Temperature Indicated - Mercury MerCruiser MCM 454 Mag MPI Service Manual

SERVICE MANUAL NUMBER 23
Code 22 (3 of 3): Throttle Position (TP) Sensor Circuit (Scan) Low Signal
Voltage
Code 23 (1 of 3): Intake Air Temperature (IAT) Sensor Circuit (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 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.
Index
90-861326--1 MARCH 1999
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STEP
Locate and repair open or short to ground in
CKT 416. Also check CKT 416E to the MAP
6
sensor for a short to ground. If a problem is
found, repair as necessary. Was a problem
found?
Locate and repair open or short to ground in
7
CKT 417. If a problem is found, repair as nec-
essary. Was a problem found?
8 Replace faulty TP sensor. Is action complete?
Repair faulty ECM connections or replace
9
faulty ECM. Is action complete?
a - Intake Air Temperature Sensor
b - To TP Sensor
c - IAT Sensor Signal
d - Sensor Ground
ACTION
a
B A
b
TROUBLE CODE DIAGNOSTICS - 7.4L MPI
VALUE YES
PROCEED TO
–
Verify V if
Repair
–
–
–
c
Page 5G-47
NO
Step 9
Step 9
–
–
d