Theory Of Operation - Mercury 150 EFI Service Manual

IGNITION

Theory of Operation

When the ignition key switch is turned to the "RUN" position, battery voltage is applied to
both the Electronic Control Module (ECM) through the purple wire and the main power
relay through the red/purple wire. As the ECM receives the "RUN" signal, it internally com-
pletes the ground circuit of the main relay, for a short period of time, energizing the fuel
pump for start-up. As the engine is cranked with the starter motor, the ECM receives the
run signal from the Crank Position Sensor (CPS) and completes the ground circuit to the
main relay for engine operation. With the main relay closed (completed circuit), D.C. cur-
rent from the battery/charging system is transferred through the 20 ampere main relay
fuse to the positive terminal of all ignition coil primary windings. The negative terminals
of the ignition coil primaries are connected to engine ground through the coils' internal
driver, which is triggered by the ECM. With the coil drivers closed, a electric magnetic field
is allowed to build up within the ignition coil. As the flywheel rotates, the CPS senses the
location of the 54 teeth on the flywheel and supplies the trigger signal information to the
ECM. The ECM utilizes the CPS information and determines when to remove the trigger
signal from the coil driver of each ignition coil. The coil driver then opens the coil primary
ground circuit, allowing it's magnetic field to rapidly collapse across the coil secondary
winding, which induces a high voltage charge (50,000 volts) that fires the spark plug.
Page 2A-12 90-883728 JULY 2001