| Subject: | Shift Stabilization and Tow/Haul Mode Feature for Hydra-Matic® 6L80/6L90 Automatic Transmissions |
| Models: | 2007-2009 Cadillac Escalade, Escalade ESV, Escalade EXT |
| 2007-2009 Chevrolet Avalanche, Silverado, Suburban, Tahoe |
| 2007-2009 GMC Sierra, Sierra Denali, Yukon, Yukon Denali, Yukon XL Denali |
| with 6L80 (MYC) or 6L90 (MYD) Automatic Transmission |
Bulletin Content
This bulletin contains information regarding Shift Stabilization and Tow/Haul mode. These features affect transmission shift patterns and the customer's perception of shift feel. Customers may notice a change in shift patterns between previously owned
vehicles without shift stabilization and current vehicles with shift stabilization.
Shift Stabilization
Applies to all above listed models.
PURPOSE
| • | Some customers may comment that upshifts are delayed or not available when in fact the transmission is operating normally with a feature known as "shift stabilization." This may contribute to "cannot duplicate/no trouble found."
It is important that the customer be educated about this feature if this occurs. |
| • | The purpose of shift stabilization is to reduce shift busyness during conditions that would otherwise produce frequent upshifts and downshifts, such as trailering, operating on grades or at higher altitudes. |
| • | Shift stabilization is an algorithm in a vehicle's software/calibration that determines if and when to prevent upshifts under load. Shift stabilization only affects upshifts and not downshifts. |
ACTIVATION
| • | This feature is internal to the vehicle's software/calibration and requires no customer action to activate. |
| • | Shift stabilization is effective in normal mode, cruise mode, and in tow/haul mode. |
FUNCTION
| • | Shift stabilization calculates the required torque at the wheels in the current gear as well as the maximum torque available at the wheels in the next higher gear. If the torque in the higher gear is not sufficient, the transmission will remain in
the current gear. If the torque in the higher gear is greater than or equal to the required torque, than the upshift is allowed. |
| • | High throttle opening will disable shift stabilization and normal downshifts will occur. |
| • | Shift stabilization occurs in the 6L80/6L90 for only 2-3, 3-4, 4-5, and 5-6 upshifts. |
| • | Shift stabilization might be disengaged in the following situations: |
| • | During cruise control operation when the "decrease" button is tapped or held, shift stabilization may be disengaged. |
| • | If a significant "bump" in the road is encountered, shift stabilization may be disengaged. |
Tow/Haul Mode
Applies to all models listed above:
PURPOSE
| • | It is important to note that the customer may accidentally engage the tow/haul button and then comment about delayed or harsh shifts when not pulling a trailer or under heavy load. Normal function of tow/haul mode when used in inappropriate conditions
may contribute to complaints that "cannot duplicate/no trouble found." |
| • | Tow /Haul mode will optimize gear selection for operation with heavy loads (such as towing): |
| • | optimize acceleration performance for launches and rolling accelerations |
| • | optimize braking performance and speed control during engine braking situations |
| • | improve vehicle speed control while requiring less throttle pedal activity |
| • | Because tow/haul mode maintains lower gears longer and increases shift pressures, it will degrade fuel economy and generate shift feel concerns if it is used inappropriately in unloaded driving conditions. |
ACTIVATION
| • | Tow/Haul is selected or de-selected via a switch on the end of the transmission shift lever for all models. A light on the instrument panel will illuminate to indicate that tow/haul has been selected. |
| • | Tow/Haul must be reselected every time the vehicle is started (if desired). |
FUNCTION
| • | Tow/Haul is designed to be most effective when the vehicle and trailer combined weight is at least 75% of the vehicle's Gross Combined Weight Rating (GCWR). |
| • | Operation of tow/haul in a lightly loaded or non-loaded vehicle will not cause damage. However, there is no benefit to the selection of tow/haul when the vehicle is unloaded. The tow/haul switch is not a performance switch. |
| • | Selection of tow/haul when unloaded may result in unpleasant engine and transmission driving characteristics and reduced fuel economy, since upshifts occur at higher engine speeds. Tow/Haul is recommend only when pulling a heavy trailer or a large
or heavy load. |
| • | Automatic downshifting (Normal - Powertrain Braking) to assist speed control will activate when Tow/Haul mode is active and the brake pedal is depressed (even lightly). Downshifts are commanded at higher speeds than normal resulting in engine braking
that assists with speed control. After a powertrain braking situation ends, upshifts will typically be commanded when the accelerator pedal is pushed down. (a quick increase in accel pedal may be required) |
| • | Starting in the 2009 model year, a similar function is available during cruise control operation (in Tow/Haul only). Automatic downshifting (Cruise - Powertrain Braking) to assist in speed control will activate when vehicle speed has increased from
the set point by a certain amount. Multiple downshift may be commanded depending on the amount of deviation of vehicle speed to the set speed. As vehicle speed slows and approaches the original set speed upshifts will be commanded to return to a normal operating
condition. |
| • | At some speeds and on some downhill grades, the transmission may not be able to command a gear low enough to slow the vehicle to the set point without overspeeding the engine. |
| • | In this case the transmission will command all available downshifts but if further engine braking is desired the driver must press the brake pedal to slow the vehicle enough so additional downshift(s) can be commanded. |
| • | Pressing the brake pedal has the normal effect of cancelling cruise control operation and in this case a transition to Normal - Powertrain Braking is completed. |
| • | As always, if cruise control operation was cancelled through usage of the brake pedal, it must be reinitiated by tapping either the Set or Resume Cruise switches. |
Refer to vehicle Owner's Manual for further information regarding tow/haul mode.
