Engine Speed (RPM) Sensor

Engine Speed (RPM) Sensor

The periphery of the flywheel/flex plate is provided with a series of holes. As it passes, each transition between hole and metal induces a voltage in the pickup coil of RPM sensor. The resulting signal is an A/C signal whose frequency is a function of the number of holes passing per second and whose voltage can vary between 0.1 V and 100 V AC, depending on the engine speed and the air gap. Voltage and frequency increases with engine speed. The engine control module (ECM) determines the engine speed and of the crankshaft by detecting the voltage pulses.

At approximately 90 degrees before TDC for cylinder 1 there is a section without any gap. When this longer metal section (= missing holes) passes the RPM sensor, voltage pulses stop and the ECM can calculate angular crankshaft position.