Ignition Theory Of Operation; General Troubleshooting; Engine Control Module (Ecm) - Mercury 15 EFI FourStroke Service Manual

ECM RPM limiter (Guardian)
Overtemperature
Low oil pressure (over 10 seconds)
Crankshaft position sensor
Primary
Ignition coil Secondary (with plug wires and caps)
Spark plug lead (without spark plug cap)

Ignition Theory of Operation

The ignition system uses a microcomputer‑controlled capacitor discharge ignition (CDI) system. This system provides quick
voltage build‑up and the strong spark required for high power and high performance engines.
This is a stator‑powered, synchronous multiport fuel injection outboard. No battery voltage source is required for the engine
operations. The flywheel must be rotating at a minimum of 250 RPM to generate the 5 VDC required to energize the engine
control module (ECM) and maintain an active warning system.
As the flywheel rotates, the stator produces alternating current, which is rectified by the ECM for the ignition and fuel delivery
systems. When the ignition driver is off, a capacitor inside the ECM stores the DC voltage. When the capacitor is charged to its
potential, an applied gate signal allows the charge to dissipate through the primary winding of the ignition coils.
A timing key cast into the flywheel rotates past the crankshaft position sensor (CPS), collapsing a magnetic field, sending a
signal to the ECM to indicate crankshaft position and engine speed. The ECM uses this data to calculate the gate signal for
dissipating the capacitor charge.
The ECM also obtains information from the engine coolant temperature (ECT) sensor, manifold absolute pressure (MAP)
sensor, and manifold air temperature (MAT) sensor to formulate the correct ignition timing. The output from an oil pressure
switch indirectly affects ignition timing, as the ECM will limit engine speed, if the oil pressure is too low.

General Troubleshooting

For general troubleshooting information, including connecting and using the CDS G3 diagnostic tool, refer to Section 1E ‑
General Troubleshooting.

Engine Control Module (ECM)

This engine control module (ECM) is powered by the stator, and does not require a 12‑volt battery to function. As the flywheel
is rotated by the recoil rope or electric starter, the stator begins to generate voltage to energize the ECM. The ECM rectifies the
applied voltage to keep itself powered as well as to supply voltage to the various sensors, ignition coils, and high‑pressure fuel
pump. The ECM does not supply power for the optional electric start or power tilt circuits.
The ECM receives information from the engine sensors. It uses this data to calculate ignition timing, fuel injection timing, and
fuel injection pulse width (duration). The ECM then sends signals to the actuators to turn them on or off according to the values
calculated for optimum performance.
The ECM will generate a warning horn and warning light if the coolant temperature exceeds its upper limit, the oil pressure
switch indicates low pressure, or a sensor's value is outside of its parameters.
The ECM stores past running information and failure codes. Engine operating information and stored running history can be
monitored and reviewed using CDS G3. An adapter harness connects the CDS G3 computer to the ECM diagnostic port.
CDS G3 Diagnostic Tool Kit
CDS G3 Cable Kit
90-8M0125265 eng NOVEMBER 2017
Parameter Specification
2800 RPM
85 °C (185 °F)
26.3 kPa (3.8 psi)
148–222 Ω
0.26–0.35 Ω
16.8–20.2 kΩ
3.0–7.5 kΩ
62967
8M0114141
8M0137534
© 2018 Mercury Marine
Ignition
Page 2A-3