2004 GMC Sierra 1500 Pickup Owner's Manual - Page 212

If your vehicle has an Allison transmission, and the fluid reaches temperatures of approximately 275°F (135°C) or greater, the driver information center will display a TRANS HOT IDLE ENGINE warning message and a chime will sound until the temperature falls below 265°F (129°C). Pull the vehicle off the roadway when it is safe to do so. Set the parking brake, place the transmission in PARK (P) and allow the engine to idle until the transmission temperature falls below 265°F (129°C). If the transmission continues to operate above 265°F (130°C), contact your nearest dealer or the GM Roadside Assistance Center. See Roadside Assistance Program on page 7-6. The following situations can cause the transmission to operate at higher temperatures: Towing a trailer Hot outside air temperatures Hauling a large or heavy load Low transmission fluid level High transmission fluid level Restricted air flow to the radiator and the auxiliary transmission oil cooler (if equipped). Notice: If you drive your vehicle with the transmission temperature gage above normal operating range, you can damage the transmission. This could lead to costly repairs that would not be covered by your warranty. Do not drive your vehicle while the transmission temperature gage reading is above normal. See your dealer for service. A temporary solution to hotter transmission operating temperatures may be to let the transmission cool down. If the transmission is operated at higher temperatures on a frequent basis, see Scheduled Maintenance on page 6-4 for the proper transmission maintenance intervals. 3-48