Computers and Control Systems: Description and Operation

MPI System:

The Computerized Engine Control system is built around the electronic MPI Control Unit. The MPI Control Unit constantly monitors return signals from various sensor inputs and then responds by opening or grounding an output device circuit within the MPI Control Unit. As operating conditions change, the MPI Control Unit makes adjustments to maintain optimum performance and fuel economy.
This vehicle uses an oxygen sensor to provide the MPI Control Unit with information concerning air/fuel mixture. The O2 sensor is threaded into the exhaust manifold and measures the level of oxygen within the exhaust gas. A voltage is generated and sent to the MPI Control Unit as an input signal. The MPI Control Unit reads this signal input and the return signals from the other sensors, then will either richen or lean the air/fuel mixture.


INPUTS
^ Power Supply (Ignition Switch Coupled)
^ Power Supply (Battery Backup)
^ Air Flow Sensor
^ Barometric Pressure Sensor
^ Intake Air Temperature Sensor
^ Engine Coolant Temperature Sensor
^ Throttle Position Sensor
^ [1][2]Idle Position Switch
^ No. 1 Cylinder TDC Sensor
^ Crank Angle Sensor
^ Oxygen Sensor
^ Vehicle Speed Sensor
^ A/C ON-OFF Switch
^ Inhibitor Switch (A/T)
^ Ignition Switch
^ Ignition Switch START terminal (start signal)
^ Power Steering Oil Pressure Switch


OUTPUTS
^ Fuel Injector Pulse Width
^ Idle Speed Control (ISC) Servo
^ Ignition Timing Power Transistor
^ Air-Conditioner Power Relay
^ Fuel Pump Control Relay
^ Evaporative Emissions Purge Control Solenoid Valve
^ EGR Control Solenoid Valve (M/T models only)
^ Self-Diagnosis Output
^ Malfunction Indicator light

Instrument Panel:

MALFUNCTION INDICATOR LIGHT
If a component or system malfunction is detected, the MPI Control Unit will illuminate the "MALFUNCTION INDICATOR LIGHT" and record the corresponding trouble code(s) in its memory. However, when an irregular signal returns to normal and the engine control unit judges that it has returned to normal, the malfunction indicator light will go out. When the ignition switch is turned off, the light will go out. Even if the ignition switch is turned on again, the light will not illuminate until the irregularity is detected once again.
Under normal conditions, the malfunction indicator light is lit for 2.5 seconds immediately after the ignition switch is turned on. This is to indicate that the light is operating normally.