Stepper motor

STEPPER MOTOR

This is contained within the throttle body and through a reduction gear operates a cam and push rod. The push rod is in direct contact with the throttle disc spindle lever and allows idle and fast idle speeds to be controlled by the ECM. The stepper motor maximum movement is 3.75 revolutions accomplished by 180 steps of 7.5, a reduction gear reduces this to 150 cam movement.

Stepper Motor Operation

To determine the required amount of stepper motor movement the ECM receives signals from:

1. Crankshaft position sensor (engine speed) - U/P and W/U wires
2. Manifold absolute pressure sensor (engine load) - integral part of ECM
3. Engine coolant temperature sensor (engine temperature) - K/G wire, K/B wire-earth
4. Battery voltage (state of battery charge)
5. Ignition supply (ignition switched off)

With the aid of these inputs the ECM can index the stepper to compensate for all prevailing conditions.

The stepper motor receives battery supply on the N/K wire via the main relay contacts and index positions on the O/S (phase 1), K/U (phase 2), O/G (phase 3) and O/U (phase 4) wires.

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 the coolant temperature 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 engine speed is below a predetermined set point.