Mercury MerCruiser MCM 454 Mag MPI Service Manual page 481

MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
The Manifold Absolute Pressure (MAP) sensor is a pressure transducer that measures the
changes in the intake manifold pressure. The pressure changes as a result of engine load
and speed change, and the MAP sensor converts this to a voltage output.
454 / 502 Mag Style Shown
A closed throttle on engine coast-down would produce a relatively low MAP output voltage,
while a wide open throttle would produce a high MAP output voltage. This high output
voltage is produced because the pressure inside the manifold is the same as outside the
manifold, so 100% of outside air pressure is measured. When manifold pressure is high,
vacuum is low. The MAP sensor is also used to measure barometric pressure under certain
conditions, which allows the ECM to automatically adjust for different altitudes.
The ECM sends a 5 volt reference signal to the MAP sensor. As the manifold pressure
changes, the electrical resistance of the MAP sensor also changes. By monitoring the
sensor output voltage, the ECM knows the manifold pressure. A higher pressure, low
vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low
voltage) requires less fuel. The ECM uses the MAP sensor to control fuel delivery and
ignition timing.
A failure in the MAP sensor circuit should set a Code 33 (also a code 34 on 7.4L MPI Models
only).
KNOCK SENSOR
On MEFI 1 and MEFI 2, the knock sensor is mounted on the lower right side of the engine
block. MEFI 3 models do not have a knock sensor module mounted on the engine; it is lo-
cated internal to the ECM.
When abnormal engine vibrations (spark knock) are present, the sensor produces a voltage
signal which is sent to the KS Module and then to the ECM. The ECM uses this signal to
aid in calculating ignition timing.
Index
Page 5B-22
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a - Manifold Absolute Pressure (MAP) Sensor
b - Electrical Connector
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SERVICE MANUAL NUMBER 23
73046
73051
90-861326--1 MARCH 1999