Speed Sensor
The speed sensor is secured by special shouldering bolts to the flywheel/drive plate housing.
The speed sensor senses TDC and engine speed by detecting the flywheel teeth as they pass the pick-up coil during engine operation. See Fig 1 . The flywheel has a large trigger tooth and notch located 12 small teeth before each TDC position. When a small tooth and notch pass the magnetic core in the sensor, the concentration and collapse of the magnetic field created induces a small voltage spike into the sensor pick-up coil windings. These small voltage spikes are sent to the ECU, allowing the ECU to count the teeth as they pass the sensor.
When a large tooth and notch pass the magnetic core in the sensor, the increased concentration and collapse of the magnetic field induces a higher voltage spike than the smaller teeth. The higher spike indicates to the ECU that a piston will soon be at TDC position, 12 teeth later.
Ignition timing for the cylinder is either advanced or retarded as necessary by the ECU based upon the inputs from all sensors.