PISTON 8D9912 - Caterpillar

Engine speed is sensed by an electronic sensor. The same sensor is used for monitoring the position of the camshaft and the injection timing control. The speed/timing sensor has a self-adjusting pickup head. The signals are created as the timing reference ring rotates past the pickup (sliphead) of the speed/timing sensor. The timing reference ring is mounted on the rear of the left camshaft. A unique tooth pattern on the timing reference ring allows the ECM to determine the crankshaft position, the direction of rotation, and the speed. The ECM supplies the speed/timing sensor with 12.5 1.0 VDC which is required for proper operation.The output of the speed/timing sensor is a series of pulses. The frequency of the pulses is dependent upon the speed of the engine. The ECM interprets the frequency of the pulses as engine speed. The shape of the pulses is dependent upon the rotation position of the camshaft. The ECM reads the shape of the pulses in order to determine the position of the camshaft.Primary and Secondary Engine Speed/Timing Sensor
The Secondary Engine Speed/Timing Sensor provides continuous operation of the engine if the Primary Engine Speed/Timing Sensor fails. To prevent any unnecessary down time, the primary sensor should be repaired or replaced at the earliest possible convenience.