Your Cummins diesel engine uses an Intake Heater Grid Relay to control the activation of the engines Heater Grid Element during Cold Weather Startups.
The intake manifold of your Cummins diesel engine uses an air heater element assembly which is located at the top of the engines intake manifold. The air heater element is used to pre-heat the incoming air to the intake manifold. This is done to help engine starting and improve driveability with cool or cold outside temperatures. Electrical supply for the air heater element is controlled by the ECM / Engine Control Module through the engines heater relays.
The Cummins ECM uses the signal from the intake manifold air temperature sensor to determine proper intake heater grid cycling. When air temperatures below 60° Farenhiegth are detected the intake manifold heater will run for 10 seconds or longer to aid in Cold temperature starting. If temperatures are frigid, the amount of time the heater grid is activated can be longer. During this period the "Wait-to-Start" light is illuminated. When the heater grid cycle has finished, the "Wait
To Start light" and heater will go off. The heater grid element will continue to cycle on and off after the engine is running. This will occur until the intake manifold reaches 60° or the truck reaches about 15 mph.
When the engine is running and cold, it is okay to see the voltmeter fluctuate up and down as the heaters cycle on and off. At around 15mph, the heater cycle is disabled. If a Heater Relay sticks on while the engine is running, the voltmeter will slowly drop below 9V and eventually the "Check Gauges" light and chime will activate. If a relay sticks when the engine is off, the heater will quickly drain the battery, and the heater may get hot enough to start a fire.