Idle Air Control (Iac) - Mercury 25 EFI Service Manual

Fuel System Operation
• Controls RPM limit function.
• Records engine running information.
ECM Systems Control Matrix
ECM Control Item
Ignition timing
Fuel injection

Idle air control (IAC)

Tachometer
Warning horn
Warning lamp
Memorizes
operation data
Idle Air Control (IAC)
The idle air control (IAC) is an electrically operated spring‑loaded solenoid valve which controls the amount of air bypassing the
closed throttle shutter. Signals from the ECM regulate the duty cycle that the IAC valve remains open, or closed. The IAC
controls three operating functions:
• Provides additional intake air for engine during start‑up, and allows increased idle RPM during engine warm‑up.
• Controls idle speed according to the varying engine loads, and running conditions.
• Functions as an electronic dashpot by providing additional bypass air as the throttle quickly closes during a rapid
deceleration, preventing engine stalling.
Page 3A-6
Controls the optimum ignition timing according to the current operating conditions
Controls the fuel injection pulse width (duration) for optimum efficiency according to current operating
condition
Stabilizes the engine RPM when idling and during quick engine RPM deceleration by managing the
duty cycle driving the IAC in order to control bypass air entering the intake manifold
Sends out six tachometer pulses per engine revolution (12 pole)
After Starting:
Short beep for two seconds after the engine is initially started indicating all sensors are functioning
Continuous Sound:
During engine over RPM, engine coolant temperature is over 90 °C (194 °F), engine oil pressure is
24.5 kPa (3.6 psi).
Intermittent Sound:
MAP sensor or engine coolant temperature sensor is defective or is disconnected
After Starting:
Illuminated for five seconds after the engine is initially started indicating all sensors are functioning
Continuous Illumination:
During engine over RPM, engine coolant temperature is over 90 °C (194 °F), engine oil pressure is
24.5 kPa (3.6 psi).
Intermittent Illumination:
MAP sensor or engine coolant temperature sensor is defective or is disconnected
Engine running time
Over temperature and time of occurrence
Engine Guardian (over RPM, and RPM reduction to 2800 or less)
Sensor failures
© 2016 Mercury Marine
Description Parameter
21101
90-8M0105568 eng MARCH 2016