2000 Chevy Silverado Hard Shifting

We used a match book to set the points and adjusted the carb by ear, but those days are long gone. Enough for nostalgia, on to your problem. When you get back down to a lower altitude does it still shift normally or is there another "adjustment" period? If you had something wrong internally with the tranny, altitude wouldn't make any difference. Let me know on the above and in the interum, I'll do a little research. I've heard of this before, but can't remember what caused it.

Charles, a couple more questions. When the shifting problem exists, does it shift hard into all gears or just 1-2 or 3-4 etc? Also, is down shifting ok, or is that also hard?

XTooltipElement
Service Information 2000 Chevrolet Chevy K Silverado - 4WD | Sierra, Silverado, Suburban, Tahoe, Yukon (VIN C/K) Service Manual | Transmission/Transaxle | Preliminary Information | Document ID: 1341281

#99-07-30-022A: Info - Shift Stabilization and Tow/haul Mode Feature for Hydra-Matic 4L60-E/4L65-E and 4L80-E/4L85-E Transmissions - (Jun 5, 2003)

Subject: Shift Stabilization and Tow/haul Mode Feature for Hydra-Matic 4L60-E/4L65-E and 4L80-E/4L85-E Transmissions

Models: 1999-2000 Cadillac Escalade

2002-2004 Cadillac Escalade, Escalade EXT

2003-2004 Cadillac Escalade ESV

1999-2004 Chevrolet Astro, Blazer, Express, S-10, Silverado

2000-2004 Chevrolet Suburban, Tahoe

2002-2004 Chevrolet Avalanche, TrailBlazer

1999-2004 GMC Envoy, Jimmy, Safari, Savana, Sierra, Sonoma, Suburban, Yukon

2000-2004 GMC Yukon, Yukon XL

2001-2004 GMC Yukon Denali, Yukon Denali XL

2003-2004 GMC Sierra Denali

1999-2004 Oldsmobile Bravada

2003-2004 HUMMER H2

with Hydra-Matic 4L60-E/4L65-E (RPO M30/M32) or 4L80-E/4L85-E (RPO MT1/MN8) Automatic Transmission

and 4.2L, 4.3L, 4.8L, 5.0L, 5.3L, 5.7L, 6.0L or 6.5L Engine (VINs W, V, M, T, R, U, N, F -- RPOs LL8, L35, LR4, L30, LM7, L31, LQ4, LQ9, L65)

This bulletin is being revised to add model years and applications. Please discard Corporate Bulletin 99-07-30-022 (Section 07 - Transmission/Transaxle).

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
"Â 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.

"Â 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
"Â This feature is internal to the vehicle's software/calibration and requires no customer action to activate.

"Â Shift Stabilization is effective in normal mode, cruise mode, and in Tow/haul mode.

FUNCTION
"Â 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.

"Â High throttle opening will disable Shift Stabilization and normal downshifts will occur.

"Â 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
Applies to the following models:

"Â 2000-2004 C/K Utility Models

"Â 1999-2004 Chevrolet and GMC C/K Pickup Models (Silverado and Sierra)

"Â 1999-2002 GMC S/T Utility Models (Jimmy and Envoy-Old Style)

PURPOSE
"Â 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.

"Â Provide the same solid shift feel when pulling a heavy trailer or a large or heavy load as when the vehicle is unloaded.

"Â Improve control of vehicle speed while requiring less throttle pedal activity when pulling a heavy trailer or a large or heavy load.

ACTIVATION
"Â 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.

"Â Tow/Haul must be reselected every time the vehicle is started (if desired).

FUNCTION
"Â 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).

"Â 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.

"Â 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.

"Â 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).

Refer to vehicle Owner's Manual for further information regarding tow/haul mode.

GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.

WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION

© 2008 General Motors Corporation. All rights reserved.

With the altitude change, there is a change in engine loaddue to the "thinner" air.

Xxxxxxxx