All models except 8HP45/845RE and 8HP70 8-speed transmissions require the transmission fluid to be drained and replaced every 60,000 miles, especially if the vehicle is used under severe conditions such as frequent towing or off-road use. To perform this procedure, ensure that the engine has completely cooled down. Purchase the specified transmission fluid and a new filter. You will also need a floor jack, jack stands, a drain pan, newspapers, and clean rags. Raise the vehicle and support it securely on jack stands. Place the drain pan underneath the transmission pan and remove the rear and side pan mounting bolts, loosening the rear pan bolts. Carefully pry the transmission pan loose with a screwdriver to allow the fluid to drain. Remove the remaining bolts, pan, and gasket. Clean the gasket surface of the transmission pan thoroughly. Drain the fluid from the pan, clean it, and remove any debris caught by the magnets if applicable. Remove the filter and seal from the valve body inside the transmission, ensuring all traces of old gasket material are removed. On certain transmissions, remove the cooler return filter and replace it with a new one. Install a new gasket and filter, making sure they are the correct type. Install a new gasket on the transmission pan and tighten each bolt gradually to the specified torque. Lower the vehicle and add the specified amount of automatic transmission fluid through the filler tube. Run the engine at a fast idle, moving the gear selector through each range and back to Park. Check the fluid level and add more if necessary. Check for leaks during the first few trips and recheck the fluid level when the transmission is hot. For 8HP45/845RE and 8HP70 8-speed transmissions, the transmission oil pan and filter are an integrated assembly and cannot be serviced separately. Raise the vehicle and place a drain pan under the drain plug. Allow the transmission fluid to drain out and reinstall the plug. Remove the transmission pan mounting bolts and lower the pan, removing the gasket. If replacing the filter, install a new transmission oil pan. Install the pan and gasket, tightening the bolts in a circular pattern. Follow the procedure for adding fluid and checking the fluid level. Finally, check for leaks during reinstallation.
