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 MS41.X 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 and in addition to the functions just mentioned, this will have an effect on the air conditioning 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 (the adaptations must be cleared using the DIS/MoDIC). If the Throttle Position input is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate when the OBD II criteria is achieved. The ECM will maintain engine operation based on the Hot-Film Air Mass Sensor and the Engine RPM Sensor.
The Idle Speed Control Valve is controlled by the ECM modulating the ground circuits to the two windings for opening and closing. By varying the duty cycle applied to the windings, 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 when the OBD II criteria is achieved.
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 (friction 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 stabilizaton for the increased load on the engine.
The Hot-Film Air Mass 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 when the OBD II criteria is achieved. The ECM will maintain engine operation based on the Throttle Position Sensor and Engine Speed Sensor.
The Air Temperature Signal allows the ECM to make a calculation of intake air temperature. 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 when the OBD II criteria is achieved.