Temperature Indicated - Mercury MerCruiser MCM 454 Mag MPI Service Manual

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Code 23 (1 of 3): Intake Air Temperature (IAT) Sensor Circuit (Non-Scan) Low

Temperature Indicated

CIRCUIT DESCRIPTION:
The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to
the ECM. The ECM applies 5 volts on CKT 472 to the sensor. When the intake air is cold,
the sensor (thermistor) resistance is high. As the intake air warms up, the sensor resistance
becomes less. see intake air temperature sensor chart under "Diagnostic Aids". At normal
operating temperature 160 F-180 F (71 C-82 C), the voltage will measure about 1.5-2.0
volts.
DIAGNOSTIC AIDS:
Check for the following conditions:
Poor connection at ECM. Inspect harness connectors for backed out terminals,
improper mating, broken locks, improperly formed or damaged terminals, and poor ter-
minal to wire connection.
Damaged harness. Inspect the wiring harness for damage. If the harness appears to be
OK, observe the IAT display on the scan tool while moving connectors and wiring har-
nesses related to the IAT sensor. A change in the IAT display will indicate the location of
the fault.
If DTC 21 is also set, check for open ground CKT 813.
After repairs, clear DTC's following "Clearing Trouble Codes" procedure at the front of this
section. Failure to do so may result in DTC's not properly being cleared.
IMPORTANT: If replacing the ECT, tighten hand tight plus 2-1/2 turns maximum.
Index
Page 5G-14
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B
a - Intake Air Temperature Sensor
b - To TP Sensor
c - IAT Sensor Signal
d - Sensor Ground
a
A
b
SERVICE MANUAL NUMBER 23
c
d
90-861326--1 MARCH 1999