[header] How to change a cummins 12 valve thermostat [title] Disconnect the negative (black) cable from the engine's battery. [title] Drain the engine's coolant into a container. [step] Close the container as soon as liquid stops draining from the truck. 
[title] Lift the belt tensioner with a 3/8-inch (9.53 mm) breaker bar and remove the drive belt. [title] Disconnect the engine's upper radiator hose from the thermostat's cover.