My 2020 3500 HO diesel with the aisin tranny is the worst vehicle I have ever owned nothing but problem since 450klms 32000klms now and the vehicle has been in and out of the shop for weeks and weeks . The transmission doesn't engage into first gear just out of the blue randomly just won't move i was almost seriously t-boned yesterday the truck stopped just in the nick of time. its like the transmission drops into neutral when I come to a stop and try to drive again the motor reves up and then it makes a loud bang and drives also does this randomly while driving . Also violently downshifts at 60klms causing the seat belts to lock and the truck feels like you have slammed on the brakes
I have the same symptom's that you described here above, and I believe that soon or later any other Aisin owner will experience the same.
The Aisin transmission is reacting to gear changes a lot slower than the 68rfe under a stop and go condition (I had a 3500 with a 68rfe for 3 years prior of the Aisin), consequently it will fall into a false neutral for a few seconds especially under conditions like: you're climbing a hill and there is a stop sign right in the middle (assuming that you're making a full stop), or you're in a hurry and you're hard braking to a full stop and then you're immediately applying the gas pedal........ The Aisin won't perform well like this, at all.
Eventually I did adjust my driving behavior to predict those occurrence's by simply coming to a full stop at a much slower speed, and wait for a couple of seconds before accelerating. Actually, if, while you're stopped (under those fast stops and go circumstances) and you do release lightly the brake pedal: you can sense the torque converted starting to grab and then you will know that you're engaged on first gear, and from that point you can just floor it if you wish to do so without any 1st gear slamming consequences.
There's always a possibility that you have a defective transmission, and I hope you don't have.
I thought that I had a defective tranny the first time it happened to me, by adjusting my driving behavior (against my own principles) it took care of the problem instantly.
