Mercury MerCruiser MCM 454 Mag MPI Service Manual page 786

SERVICE MANUAL NUMBER 23
Code 15 (2 of 3): Engine Coolant Temperature (ECT) Sensor Circuit (Scan)
High Temperature Indicated
DIAGNOSTIC AIDS:
Check for the following conditions:
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.
TEST DESCRIPTION:
Index
90-861326--1 MARCH 1999
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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 ECT display on the scan tool while moving connectors and wiring har-
nesses related to the ECT sensor. A change in the ECT display will indicate the location
of the fault.
The scan tool displays engine coolant temperature in degrees celsius and fahrenheit. If
the engine is cold (not running within 8 hours), the scan tool should display a ECT sensor
value within a few degrees of outside air temperature. This may help aid in diagnosing a
"shifted" coolant sensor. After engine is started, the temperature should rise steadily and
then stabilize at operating temperature when the thermostat opens.
Check harness routing for a potential short to ground in CKT410.
Number(s) below refer to step number(s) on the diagnostic table.
Step 2. DTC 15 will set if signal voltage indicates a coolant temperature above 266 F
(130 C).
Step 3. This test simulates a DTC 14. 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 coolant tem-
perature greater than the specified value?
3 1. Ignition "OFF."
2. Disconnect ECT harness connector.
3. Ignition "ON," engine "OFF."
Does scan tool display a coolant tempera-
ture below the specified value?
ACTION
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
Page 5G-41
NO
Go to
OBD
System
Check
Step 4
Step 5