Ignition System: Description and Operation

Fig. 1 Flywheel trigger tooth locations. 4-150 engine:

Fig. 2 Flywheel trigger tooth locations. 6-243 engines:

Fig. 3 TDC Sensor location:

The Ignition Control Module (ICM) incorporates a solid state ignition circuit. The ignition coil is mounted to the ICM and can be removed and serviced separately. This module electronically controls ignition advance and retard. The ICM receives input signals from the Electronic Control Unit (ECU) in the form of a 5 volt square wave. When the leading edge of the 5 volt square wave contacts the ICM ignition circuitry, the ICM will charge the coil primary windings. At a point when coil saturation occurs, the ICM circuitry will open the primary windings, causing the magnetic field to collapse. This will induce high voltage in the secondary coil windings which is routed to the spark plug through the coil wire, distributor cap, rotor and spark plug wire. The ICM also provides a Tach output signal for the diagnostic connector and tachometer.
On 4-150 engines, the flywheel, Fig. 1, has two teeth machined off 180° apart. These machined off teeth are position at 90° before top dead center and 90° before bottom dead center. On 6-243 engines, the flywheel, Fig. 2, has a large trigger tooth and notch located 70° (12 small teeth) before top dead center and 70° before bottom dead center. The TDC sensor, Fig. 3, which is mounted to the flywheel housing, senses TDC, BDC and engine speed from the teeth on the flywheel. Special shouldered bolts are used to secure the TDC sensor, which is non-adjustable.