Engine Control Module: Description and Operation

The MPI Control Unit consists of an 8-bit microprocessor, random access memory (RAM), read only memory (ROM) and an Input/Output (I/O) interface.

By increasing or decreasing the fuel injector on-time (pulse width), the Control Unit is able to provide a precise air/fuel ratio for the driving condition. Opening and grounding the Power Transistor circuit controls the Ignition System dwell and timing advance/retard. Vehicle idle speed is controlled through the idle speed control servo, and fuel pump drive power is supplied through the MPI control relay. To regulate exhaust emissions, the MPI Control Unit internally grounds the solenoid circuits, which supply vacuum for canister purging or to the EGR Valve (applicable to California model vehicles). Also if needed, the MPI Control Unit may temporarily interrupt the power supply to the A/C power relay.

Prior to adjusting the fuel injector pulse width the control unit compares the pre-programmed information (ROM) with returning input signals from the Air Flow Sensor, Barometric Pressure Sensor, Intake Air Temperature Sensor, Engine Coolant Temperature Sensor, Throttle Position Sensor, Idle Position Switch, Crank Angle Sensor, Top Dead Center (TDC) Sensor, and the Oxygen Sensor.

Ignition timing is influenced by ROM and the Air Flow Sensor, Barometric Pressure Sensor, Intake Air Temperature Sensor, Engine Coolant Temperature Sensor, Throttle Position Sensor, Idle Position Switch, TDC Sensor, Crank Angle Sensor, the Vehicle Speed Sensor, the Inhibitor Switch (on automatic transmission equipped models), and the Ignition Switch Start Signal.

The Idle Speed Control Servo adjusts the idle speed according to engine load which is translated into stepper motor positions by the Control Unit. This is done by input signals from Engine Coolant Temperature Sensor, Throttle Position Sensor, Idle Position Switch, ISC Motor Position Sensor, Crank Angle Sensor, Vehicle Speed Sensor, A/C Select Switch, Inhibitor Switch (on automatic transmission equipped models), Power Steering Oil Pressure Switch, the Ignition Switch and the Ignition Switch Start Signal.

Emission Controls include the Purge Control Solenoid and the EGR Valve Solenoid. Both components are Control Unit activated, but depend on different sensor input signals. The Purge Control Solenoid circuit is energized once the Control Unit evaluates input from the Air Flow Sensor, Barometric Pressure Sensor, Intake Air Temperature Sensor, and the Engine Coolant Temperature Sensor, and the A/C Select Switch. The EGR Valve Solenoid is activated with input from the Air Flow Sensor, the Engine Coolant Temperature Sensor, and the Crank Angle Sensor.

Power to the Fuel Pump Drive Terminal is supplied after the Control Unit receives a signal input from the Ignition Switch and the Crank Angle Sensor. The A/C power Relay is closed allowing current to flow depending upon the Throttle Position Sensor (TPS), the Crank Angle Sensor, the A/C Select Switch, and the Inhibitor Switch.