Mercury 3.0 Liter Service Manual download pdf (Page 19)

90-898303 2-14

EFI Electrical Components

EFI Electronic Control Module

The ECM continuously monitors various engine conditions (temperature,

throttle opening) and climate conditions (induction air temperature, barometric

pressure, and altitude level) needed to calculate fuel delivery (pulse width

length) of injectors. The pulse width is constantly adjusted (rich/lean

conditions) to compensate for operating conditions, such as cranking, cold

starting, climate conditions, altitude, acceleration and deceleration; allowing

the outboard to operate efficiently at all engine speeds.

SENSOR INTERACTION WITH THE ECM

The ECM relies on sensor feedback to provide proper fuel rates and timing

advance for optimum engine performance under all conditions.

IMPORTANT: DO NOT run engine for extended periods of time with

sensors disconnected or bypassed (shorted). Serious engine damage

may result.

AIR TEMPERATURE SENSOR

The air temperature sensor transmits manifold air temperature, through full

RPM range, to the ECM. As air temperature increases “sensor” resistance

decreases causing the ECM to decrease fuel flow (leaner mixture).

Disconnecting the air temp sensor (open circuit) will increase fuel flow (richen

mixture) by 10%. Bypassing air temp sensor (short in circuit) will cause fuel

flow to decrease 10%.

The air temperature sensor circuit can be tested using the EFI tester. The air

temperature sensor can be tested following air temperature sensor test.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR

The MAP sensor is a non-serviceable sensor mounted in the ECM box. The

MAP sensor is used to sense changes in manifold absolute pressure and is

connected to the intake manifold by the way of a vacuum hose. The MAP

sensor is functioning through the full RPM range and is continually signaling

induction manifold pressure readings to the ECM. The ECM in turn determines

fuel flow as signals are received. Drawing a vacuum on the MAP sensor hose

will create a lean fuel condition altering engine operation. If no change occurs

when drawing vacuum, MAP sensor is not functioning properly.

NOTE: MAP sensor can be tested with the EFI tester.

ENGINE HEAD TEMPERATURE SENSOR

The Engine Head Temperature Sensor provides the ECM signals related to

engine temperature to determine level of fuel enrichment during engine warm

up. The ECM is receiving information at all engine temperatures but stops fuel

enrichment at an engine temperature of 90° F (32° C). An open circuit on the

temperature sensor will increase fuel flow up to 40% but will not be affected at

wide open throttle. If no change occurs when sensor is disconnected, sensor

may not be functioning properly. The engine head temperature sensor can be

tested following Engine Head Temperature Sensor Test.

NOTE: If sensor does not make clean contact with cylinder head a rich

condition may exist.

Notes

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Mercury 3.0 Liter Service Manual Page 19

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