How the Idle Air Control (IAC) Works and Is Adjusted

Introduction : Idle Air Control

At idle, the intake air quantity is regulated by：

1.A bypass Idle Air Control Valve (IACV),The following is an illustration.

2.Electronic Throttle Control (ETC),The following is an illustration.

Digital Linear Actuators : Description

Nominal Operating Voltage: 12 Vdc
Min. / Max. Operating Voltage: 7.5 Vdc / 14 Vdc
Resistance per Winding (@ 27°C): 53 Ω +/- 5.3
Minimum Resistance (@ -40°C): 35 Ω
Inductance per Winding (@ 25°C): 33.5 mH +/- 6 (1 kHz)
Step Increment: 0.04167 mm
Maximum Travel: 8.5 mm (204 steps)
Operating Temperature Range: -40°C to 125°C (150°C peak)
Minimum Storage Temperature: -40°C
Weight: 110 g
Mounting Flange Type: Asymmetrical
Resonance Frequency Range: 70 to 120 pps

Recommendations for IAC Installation on the Throttle Body

1.The Actuator Shaft should be at horizontal or below Horizontal. This will insure that condensates do not collect in the actuator housing.

2.The outlet of the actuator connector should be oriented so to avoid water collection into the connector.

3.The air flow must be in the direction shown by the arrows

4.The Idle Air Bypass flow should be controlled by the position of the actuator pintle relative to the seat (see  below). Therefore, the smallest flow section of the complete bypass channel must be the flow section created by the pintle and its seat.As a good practice recommendation, the inlet and outlet sections of the bypass channel should be larger than the seat section by a minimum of 50%.

5.Whenever possible, avoid：
Sharp corners
Sharp flow direction change
Multiple flow direction change (elbows)
Rough surface finish and burrs

6.The dimension “S” represents the nominal diameter of the seat (i.e. 8, 10, 12 mm).The O-ring is designed to provide radial compression sealing with the above recommended seat design. Please note the an axial compression sealing is not recommended.

Recommendation for Use - System Supplier

1.Restore power to the Actuator with the same polarity present when the power was last turned off.If not, an uncontrolled / unpredictable motion of ±2 steps may occur.

2.Park the pintle within the first half stroke of the actuator (from the seat) in order to avoid walkout (Walkout phenomenon is defined as the drifting forward of the pintle due to external vibration combined with internal spring force (actuator power is turned off).Maintain current to the coils at key-off until engine vibrations cease and Manifold Absolute Pressure (MAP) has settled to atmospheric pressure. If not, the actuator may change position when the power is removed due to vibration.  The precautions will prevent the tendency for the actuator to change position in the pintle extend direction.

3.If the supplied voltage is below specified minimum operating voltage for the given ambient temperature condition, the actuator should not be stepped, but kept in position holding.Contact VDO for the voltage data specific to your application

4.If the supplied voltage is above the specified maximum operating voltage for the given ambient temperature condition, the actuator should be powered off in order to avoid potential damage.Contact VDO for the voltage data specific to your application

5.1 open/shorted coil: keep power on the remaining coil to hold position.2 open/shorted coils: de-energize both coils.

6.A small band of unstable driving speed is inherent to stepper motors (due to the electro-mechanical resonance frequency). The motor stepping logic should not allow the actual driving speed to be or to shift into the unstable range : [70  - 120] steps/sec,If not, a potential of loss steps may occur (in multiples of 4).

7.Use a time delay (“pause”) whenever stopping and restarting motion. Use a time delay whenever direction is reversed.The time delay should be greater or equal to the lesser of :
- Normal step time step + 12.5 ms
- 20 ms.
If not, loss steps may occur (in multiples of 4).

8.The bounce-back phenomenon occurs when the stepper motor kinetic energy of the rotor reaches a critical level while the pintle is extended in the valve seat. When contacting the seat, the “rebound” energy causes the stepper motor to drive backward while being commanded forward. The result is a speed that is three times the driving speed but in the opposite direction from that commanded.The initialization sequence requires closing the actuator to obtain a reference position. The following specific sequence must be used during this initialization sequence in order to avoid bounce-back :The total number of steps (TS) shall consist of a total number of 261 steps using a “stutter-step” drive sequence, of which the first 252 (= 261-9) are at the specified speed (SR1) and the last 9 steps are at the specified speed (SR2). “Stutter-step” drive sequence is defined as a pause of one step after every third step (3 steps, pause, 3 steps, pause, etc.). See flow chart next page.

9.Initialization : Flow Chart
SR1 = application speed in closing direction
SR2 = 30 steps/sec
TS = 261 steps
Note:
If an estimation of the actuator position exists, the total number of steps (TS) to achieve initialization may be reduced to the sum of :number of steps required to reach the seat from the estimated position
an overdrive number of steps due to uncertainty in the     estimated position (typically 20 steps overdrive)
This may be useful to reduce the time required to perform an initialization sequence if done preventively at each key-off.