Theory Of Operation; Ignition Component Description; Electronic Control Module (Ecm) - Mercury Optimax 115 Service Manual

IGNITION

Theory of Operation

When the ignition key is turned to the RUN position, battery voltage is applied to the main
relay through the PURPLE wire. When the Electronic Control Module (ECM) receives a
signal from the Crank Position Sensor, the main relay ground circuit is completed through
the ECM. The main relay is then closed and D.C. current from the battery or charging sys-
tem is transferred through the main relay 20 Amp fuse to the positive terminal of all 6 igni-
tion coil primary windings. The negative terminal of the coil primary is connected to engine
ground through the Electronic Control Module (ECM). When this circuit is closed, a mag-
netic field is allowed to be built up in the ignition coil. The Crank Position Sensor senses
the location of the 24 teeth on the flywheel and supplies a trigger signal to the ECM. When
the ECM receives this signal, the ECM will then open the ground circuit of the coil primary.
The magnetic field in the ignition coil primary will then collapse cutting across the coil sec-
ondary winding creating a high voltage charge (50,000 volts) that is sent to the spark plug.

Ignition Component Description

Electronic Control Module (ECM)

The ECM requires 8 VDC minimum to operate. If the ECM should fail, the engine will stop
running.
The inputs to the ECM can be monitored and tested by the Digital Diagnostic Terminal
91-823686A2 using adaptor harness 84-822560A5.
The ECM performs the following functions:
Calculates the precise fuel and ignition timing requirements based on engine speed,
throttle position, manifold pressure and coolant temperature.
Controls fuel injectors for each cylinder, direct injectors for each cylinder and ignition
for each cylinder.
Controls all alarm horn and warning lamp functions.
Supplies tachometer signal to gauge.
Controls RPM limit function.
Monitors shift interrupt switch.
Records engine running information.
Page 2A-6 90-855347R1 JANUARY 1999