IMPORTANT : Before diagnosing IAC motor, several things must be checked and verified; the throttle plate should
be free of carbon buildup and should open and close freely, the minimum air rate (minimum throttle
opening) should be set according to manuf acturer’s specifications, and check f or vacuum leaks or
false air leaks.
• Troubleshooting Tips
If the engine idle speed doesn’t change corresponding with the change of the PCM’s command signal, suspect a
bad IAC motor or clogged bypass passage.
IAC (Idle Air Control) Solenoid
• Theory of Operation
Idle air control solenoids keep the engine idling as low as possible, without stalling, and as smoothly as possible
when accessories such as air conditioning compressors, alternators, and power steering load the engine.
Ford’s IAC solenoids are driven by a DC signal with some AC superimposed on top. The solenoid opens the throttle
plate in proportion to the DC drive it receives from the PCM. The DC drive is applied by holding one end of the
solenoid coil at battery positive while pulling the other end toward GND. The DC voltage at the driven pin decreases
as the solenoid drive current is increased.
• Symptoms
Erratic high or low idle, stalling, high activity but no change in idle
• Test Procedure
1. Connect the CH A lead to the IAC control signal from the PCM and its ground lead to the chassis GND.
2. Run the engine at idle while turning accessories (A/C, blowers, wipers, etc.) on and off. If the vehicle has an
automatic transmission, put it in and out of drive and park. This will change the load on the engine and cause the
PCM to change the output command signal to the IAC solenoid.
3. Make sure that the amplitude, frequency, and shape are all correct, repeatable, and consistent for the various idle
compensation modes.
4. Make sure that idle speed responds to the changes in the IAC drive.
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The idle control output command from the PCM should change when accessories are
switched on and off or the transmission is switched in and out of gear.
The pulse width modulated signals f rom t he P CM should control the speed of the
motor, and in turn the amount of air bypassing the throttle plate.
The turn off spikes may not be present in all IAC drive circuits.
• Troubleshooting Tips
If the waveform has runted (shortened) spike heights, it indicates a shorted EGR vacuum solenoid.
If the waveform has a flat line (no signal at all), it indicates a PCM failure, PCM’s EGR conditions not met, or wiring
or connector problem.
Too much EGR flow can make the vehicle hesitate, loose power, or even stall. Not enough EGR flow can result in
emissions with excessive NOx and engine detonation (pinging).
IAC (Idle Air Control) Motor
• Theory of Operation
Idle air control valves keep the engine idling as low as possible, without stalling, and as smoothly as possible when
accessories such as air conditioning compressors, alternators, and power steering load the engine.
Some IAC valves are solenoids (most Fords), some are rot ating mot ors (European Bosch), and some are gear
reduction DC stepper motors (most GM, Chrysler). In all cases, however, the PCM varies the amplitude or pulse
width of the signal to control its operation and ultimately, idle speed.
Rotating IAC motors receive a continuous pulse train. The duty cycle of the signal controls the speed of the motor,
and in turn the amount of air bypassing the throttle plate.
• Symptoms
Erratic high or low idle, stalling, high activity but no change in idle
• Test Procedure
1. Connect the CH A lead to the IAC control signal from the PCM and its ground lead to GND.
2. Run the engine at idle while turning accessories (A/C, blowers, wipers, etc.) on and off. If the vehicle has an
automatic transmission, put it in and out of drive and park. This will change the load on the engine and cause the
PCM to change the output command signal to the IAC motor.
3. Make sure that idle speed responds to the changes in duty cycle.
4. Use the Glitch Snare mode to check for signal dropouts.
• Reference Waveform
VEHICLE INFORMATIONS
YEAR
: 1989
MAKE
: BMW
MODEL
: 525 I
ENGINE
: 2.5 L
FUELSYS : Multiport Fuel Injection
PCM_PIN : 22 WhtGrn wire
STATUS
: KOER (Key On Running)
RPM
: Idle
ENG_TMP : Operating Temperature
VACUUM : 15 In. Hg
MILEAGE : 72822
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FREQ = 100 Hz
DUTY = 52.5 %
PCM turns
circuit on
PCM turns
circuit off
