Shift Stabilization/Tow/Haul Mode Feature

This article has been updated with bulletin No: 99-07-30-022A.

SHIFT STABILIZATION AND TOW/HAUL MODE FEATURE FOR HYDRA-MATIC 4L60-E/4L65-E AND 4L80-E/4L85-E TRANSMISSIONS

PURPOSE
^ 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 delay 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 4L60-E for only 2-3 and 3-4 shifts and in the 4L80-E for 1-2, 2-3, and 3-4 shifts.

TOW/HAUL MODE

PURPOSE
^ Reduce the frequency and improve the predictability of transmission shifts when pulling a heavy trailer or a large or heavy load. This is most apparent in city traffic. Without tow/haul, the transmission may upshift on a closed throttle decel. With tow/haul, the transmission will hold the current gear on a closed throttle decel. This reduces upshifts and downshifts.
^ Provide the same solid shift feel when pulling a heavy trailer or a large or heavy load as when the vehicle is unloaded.
^ Improve control of vehicle speed while requiring less throttle pedal activity when pulling a heavy trailer or a large or heavy load.

ACTIVATION
^ Tow/Haul is selected or de-selected via a switch on the end of the transmission shift lever for all models except the GMC Jimmy and Envoy, their switch is mounted on the shift console. 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, this situation will cause a firm shift. 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. Tow/haul is recommend only when pulling a heavy trailer or a large or heavy load.
^ Examples of altered shift speeds when in tow/haul:
- Minimum 1-2 shifts pushed out approximately 13 km/h (8 mph)
- Minimum 2-3 shifts pushed out approximately 24 km/h (15 mph)
- Minimum 3-4 shifts pushed out approximately 48 km/h (30 mph)
- Tow/haul only affects shift points below 88 km/h (55 mph).