The crankshaft speed sensor or the engine speed sensor on your 12 valve Cummins is in charge of emitting a signal for a few different vehicle operations. It creates a signal for the tach, alternator, air conditioning, and shifting in automatics. If you are experiencing any issues with those components, it may be your crankshaft speed sensor. Replace your failed sensor with a genuine Cummins Replacement Crankshaft Speed Sensor. This sensor is a direct replacement for trucks with the rectangular harness plug.