Mercury MerCruiser MCM 454 Mag MPI Service Manual page 480

SERVICE MANUAL NUMBER 23
ENGINE COOLANT TEMPERATURE (ECT) SENSOR
The Engine Coolant Temperature (ECT) Sensor is a thermistor (a resistor which changes
value based on temperature) immersed in the engine coolant stream. Low coolant tempera-
ture produces a high resistance, while high temperature causes low resistance.
The ECM supplies a 5 volt signal to the ECT through a resistor in the ECM and measures
the voltage. The voltage will be high when the engine is cold, and low when the engine is
hot. By measuring the voltage, the ECM knows the engine coolant temperature. Engine
coolant temperature affects most systems the ECM controls.
A failure in the ECT circuit should set Code 14 (Code 15 on 7.4L MPI Models only). Remem-
ber, this code indicates a failure in the coolant temperature sensor circuit, so proper use of
the chart will lead to either repairing a wiring problem or replacing the sensor.
INTAKE AIR TEMPERATURE (IAT) SENSOR
The Intake Air Temperature (IAT) sensor is a thermistor mounted on the underside of the
plenum. Low temperature produces a high resistance, while high temperature causes a low
resistance.
The ECM supplies a 5 volt signal to the sensor through a resistor in the ECM and measures
the voltage. The voltage will be high when the intake air is cold, and low when the intake
manifold air is hot.
A failure in the IAT sensor circuit should set a Code 23 (also a code 25 on 7.4L MPI Models
only).
Index
90-861326--1 MARCH 1999
Downloaded from https://needmanual.com/
a - Engine Coolant Temperature (ECT) Sensor
b - Harness Connector
c - Locking Tab
a - Intake Air Temperature (IAT) Sensor
b - Harness Connector
c - Locking Tab
MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
b
c
c
b
a
73052
a
73047
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