Diagnostic Testing Using A Scan Tool; Temperature Indicated - Mercury MerCruiser MCM 454 Mag MPI Service Manual

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Diagnostic Testing Using A Scan Tool (Scan)
Code 14 (1 of 3): Engine Coolant Temperature (ECT) Sensor Circuit (Scan) -
Low Temperature Indicated
CIRCUIT DESCRIPTION:
The Engine Coolant Temperature (ECT) Sensor uses a thermistor to control the signal volt-
age to the ECM. The ECM applies a voltage on CKT 410 to the sensor. When the engine
coolant is cold, the sensor (thermistor) resistance is high, therefore, the ECM will see high
signal voltage. As the engine coolant warms, the sensor resistance becomes less, and the
voltage drops. At normal engine operating temperature, 160-180 F (71-82 C), the voltage
will measure about 1.5 to 2.0 volts.
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 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.
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 33 is also set, check for open ground CKT 814.
Index
Page 5G-38
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a
a - Engine Coolant Temperature (ECT) Sensor
b - To Map Sensor
c - Engine Coolant Temperature (ECT)
d - Sensor Ground
b
SERVICE MANUAL NUMBER 23
c
d
90-861326--1 MARCH 1999