New Calibration Info for Allison MW7 6-Speed Automatic Transmission Overheat Condition in Cold Ambient Temperatures (Less Than -40C, -40F) on Vehicles w/Aftermarket Sump Heater, 6.6L Duramax Diesel

Some customers, in colder climate areas, may elect to have an aftermarket transmission sump heater installed to assist in vehicle cold start warm up. Combining a transmission sump heater with systems incorporated in the vehicle to enhance the heater performance may cause the transmission to overheat/fail when the vehicle is started and left idling in the elevated idle mode.

To address this issue, a new transmission calibration has been released to alter the operating temperature range of the transmission component of elevated idle when a vehicle is equipped with a transmission sump heater. The calibration is made available on a Customer Pay basis as it is required based on the usage of an aftermarket automatic transmission sump heater. See Calibration Information Below.

A customer letter will be sent to all customers of record in cold climate areas to make them aware of this condition and a copy is included for your information. The following areas in the United States and Canada have temperature ranges that meet the criteria of temperatures less than -40°C (-40°F):

Please provide a copy of this information to customers of new vehicles built within the VIN range above that are purchased after the publication of this bulletin.

An updated TCM calibration is now available to address this concern. This calibration, or any that follow, is designed to address this concern. Refer to Service Programming System (SPS) using the appropriate Service Information (SI) procedures. The new TCM calibrations will be available to dealerships in the following TIS2WEB version (website version of TIS):

2006 and 2007 vehicles - TIS2WEB version 12 for 2006. Available now.

If you cannot access the calibration, call the Techline Customer Support Center at 1-800-828-6860 (English) or 1-800-503-3222 (French) and it will be provided. As always, make sure your Tech 2® is updated with the latest software version.

December, 2006

Dear Valued General Motors Customer:

Subject:

Added after market transmission sump heaters used in:

Equipped with 6.6L Duramax™ Diesel Engine (VINs D, 2 -- RPO's LBZ, LLY)

and Allison® LCT 1000 (RPO MW7) Six Speed Automatic Transmission

General Motors is committed to customer satisfaction and we want you to have absolute confidence in our products. Our records indicate you may currently own one of the vehicles listed above. This letter is to advise you of a potential product concern, which could occur if an aftermarket transmission sump heater has been added to your vehicle.

If an aftermarket transmission sump heater has been added to your vehicle, it may be susceptible to transmission failure when all of the following conditions exist:

When a vehicle has been exposed to this extreme cold temperature for an extended period of time, the fluids become very thick. An aftermarket transmission sump heater warms the transmission fluid in the transmission, allowing it to operate. However, since the aftermarket heater does not warm all the transmission fluid, such as that fluid located in the cooler lines and transmission oil cooler located in the radiator, that fluid remains very thick and cannot flow. When the Elevated Idle Mode is engaged the transmission temperature could increase to a level that may cause damage to the transmission.

If you own one or more of the vehicles described above with an aftermarket transmission sump heater, we recommend you have the Transmission Control Module (TCM) recalibrated to accommodate the effect of the aftermarket sump heater and eliminate this potential over temperature concern. This recalibration is beneficial only on vehicles located in extreme cold climates with all the conditions listed above. Disabling the Elevated Idle Mode is an interim action that will protect the transmission prior to the TCM recalibration being performed. Elevated Idle can be engaged or disengaged using the driver information center controls or by the following method.

1. Turn the ignition to RUN, with the engine off.
2. Press the accelerator pedal to the floor and hold while quickly pressing the brake pedal three times in less than eight seconds.
3. Release the accelerator pedal and start the engine.

Please contact your local GM Dealership at your earliest convenience and reference this letter when scheduling an appointment. Since this recalibration is only necessary to accommodate the effect of an added aftermarket transmission sump heater, you would be responsible for the cost to recalibrate the TCM. Your dealer can advise you of the cost to perform this service.

Damages resulting from aftermarket modifications are not covered by your vehicle’s GM Warranty.

We regret any inconvenience this may cause; however, we are making this correction available in the interest of your continued satisfaction with General Motors products.