Mercury MerCruiser MCM 454 Mag MPI Service Manual page 701

TROUBLE CODE DIAGNOSTICS - 454/502 MAG MPI & 8.2L MPI
Diagnostic Testing Without A Scan Tool (Non-Scan)
Code 14 (1 of 3): Engine Coolant Temperature (ECT) Sensor Circuit
(Non-Scan)
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.
DIAGNOSTIC AIDS:
An intermittent problem may be caused by a poor or corroded connection, rubbed through
wire connection, a wire that is broken inside the insulation, or a corroded wire.
Any circuitry that is suspected as causing the intermittent complaint should be thoroughly
checked for backed-out terminals, improper mating, broken locks, improperly formed or
damaged terminals, poor terminal-to-wiring connections, corroded terminals and/or wiring,
or physical damage to the wiring harness. After repairs, clear codes following "Clearing
Codes" procedure at the front of this section. Failure to do so may result in codes not
properly being cleared. Check harness routing for a potential short to ground in CKT 410.
See "Intermittents" in "Troubleshooting."
Index
Page 5F-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