8L90 1,2 Shift Issue

[MobileHomie]

2017 Yukon Denali 6.2 with 8L90.
This morning the 1,2 shift, 1st goes kind of long then goes to nuteral, this lets the engine rev up then bangs into 2nd. Then I put in tow mode and it acted much better.
This was at low speed, very gentle on the throttle, the the engine and trans temps very low.
Maintenance history on the rig, I bought it used with 32,000 miles in 2020, had the transmission serviced around 36,000 at a dealership. 5k oil changes, at 63k lifter failure lead to an AFM delete, stage one cam and I had that shop service the transmission again while the engine was out. The rig now has 83,700.
With the different action in tow mode I wonder if it may be software related?

 

[B-train]

Could be, the line pressure was probably increased for tow/haul. When was it serviced last? I'd recommend a drop the pan, replace the filter, and fill with new fluid. Has it had the TSB fluid flush performed? If it's mostly the original fluid in the pan, you may have issues due to that. There is an updated fluid for those.

 

[Joseph Garcia]

@NickTransmissions

 

[NickTransmissions]

2017 Yukon Denali 6.2 with 8L90.
This morning the 1,2 shift, 1st goes kind of long then goes to nuteral, this lets the engine rev up then bangs into 2nd. Then I put in tow mode and it acted much better.
This was at low speed, very gentle on the throttle, the the engine and trans temps very low.
Maintenance history on the rig, I bought it used with 32,000 miles in 2020, had the transmission serviced around 36,000 at a dealership. 5k oil changes, at 63k lifter failure lead to an AFM delete, stage one cam and I had that shop service the transmission again while the engine was out. The rig now has 83,700.
With the different action in tow mode I wonder if it may be software related?

I saw this yesterday; I'm not familiar with 8-speeds as I haven't started working on those yet. I believe Tow-Haul mode raises shift points to keep you in gear so as to maintain higher engine RPMs and circuit line pressure all of which is intended to prevent slipping when towing a heavy load. That would explain why Tow-Haul mode is producing better shifting. Long 1-2 shift could be any number of things so my next step would be to plug in a two-way scan tool and look at shift speeds, TCC information as well as TPS, etc. this may give you a starting point, especially if one or more transmission PIDs looks way out of wack.

If you don't have a two-way scanner nor access to one, I'd take it back to the dealer where you had it serviced last so they can diagnose it.

 

[MobileHomie]

Thank you Nick! The dealer is a long way away for diagnostics. I am not sure if it's two way, I have a innova 5510 scanner and have no idea how to use it. I am not opposed to buying one though.
The good thing is the condition did not repeat today!

 

[NickTransmissions]

Thank you Nick! The dealer is a long way away for diagnostics. I am not sure if it's two way, I have a innova 5510 scanner and have no idea how to use it. I am not opposed to buying one though.
The good thing is the condition did not repeat today!

Look up the scanner's specs/features on the Innova website and/or watch some YT vids. It's a fairly popular scan tool and Innovas are sold at every Autozone so I'd imagine you can get the info you need...

 

[MobileHomie]

Looks like I'm in the market for a good bi-directional scan tool.
Anyone have suggestions for one I can grow into?