Mercury MerCruiser MCM 454 Mag MPI Service Manual page 760

SERVICE MANUAL NUMBER 23
Code 23 (2 of 3): Intake Air Temperature (IAT) Sensor Circuit (Non-Scan) Low
Temperature Indicated
TEST DESCRIPTION:
Index
90-861326--1 MARCH 1999
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Number(s) below refer to step number(s) on the diagnostic table.
STEP 4. Check the harness terminals thoroughly for loose connections. If the resistance
of the IAT sensor is monitored, the resistance should steadily decrease as the intake air
warms up. The resistance reading should stabilize when the engine reaches normal op-
erating temperature.
STEP 2. This step checks if there is a problem with the ECM and wiring or if the problem
is the IAT sensor.
STEP 3. This step will isolate the problem to CKT 472 (5 volt reference) or to CKT 813
(sensor ground).
STEP
Was the "On-Board Diagnostic" (OBD) System
1
Check Performed?
2 1. Ignition "OFF."
2. Disconnect IAT sensor harness terminals.
3. Ignition "ON," engine "OFF."
4. Connect DVOM across the sensor harness
terminals.
Is voltage above the specified value?
1. Connect positive (+) DVOM lead to harness
3
terminal "B" CKT 472 (5 volt reference).
2. Connect negative (–) DVOM lead to a
known good ground.
Is voltage above the specified value?
Locate and repair intermittent faulty connec-
4
tions. If OK, replace faulty IAT sensor. Is action
complete?
Locate and repair open CKT 472. If a problem
5
is found repair as necessary. Was a problem
found?
Locate and repair open ground CKT 813. If a
6 problem is found, repair as necessary. Was a
problem found?
Repair faulty ECM connections or replace
7
faulty ECM. Is action complete?
ACTION
TROUBLE CODE DIAGNOSTICS - 7.4L MPI
VALUE YES
PROCEED TO
– Step 2
4 Volts Step 4
4 Volts Step 6
–
–
Verify
Re air Repair
–
–
Page 5G-15
NO
Go to
OBD
System
Check
Step 3
Step 5
–
Ste 7 Step 7
–