The automatic transmission fluid inside of a Duramax diesel engine is designed to protect all of the internal gears, bearings and seals. When the diesel engine is running and the transmission is engaged, the automatic transmission fluid is pumped out of the transmission pan and into the transmission. The transmission fluid then bonds to all of the internal parts to prevent excessive heat friction. Change the automatic transmission fluid inside of your Duramax diesel engine between 50,000 and 75,000 miles to prevent the fluid from breaking down.
Drive the Duramax around for about 20 minutes to heat up the transmission fluid. The hot transmission fluid will give you a more accurate reading on the dipstick. Park the Duramax on level ground and open the hood.
Locate the automatic transmission fluid dipstick on the rear of the engine. Pull the dipstick out and look at where the fluid level is. Wipe away the excess fluid from the dipstick with a shop rag.