Servicing the automatic transmission in your GMC is crucial to the life of the truck. GMC is notorious for being run hard. The best way to show appreciation to your GMC truck is to make sure your fluids are fresh, clean and changed regularly. Changing your transmission fluid every 15,000 miles will ensure your truck shifts smoothly and will keep all the sediment and debris out of your transmission pan.
Locate the transmission behind the motor. The transmission contains a pan underneath that is held on by numerous bolts. Some transmissions have a drain plug you can can open with a crescent wrench. If the drain pan does not have a drain plug, you will need to remove the pan. Locate each bolt and unscrew it with a socket wrench. Unscrew one side of the pan to allow some of the fluid to pour out into your catch pan. You can proceed to remove the remaining bolts. Pull the pan down, and if the pan is sticking to the transmission, you can hit it with a rubber mallet to help it break loose from the seal. Pour the transmission fluid into a disposable container. You will need to find a suitable place to dispose of the old fluid. Scrape the old gasket material off the top of the drain pan and the bottom of the transmission where the drain pan was mounted. Unbolt the filter with a socket wrench and dispose of the filter with the transmission fluid. Ensure the o-ring is removed from the area where the filter was sitting. Clean the pan out before re-installing by spraying it with brake cleaner. This is the only suitable chemical to use for cleaning the transmission pan. Ensure all the debris and sediment are removed.
Screw the new transmission filter back on the transmission. Ensure the new o-ring is positioned correctly. You will need a fresh gasket to lay on the surface of the drain pan. Ensure it is the exact length and that the bolt holes match up to the already bored holes in the gasket. You can apply a small bead of black RTV silicone to ensure the gasket stays on the drain pan surface while you are mounting it back to the transmission. Place the drain pan up to the transmission and start each bolt. Do not tighten until all the bolts are in place. Ensure each bolt is tight, as you may want to double check each bolt when you are done tightening them.
Open the hood to access the transmission fluid dipstick. Pull the dipstick from the tube and place it to the side. The transmission dipstick will have a red cap and be located toward the back right of the motor. Place a funnel into the tube and proceed pouring the first quart of transmission fluid. The system will generally hold 4 to 12 quarts. Consult your owner's manual to be positive. Remember that your torque converter will hold up to 4 quarts as well. The best strategy is to pour 4 quarts of transmission fluid and then place the transmission dipstick back in to the tube. Check the level, and if full, you can leave the dipstick in the tube. If the level is not at the full mark, then you will want to pour in half-quart increments until the transmission shows full. Drive your vehicle and run it through all the gears, then check again while it is running. Top off the level if it is low.
