General System Description

Multi-Point Fuel Injection System Schematic (1989 2.5 Turbo):

The electronic, multi-point fuel injection system uses a digital pre-programmed Single Module Engine Controller (SMEC) to control air-fuel mixtures delivered to the engine, ignition timing, idle speed, various emission control devices, the engine cooling fan, the charging system and on turbocharged models, the turbocharger wastegate. The SMEC monitors inputs from various engine and vehicle sensors, computes ideal operating parameters for the controlled subsystems based on these inputs, and controls operation of the subsystems. Sensors used to control system operations include: an exhaust gas oxygen sensor, Manifold Absolute Pressure (MAP) sensor, throttle position sensor, coolant and charge temperature sensors, detonation sensor, vehicle speed sensor, and the brake, speed control, neutral safety and A/C compressor clutch switches and auto shut down relay.
All inputs to the SMEC are converted to signals. These signals cause the SMEC to alter fuel injector delivery and/or ignition timing to maintain ideal air fuel mixtures and ignition timing for all vehicle operating conditions.
In addition, the SMEC tests many of its input and output circuits, and if a fault is found in one of the monitored circuits information relating to the malfunction is stored in the SMEC memory. Information relating to monitored circuit malfunctions can be accessed by the technician through the instrument panel mounted check engine lamp, or by connecting a diagnostic read-out instrument to the system. Both access methods provide numerical type codes which relate to specific circuit malfunctions.
If the SMEC detects a fault in a signal sent from the MAP sensor, throttle position sensor or coolant temperature sensor, or if battery voltage drops or climbs abnormally, the system goes into ``limp-in'' mode. During limp-in mode, the SMEC replaces the failed signal with a modified signal, enabling the vehicle to remain drivable. Also during limp-in mode, the check engine lamp is illuminated.