Engine Speed Sensor
Engine speed sensor is attached to bellhousing. It senses and counts teeth on flywheel gear ring as they pass during engine operation. Signal from speed sensor provides ECU with engine speed and crankshaft angle. On flywheel gear ring, large trigger tooth and notch is located 90 degrees before each TDC point. Each trigger tooth is followed by 12 smaller teeth and notches before TDC point is reached.
As each of 12 small teeth and notches pass magnet core in speed sensor, concentration and collapse of magnetic field induces slight voltage (spike) in sensor pick-up coil winding. See Fig 1 . Larger trigger teeth and notches induce higher voltage (spike) in sensor pick-up coil winding. These voltage spikes enable ECU to count teeth as they pass speed sensor.
Higher voltage spike (from larger tooth and notch) indicates to ECU that piston will be at TDC position after 12 smaller voltage spikes have been counted. ECU will then either advance or retard ignition timing depending upon remaining sensor inputs.