Idle Speed Control

Idle Speed Control

Once the engine has reached normal operating temperature, the idle speed is controlled by the ECM. When the idle drops below the specified idle speed (due to increased mechanical or electrical loads), the ECM indexes the stepper motor to restore the correct idle speed.

Idle speed control will only occur when the engine speed is below a predetermined set point.

On return to idle, the ECM initially implements a slightly higher idle speed. This prevents the engine stalling when the throttle is closed.

Cranking Position

During cranking, the ECM indexes the stepper to a position for a ’throttle off’ start. This position is dependent upon coolant temperature.

Over-run Control

During over-run conditions, the ECM indexes the stepper motor, opening the throttle disc. This increases the air flow into the engine reducing hydrocarbon emissions.

Low Battery Voltage Compensation

When the battery voltage drops below a preset value, the ECM indexes the stepper motor increasing the idle speed, consequently increasing alternator output.

Ignition Off Position

When the ignition is switched off, the ECM keeps the main relay energised for 30 seconds. During this time the stepper motor is indexed to 180 steps (fully open).

The MEMS idle control is an adaptive system and the ECM learns the engine load and wear characteristics over a period of time. The amount of stepper motor steps required to maintain the specified idle consequently will differ from model to model. In the event of a new ECM or an ECM from another vehicle being fitted, it will take a short time of normal driving for the ECM to learn the load and wear characteristics of that engine.