Harsh Shifting on the 8L90 Automatic Transmission

There may be several harsh shifting conditions on some 2015-2016 Silverado, Sierra, Yukon and Escalade models equipped with the 5.3L engine (RPO L83) or 6.2L engine (RPO L86) and the 8L90 automatic transmission (RPOs M5U, M5X). The shift conditions may include:

• Harsh 1-2 upshift (except for the first 1-2 upshift of the day)
• Harsh 3-1 downshift when de-accelerating to a stop
• Harsh downshift under heavy throttle apply
• Active Fuel Management (AFM) V4 to V8 transition harshness (6.2L engine only)
• Coast-down downshifts (6.2L engine only)

 

There is new Engine Control Module (ECM) and/or Transmission Control Module (TCM) software that is available to improve these conditions. Use the Service Programming System (SPS) and follow the on-screen instructions to complete the programming. If both controller options are listed in SPS, select K20/K71 Transmission Control Module.

 

Do not install this calibration if the vehicle is not exhibiting these conditions. It is not intended to be installed on these vehicles for any other conditions.

 

TIP: Installation of the new TCM software will require that the Transmission Service Fast Learn (SFL) procedure be performed using GDS 2 or SPS through the TIS2Web application. The transmission may exhibit poor shift quality until the clutch values are learned. Performing the SFL will reset and relearn all the shift adapts.

 

After the programming of the TCM (Fig. 16) is complete, evaluate the shift quality of the transmission. Refer to Bulletin #16-NA-019 for more information about the transmission adaptive functions, how to learn the clutches and improve shift quality.

 

TIP: Due to the installation of the new TCM software, Bulletin #14-07-30-001 should not be used on 2015 model year vehicles. It will not allow for proper clutch learning.

 

F16 TCM

Fig. 16

 

Transmission Shift Adapts

 

The 8-Speed automatic transmission (Fig. 17) uses a line pressure and volume control system during upshifts to compensate for new transmission build variation as well as the normal wear of transmission components. The variation from new and normal wear of the apply components within the transmission over time can cause shift time (the time required to apply a clutch) to be longer or shorter than desired.

 

2015 Hydra-Matic 8L90 (M5U) Eight Speed RWD Automatic Transaxle

Fig. 17

 

In order to compensate for these changes, the TCM adjusts the pressure commands to the various pressure control (PC) solenoids to maintain the originally calibrated shift timing. Referred to as “adaptive learning,” this adjusting process ensures a consistent shift feel and increases transmission durability. Transmission adapts can be reset and relearned using the Transmission Service Fast Learn procedure that is completed in the service stall.

 

When the Service Fast Learn is complete, perform a test drive and note any soft or harsh shifts. Within GDS 2, a Transmission Service Fast Learn Data page is available to aid in performing adaptive learning by showing throttle percentage, engine speed, transmission fluid temperature, and gear command.

 

– Thanks to Matt Bunting and Dave Peacy

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