Mercury MerCruiser MCM 454 Mag MPI Service Manual page 706

SERVICE MANUAL NUMBER 23
Code 23 (1 of 2): Intake Air Temperature (IAT) Sensor Circuit (Non-Scan)
CIRCUIT DESCRIPTION:
The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to
the Electronic Control Module (ECM). The ECM applies a voltage (about 5 volts) on CKT
472 to the sensor. When the air is cold, the sensor (thermistor) resistance is high; therefore,
the ECM will see a high signal voltage. If the air is warm, the sensor resistance is low; there-
fore, the ECM will see a low voltage.
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. If Code 21 is also set, check CKT 813 for faulty wiring or
connections. Check terminals at sensor for good contact.
TEST DESCRIPTION:
Number(s) below refer to step number(s) on the diagnostic table.
Step 2. A Code 23 will set due to an open sensor, wire, or connection. This step will deter-
mine if the wiring and ECM are OK.
Step 3. If the resistance is greater than 25,000 ohms, replace the sensor.
Index
90-861326--1 MARCH 1999
Downloaded from https://needmanual.com/
a - Intake Air Temperature Sensor
b - To TP Sensor
c - IAT Sensor Signal
d - Sensor Ground
TROUBLE CODE DIAGNOSTICS - 454/502 MAG MPI & 8.2L MPI
a
B A
b
c
d
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