2017 Sierra 8L90 fast learn, few min in it slowly revs to 6k RPM before I hit the key

[Dominatorstang]

Curious if anyone else had run into this. The transmission has been fine and the TCM is still locked with factory tune. The L86 engine has mild modifications and a tune that have been working fine.

I was just doing a few software updates on other components with a VX Diag and Techline Connect. Came down to the transmission setup to see if it had any new calibrations, but the only options it gave me in reprogram was asking which component I replaced, so I left it alone.

Going into Setup for the transmission, the first thing it wanted to do was the fast learn. I did the normal prep of parking brake, put in drive, holding brake. I feel transmission doing the normal thing of applying and dropping out different clutches at RPM's around idle. Then it revs out to 1500 or so for a sec, then slowly revs further and I get nervous, so I watch closely as it takes about 5 more seconds slowly revving past 5k. I flip the ignition off just before 6k and say nope I am done.

[Dominatorstang]

I am still not sure what caused this to happen on that day, but about a week ago I was able to complete a fast learn using the HPT scanner function without any odd things happening.

[IARLLC]

I'm glad to hear that it made it through the process OK later. I had a client add a cooler to his 2017 L86/8L90E Silverado High Country rally truck. I'm not sure what he got the cooler out of but it had its own thermostat....and he installed it the wrong way. Within a few hours of the beginning of the rally he got the converter so hot that it cooked some poorly added wires across the top of the trans. Those wires melted and shorted all of the other wires they touched back there. His boss begged me to leave the 2 trucks that I was responsible for to help put him back on the trail/desert. WHAT A MESS! I got it running but the trans acted weird. Removed the thermostat from the cooler...a little better. I tried to run a fastlearn. Imagine a desert rally with over 300 drivers and crew trying to sleep in their tents in the middle of the night while this open exhaust L86 grunting through the gears...then it did what yours did...rpm went up and up and up. Between the laptop, harness, aftermarket steering wheel, window net in my face, very great concern that everybody would kill me for the noise....then BANG!!! I did not get to the key before it engaged the gear like a high rpm neutral-drop. None of us could believe that it did not take out a u-joint or rearend. After a few more hours I tried again and again that gear did not engage. Maybe the guy should not be allowed to work on cars but I gotta hand it to him for finishing 5 days/1250km in deep sand, sharp rock cliffs, etc in 3rd gear

Good job catching it before it went BANG!