Motor strategy

The step motor is very precise and offers high resolution (about 220 steps per second). Pulses sent by the electronic control unit to the engine are converted from rotary motion into linear movement (about 0.04 mm/step) via a worm screw mechanism. The plunger then works to alter the cross section of the by pass port. The constant minimum air flow (Qo) due to air leaking beneath the throttle valve is adjusted during production and ensured by a tamperproof cap. Maximum air flow (Q2) occurs when the plunger is fully retracted (about 220 steps corresponding to 8.9 mm). Air flow follows the law shown in the graph alongside for intermediate values.

Motor strategy

The number of working steps depends on engine condition: - Ignition stage When the ignition key is inserted, the control unit makes the step motor take up a position dependent upon coolant temperature and battery voltage. - Warm-up stage RPM is adjusted according to coolant temperature - With engine warm: Idle management depends on a signal from the engine rpm sensor. The control unit manages sustained idle mode when external loads are activated. - During deceleration The electronic control unit identifies deceleration stage from throttle potentiometer position. It controls step motor position via the idle flow law (dashpot law), i.e. it slows the return of plunger (3) to its seat so that a quantity of air bypassed from hole (2) reaches the motor and reduces levels of pollutants in exhaust gases.

Recovery

Actuator operation disabled, fuel vapour recirculation self-adaptation inhibited and speed limited to 1200 rpm.

Removing-refitting engine idle speed actuator (step motor)

Procedure for premodification cars - disconnect the battery negative terminal. - unscrew both retaining bolts and withdraw the actuator; - check the condition of the o-ring and remove any impurities from the seat in the case; - refit the actuator and check that the plunger fits properly into its seat without forrcing.

**Step motor bolt tightening torque 0.35 - 0.45da Nm** NOTE The engine idle speed actuator is built into the throttle body in postmodification cars.

Connector wiring

The numbers in squares indicate the control unit pins.