Coolant Temperature Compensation

Coolant Temperature Compensation

During cold running conditions the ECM controls the stepper motor to provide a fast idle condition (elevated idle). The ECM determines the amount of fast idle required by sensing engine coolant temperature via a coolant sensor located in the thermostat housing.

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 throttle pedal is OFF, indicated by throttle position sensor. 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 energises for 30 seconds. During this time the stepper motor is indexed to 180 steps (fully open).