The present invention relates to an automatic inter-axle differential lock actuation sensing method for a vehicle having a tandem drive axle. The inter-axle differential lock actuation sensing method comprises providing a clutch locking mechanism of an inter-axle differential for a vehicle, the clutch locking mechanism having a first set of teeth in a fixed position and a second set of teeth on a sliding clutch, locating a speed sensor such that when the clutch mechanism is disengaged the sensor measures a speed of the sliding clutch, and when the clutch mechanism is fully engaged the sensor measures zero speed even though the sliding clutch continues to rotate, providing a speed of the vehicle from a vehicle data link, and using the speed sensor-speed and the vehicle speed to determine status of clutch mechanism engagement.