Mercury MerCruiser MCM 454 Mag MPI Service Manual page 683

GENERAL SYSTEM DIAGNOSTICS
Chart A-8 (1 of 2): Idle Air Control (IAC) Functional Test
a
CIRCUIT DESCRIPTION:
The ECM controls idle speed to a calibrated "desired" RPM based on sensor inputs and
actual engine RPM. The ECM uses four (4) circuits to move the Idle Air Control (IAC) valve.
The movement of the IAC valve varies the amount of air flow bypassing the throttle plates.
The ECM controls idle speed by determining the position of the IAC valve.
DIAGNOSTIC AIDS:
An intermittent may be caused by a poor connection, rubbed through wire insulation or a
wire broken inside the insulation. Check for the following items:
Poor connection or damaged harness. Inspect the ECM harness and connectors for
improper mating, broken locks, improperly formed or damaged terminals, poor terminal
to wire connection, and damaged harness.
Check for vacuum leaks, disconnected or brittle vacuum hoses, cuts, etc. Examine man-
ifold and throttle body gaskets for proper seal. Check for cracked intake manifold.
Check for poor connections, opens, or short to grounds in CKT's 441, 442, 443, and 444.
This may result in improper idle control.
An IAC valve which is "frozen" and will not respond to the ECM, a throttle stop screw
which has been tampered with, or a damaged throttle body or linkage may cause
improper idle.
TEST DESCRIPTION:
Step 2 determines if the IAC valve is functioning properly.
Step 4 determines if the circuitry or the IAC valve is faulty.
Index
Page 5E-90
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a - Idle air control (IAC) valve
b - IAC Coil "a" High
c - IAC Coil "a" Low
d - IAC Coil "b" High
e - IAC Coil "b" Low
441 LT BLU/WHT
442 LT BLU/BLK
443 LT GRN/WHT
444 LT GRN/BLK
SERVICE MANUAL NUMBER 23
b
c
d
e
90-861326--1 MARCH 1999