2003 GMC Sierra 1500 Pickup Owner's Manual - Page 403

Automatic Transmission Fluid (Allison Transmission) When to Check and Change A good time to check your automatic transmission fluid level is when the engine oil is changed. Change both the fluid and filter every 25,000 miles (41 500 km) if the vehicle is mainly driven under one or more of these conditions: In heavy city traffic where the outside temperature regularly reaches 90°F (32°C) or higher. How to Check Because this operation can be a little difficult, you may choose to have this done at the dealership service department. If you do it yourself, be sure to follow all the instructions here, or you could get a false reading on the dipstick. Notice: Too much or too little fluid can damage your transmission. Too much can mean that some of the fluid could come out andfall on hot engine part or exhaust system parts, starting a fire. Too little fluid could cause the transmission to overheat. Be sure to get an accurate reading if you check your transmission fluid. Wait at least 30 minutes before checking the transmission fluid level if you have been driving: When outside temperatures are above 90°F (32°C). At high speed for quite a while. In heavy traffic - especially in hot weather. While pulling a trailer. In hilly or mountainous terrain. 0 When doing frequent trailer towing. Uses such as found in taxi, police or delivery service. If you do not use your vehicle under any of these conditions, change the fluid and filter every 50,000 miles (83 000 km). See Pat? A: Scheduled Maintenance Services on page 6-4. 5-28