Stepper Motor

Back Up Values : Coolant sensor - 60oC Inlet air sensor - 35oC Manifold absolute pressure - the ECM implements air fuel ratio relating to engine speed and throttle position.

Throttle position sensor The throttle position (TP) sensor is a potentiometer attached to the throttle housing and is directly coupled to the throttle disc. Closed throttle is detected by the TP sensor which initiates idle speed control via the idle air control valve (IACV).

The TP sensor receives a 5 volt supply from the ECM on the Y/P wire and is supplied with an earth on the K/B wire. The sensor then provides a signal which is proportional to throttle disc position on the Y/G wire to the ECM.

Exhaust emission control (closed loop) The ECM uses a signal from the oxygen sensor to control exhaust emissions. This system controls the air/fuel ratio to 14.7 : 1 at idle and cruise conditions.

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 7.5o, a reduction gear reduces this to 150o 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) 2 Manifold absolute pressure sensor (engine load) 3 Engine coolant temperature sensor (engine temperature) 4 Throttle position sensor (throttle closed) 5 Battery voltage (state of battery charge) 6 Ignition supply (ignition switched off)

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