Mercury MerCruiser MCM 454 Mag MPI Service Manual page 761

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Code 23 (2 of 3): Intake Air Temperature (IAT) Sensor Circuit (Non-Scan) Low
Temperature Indicated
INTAKE AIR TEMPERATURE SENSOR CHART
Code 25 (1 of 3): Intake Air Temperature (IAT) Sensor Circuit (Non-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 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.
Index
Page 5G-16
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Temperature - to - Resistance Values (Approximate)
F
210
160
100
70
40
20
0
-40
B
a - Intake Air Temperature Sensor
b - To TP Sensor
c - IAT Sensor Signal
d - Sensor Ground
IAT Sensor
C
100
70
38
20
4
-7
-18
-40
a
A
b
SERVICE MANUAL NUMBER 23
OHMS
185
450
1,800
3,400
7,500
13,500
25,000
100,700
c
d
90-861326--1 MARCH 1999