2019 GMC Sierra 1500 - 8l90

[LCH14]

Looking for help!

 

2019 GMC Sierra 1500 - 5.3l 4x4 8l90 61k miles - truck is in Arizona, was over 110 outside on Monday when this whole situation transpired. Never had any major issues with transmission until this point:

 

Truck sat outside my construction office for approx 1 hour idling before I realized, ran around the job site wrapping some things up for the day and then left. Went to get food and sat in the drive through for approx 30 minutes idling. As soon as I pull out of the drive through and onto the main road, rear tires lock up and feels like transmission fell out of the truck. Shifted to park, shut the truck off and tried again, same thing happened. Was able to reverse into parking lot (truck slammed into reverse). Let the truck sit turned off for 10 minutes before trying again, but it continued to happen. Towed truck back to my house. Highest the trans temps reached was 198.

 

Friday, after work I start looking into the situation. Truck sat in my driveway untouched all week. Truck is now driving like normal, transmission doesn’t get over 185 degrees when trying to heat it up. Got up to 60 multiples times with no issues, no issues accelerating, only down shifted hard into first one time at 185 degrees. Does anyone have any insight? Never through a code or a check engine light. Truck has never been used to tow either. No transmission services at this point.

[newdude]

1 hour ago, LCH14 said:

Looking for help!

 

2019 GMC Sierra 1500 - 5.3l 4x4 8l90 61k miles - truck is in Arizona, was over 110 outside on Monday when this whole situation transpired. Never had any major issues with transmission until this point:

 

Truck sat outside my construction office for approx 1 hour idling before I realized, ran around the job site wrapping some things up for the day and then left. Went to get food and sat in the drive through for approx 30 minutes idling. As soon as I pull out of the drive through and onto the main road, rear tires lock up and feels like transmission fell out of the truck. Shifted to park, shut the truck off and tried again, same thing happened. Was able to reverse into parking lot (truck slammed into reverse). Let the truck sit turned off for 10 minutes before trying again, but it continued to happen. Towed truck back to my house. Highest the trans temps reached was 198.

 

Friday, after work I start looking into the situation. Truck sat in my driveway untouched all week. Truck is now driving like normal, transmission doesn’t get over 185 degrees when trying to heat it up. Got up to 60 multiples times with no issues, no issues accelerating, only down shifted hard into first one time at 185 degrees. Does anyone have any insight? Never through a code or a check engine light. Truck has never been used to tow either. No transmission services at this point.

 

 

 

I'd change the fluid and start there.

 

198F is nothing for the 8L90.  They typically operate 180-200F stock.  

[Coastie05]

Yea a fluid change is in order here. Honestly, your transmission didn’t even get hot. During the summer in stop and go traffic I’ve seen my 8l80 get up to 216 and it was still shifting smoothly. 

[jl6690506]

Sorry, I accidentally posted in the wrong language. I'll repost it in English.

Edited 59 minutes ago by jl6690506
I use wrong languge

[jl6690506]

198°F isn't unusually hot for an 8L90, so I don't think temperature alone caused the issue.

Since everything returned to normal after the truck sat for several days, I'd be looking at an intermittent electronic or hydraulic control problem before assuming the transmission itself has failed.

The first thing I'd do is scan the Transmission Control Module (TCM), not just the ECM. GM transmissions can store manufacturer-specific or history codes that won't trigger a check engine light.

I'd check:

• TCM history and pending DTCs

• Commanded vs. actual gear

• Line pressure

• Shift solenoid status

• Torque converter clutch (TCC) slip

• Transmission fluid temperature

If it happens again, try to capture the data before cycling the ignition, since some intermittent faults disappear after a restart.

A professional scan tool that supports full GM transmission diagnostics makes this much easier. For example, the Foxwell NT809BT can access the TCM, read manufacturer-specific transmission codes, and display live transmission data that's not available on a basic OBD-II reader:

https://www.foxwelldiag.com/products/foxwell-nt809bt

Hopefully it's just an intermittent control issue, but I'd definitely pull the TCM data before replacing any parts.