Idle Up Control Valve: Description and Operation

Idle air control (IAC)






Idle speed is controlled by a stepper motor which consists of two coils. When energised in the correct sequence the coils move a plunger which opens and closes the throttle bypass controlling the quantity of idle air. The stepper motor controls idle speed by moving the plunger a set distance called a step. Fully open is zero steps and fully closed 180 steps. Failure of the stepper motor will result in low or high idle speed, poor idle, engine stall or non start. The fault is indicated by illumination of the malfunction indicator light (MIL) on North American specification vehicles.