Idle Speed Control

The ECM will reference all relevant sensors to achieve the optimum timing for any given condition. This electronically increases the primary coil charging time (dwell angle) as engine speed increases to maintain coil ht voltage at high engine speeds.

The ECM calculates ignition timing using inputs from the following:

• CKP sensor
• TP sensor
• ECT sensor
• IAT sensor.

The ECM calculates dwell angle using inputs from the following:

• CKP sensor
• Battery voltage.

At start up the ECM sets ignition timing by referencing the ECT sensor. After start up, the ignition timing will be controlled according to maps stored in the memory and modified according to additional sensor inputs.

The choice of ignition point is critical in maintaining engine power output with low emissions. Advancing the ignition may increase power output under certain conditions, but it also increases the amount of oxides of nitrogen (NOx) and carbon monoxide (CO) produced in the combustion chamber.

There is a narrow range of ignition points for all engine conditions which give an acceptable compromise between power output and emission control.

The ignition mapping contained within the ECM memory keeps the ignition timing within this narrow band. The ignition timing is used to control engine idle speed in conjunction with the IAC valve stepper motor.

As the MEMS 3 system does not have a knock sensor, ignition timing advance is controlled using different mapping at high engine and intake air temperatures in order to avoid detonation (pinking).

Idle Speed Control

The ECM regulates the engine speed at idling. The ECM uses two methods of idle speed control:

• Ignition timing adjustment
• IAC valve stepper motor.

When the engine idle speed fluctuates, and there are no additional loads on the engine, the ECM will vary the ignition timing and the IAC valve to regulate the idle speed.

This allows very rapid correction of out of tolerance idle speeds. When an additional load is placed on the engine, such as when the power steering is turned on full lock, the ECM uses the IAC valve stepper motor to control the idle speed to specification.

The idle speed is determined from the CKP sensor, but there are also inputs to the ECM from the following:

• Alternator
• Park/Neutral switch (EM-CVT)
• A/C system
• Cooling fan status.

If the ECM receives information from the above inputs that an extra load is being placed on the engine, it can immediately compensate and avoid engine poor idle or stall conditions.

The IAC valve stepper motor is mounted on the inlet manifold and controls a throttle valve air by-pass port.

To increase the idle speed, the stepper motor allows more air to by-pass the throttle and enter the cylinders. To decrease the idle speed, the stepper motor allows less air to enter the cylinders.

The stepper motor is a bi-polar type which consists of two windings controlled by pulse width modulated (PWM) signals from the ECM.

The position of the stepper motor is always referenced on power down of the ECM, this may take from three to five seconds. The stepper motor is also used to reduce manifold vacuum during deceleration to control emissions.