Mercury MerCruiser MCM 454 Mag MPI Service Manual page 796

SERVICE MANUAL NUMBER 23
Code 25 (2 of 2): Engine Coolant Temperature (IAT) Sensor Circuit (Scan) High
Temperature Indicated
TEST DESCRIPTION:
INTAKE AIR TEMPERATURE SENSOR CHART
Index
90-861326--1 MARCH 1999
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Number(s) below refer to step number(s) on the diagnostic table.
Step 2. DTC 25 will set if signal voltage indicates a coolant temperature above 266 F
(130 C).
Step 3. This test simulates a DTC 23. If the ECM recognizes the high voltage signal and
displays a low temperature, the ECM and wiring are OK.
STEP
Was the "On-Board Diagnostic" (OBD) System
1
Check Performed?
2 Ignition "ON."
Does the scan tool display a temperature
greater than the specified value?
3 1. Ignition "OFF."
2. Disconnect IAT harness connector.
3. Ignition "ON," engine "OFF."
Does scan tool display a temperature be-
low the specified value?
Trouble code 25 is intermittent. Locate and re-
4
pair intermittent faulty connections. Refer to
"Diagnostic Aids".
Locate and repair short to ground in CKT 472.
5 If a problem is found, repair as necessary. Was
a problem found?
6 Repair faulty IAT sensor. Is action complete?
Repair faulty ECM connections or replace
7
faulty ECM. Is action complete?
Temperature - to - Resistance Values (Approximate)
F
210
160
100
70
40
20
0
-40
ACTION
IAT Sensor
C
100
70
38
20
4
-7
-18
-40
TROUBLE CODE DIAGNOSTICS - 7.4L MPI
VALUE YES
PROCEED TO
– Step 2
266 F 266 F
(130 C)
Step 3
-22 F -22 F
(-30 C) ( )
Step 6
–
Verify
–
Repair
–
–
OHMS
185
450
1,800
3,400
7,500
13,500
25,000
100,700
Page 5G-51
NO
Go to
OBD
System
Check
Step 4
Step 5
–
Step 7
–
–