Principle Of Operation

Air flow into the engine is regulated by the Throttle Valve or the Idle Speed Control Valve. Both of these air "passages" are necessary for smooth engine operation from idle to full load. On the M1.7.2 system, the Throttle Valve is mechanically controlled  and the Idle Speed Control Valve is electrically controlled  . All of the ECM monitoring, processing and output functions are a result of regulated air flow.

The Throttle Position Sensor  is monitored by the ECM for throttle angle position and rate of movement. As the throttle plate is opened, a rising voltage signal (up to 5v) requests acceleration and at what rate. The ECM will increase the volume of fuel injected into the engine, advance the ignition timing and decrease the Idle Speed Valve opening (air is now going by the throttle plate). The "full throttle" position indicates maximum acceleration to the ECM, this will have an effect on the A/C compressor (covered in Performance Controls).

As the throttle plate is closed (integral springs), a decrease in voltage signals the ECM to activate fuel shut off if the RPM is above idle speed (coasting). The Idle Speed Control Valve will then be opened to maintain idle speed.

The ECM monitors the engine idle speed in addition the Throttle Position Sensor voltage. The voltage value is "learned" at the correct idle speed and if the voltage value has changed (mechanical wear of throttle plate or linkage), the ECM will adjust the Idle Speed Control Valve to maintain the correct idle speed based on the "new" voltage. To clear this "learned" value, disconnect the ECM for at least one minute. If the Throttle Position input is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate. The ECM will maintain engine operation based on the Air Flow Volume Sensor and the Engine RPM Sensor.

The Idle Speed Control Valve  is controlled by the ECM modulating the ground signal to the valve, opening it against spring pressure. By varying the duty cycle applied to the winding, the valve can be progressively opened, or held steady to maintain the idle speed. If the Idle Speed Control Valve circuit is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate. The valve will spring to the fixed opening, allowing the engine to idle.

There are additional factors that influence the ECM in regulating idle speed:

• The RPM sensor input allows the ECM to monitor engine speed because of loads that cause idle fluctuations due to drag on the engine: power steering, thick oil (fractional forces), etc. 
• Cold engine temperature (coolant NTC) provides higher idle speed to raise temperature sooner. 
• Vehicle speed informs ECM when the vehicle is stationary and requires idle maintenance 
• A/C on request from the climate control system (arming the ECM) and compressor engage (stabilize idle speed) acknowledgment. 
• Range selector provides a Park/Neutral input to the ECM identifying when the vehicle is in a drive gear. This signal allows idle stabilization for the increased load on the engine. 

The Air Flow Volume Sensor  sends a varying voltage (0-5v) to the ECM representing the measured amount of intake air volume. This input is used by the ECM to determine the amount of fuel to be injected. If this input is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate. The ECM will maintain engine operation based on the Throttle Position Sensor and Engine RPM Sensor.

The Air Temperature  signal allows the ECM to make a calculation of air density. The varying voltage input from the NTC sensor indicates the larger proportion of oxygen found in cold air, as compared to less oxygen found in warmer air. The ECM will adjust the amount of injected fuel because the quality of combustion depends on oxygen sensing ratio.

The ignition timing is also affected by air temperature. If the intake air is hot the ECM retards the base ignition timing to reduce the risk of detonation. If the intake air is cooler, the base ignition timing will be advanced. If this input is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate.

DISA  is controlled by the ECM activating the Change Over Solenoid below 4840 RPM.

• When activated the solenoid applies vacuum to the change over valve and the valve closes, providing the long pipe effect. 
• Above the RPM, the solenoid is switched off and the Change Over Valve springs opens, providing the short pipe effect. 
• On decel, the solenoid will not be activated until 4760 RPM. This over lap prevents repeated opening and closing of the valve while driving at a constant engine speed of 4800 RPM. 

If there is a defect in this system, the Changeover Valve will be opened to ensure intake air availability for maximum power (short pipe affect). The Vacuum Motor and valve shaft are both spring loaded to open the Changeover Valve if vacuum is not applied.