finally getting it fixed. the tranny flush didnt help me, but didnt make it worse either. its an easy fix and should fix most of your problems

 

I would have to say something is up with either your computer malfunctioning, or a lot of things gone wrong due to lack of maintenance. Possibly jumper cables hooked up backwards or broken insulation on a wire causing it to ground out. Something's up and I highly suspect it's some kind of wiring/computer issue due to the key trick not working. Even when my battery is bout dead, I can do the key trick in all of the chrystler vehicles I've ever worked on.

 

Key trick doesn't work for me either . Clean all batt / ground conections is always a good place to start , good luck