Engine Speed Sensor: Description and Operation

PURPOSE AND LOCATION
The purpose of the crankshaft speed sensor is to supply the ignition ECU with information on engine speed. The sensor is mounted close to the flywheel which has teeth machined around the circumference.

CONSTRUCTION
The sensor consists of a permanent magnet, a pole tip which concentrates a magnetic field at the end of the device, and a coil. The magnet creates a magnetic field which varies in strength when a tooth is passing the tip.

OPERATION
An altering voltage is induced in the sensor when one of the teeth on the flywheel passes the tip. This variable signal is transmitted by a wire to the ECU. In effect, the sensor supplies the ECU with a unique altering voltage (an electrical picture) from which the ECU can determine both the crankshaft position and speed.

SIGNAL
The frequency of the signal is used to determine the engine speed. To provide a reference signal for the crankshaft position, two teeth on the flywheel circumference have a gap. The ECU detects the crankshaft position by sensing the interruption in the signal which occurs as the gap passes the sensor. This referance is used to determine the top dead center (TDC) position.