Mercury MerCruiser MCM 454 Mag MPI Service Manual page 793

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Code 23 (2 of 3): Intake Air Temperature (IAT) Sensor Circuit (Scan) - Low
Temperature Indicated
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
intake air 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.
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.
If DTC 21 is also set, check for open ground CKT 813.
TEST DESCRIPTION:
Number(s) below refer to step number(s) on the diagnostic table.
Step 2. DTC 23 will set if signal voltage indicates a coolant temperature below -22 F
(-30 C).
Step 3. This test simulates a DTC 25. If the ECM recognizes the low voltage signal and
displays a high temperature, the ECM and wiring are OK.
STEP
1
2
3
Index
Page 5G-48
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ACTION
Was the "On-Board Diagnostic" (OBD) System
Check Performed?
Ignition "ON."
Does the scan tool display a temperature
less than the specified value?
1. Ignition "OFF."
2. Disconnect IAT harness connector.
3. Connect a jumper wire from harness ter-
minal "A" (CKT 472) to harness terminal "B"
(CKT 813).
4. Ignition "ON," engine "OFF."
Does scan tool display a temperature
above the specified value?
SERVICE MANUAL NUMBER 23
VALUE YES NO
PROCEED TO
Go to
OBD
– Step 2
System
Check
-22 F -22 F
(-30 C)
Step 3 Step 4
266 F
(130 C)
Step 6 Step 5
90-861326--1 MARCH 1999