Speed Sensor
The speed sensor is a nonadjustable sensor attached to the flywheel/drive plate housing with special shoulder bolts. This sensor provides Top Dead Center (TDC) and engine speed information to the ECU by counting the flywheel teeth as they pass during engine operation. The flywheel has a large trigger tooth and notch located 12 small teeth before each TDC position. See Fig 1 .
When a small tooth and notch pass the magnetic core of 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 give the ECU information for calculating engine speed.
When a large tooth and notch pass the magnetic core of the sensor, the concentration and collapse of the magnetic field created induces a higher voltage spike into the sensor pick-up coil windings. This higher voltage spike indicates to the ECU that a piston will soon be at TDC (12 teeth later).
The ECU uses speed sensor information for advancing or retarding ignition timing, dependent upon input from other sensors and switches.