Mercury MerCruiser MCM 454 Mag MPI Service Manual page 795

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Code 25 (1 of 2): Intake Air Temperature (IAT) Sensor Circuit (Scan) High
Temperature Indicated
CIRCUIT DESCRIPTION:
The Intake Air Temperature (IAT) Sensor uses a thermistor to control the signal voltage to
the ECM. The ECM applies a voltage on CKT 472 to the sensor. When the intake air is cold,
the sensor (thermistor) resistance is high, therefore, the ECM will see high signal voltage.
As the intake air warms, the sensor resistance becomes less, and the voltage drops. At nor-
mal engine operating temperature, 160-180 F (71-82 C), the voltage will measure about
1.5 to 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.
The scan tool displays intake air temperature in degrees celsius and fahrenheit. If the
engine is cold (not running within 8 hours), the scan tool should display a IAT sensor
value within a few degrees of outside air temperature. This may help aid in diagnosing a
"shifted" sensor. After engine is started, the temperature should rise steadily and then
stabilize at operating temperature.
Check harness routing for a potential short to ground in CKT 472.
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.
Index
Page 5G-50
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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