Diagnostic Testing Without A Scan Tool - Mercury MerCruiser MCM 454 Mag MPI Service Manual

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Diagnostic Testing Without A Scan Tool (Non-Scan)
Code 14 (1 of 3): Engine Coolant Temperature (ECT) Sensor Circuit
(Non-Scan) Low Temperature Indicated
CIRCUIT DESCRIPTION:
The Engine Coolant Temperature (ECT) sensor uses a thermistor to control the signal
voltage to the ECM. The ECM applies 5 volts on CKT 410 to the sensor. When the engine
coolant is cold, the sensor (thermistor) resistance is high. As the engine coolant warms up,
the sensor resistance becomes less. see engine coolant temperature sensor chart under
"Diagnostic Aids." At normal operating temperature 160 F-180 F (71 C-82 C), the voltage
will measure about 1.5-2.0 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.
If DTC 33 is also set, check for open ground CKT 814.
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.
IMPORTANT: If replacing the ECT, tighten hand tight plus 2-1/2 turns maximum.
Index
Page 5G-4
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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