General System Description

DESCRIPTION
The Powertrain Control Module (PCM) is the control center of the vehicle. It controls the following:

- On-Board diagnostics for powertrain functions.
- Fuel metering system.
- Transaxle shifting.
- Ignition timing.

It constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. The PCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (Service Engine Soon), and store diagnostic trouble codes which identify the problem areas to aid the technician in making repairs.

FUEL METERING SYSTEM
The main control sensor is the Heated Oxygen Sensor (H02S) located in the exhaust manifold. The HO2S tells the PCM how much oxygen is in the exhaust gas and the PCM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best mixture to minimize exhaust emissions is 14.7 to 1 which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a "Closed Loop" system.

TRANSAXLE SHIFTING
The Torque Converter Clutch (TCC) system uses two PCM controlled solenoids for engagement; a Pulse Width Modulated (PWM) solenoid valve, and an apply solenoid valve in the automatic transaxle to couple the engine flywheel to the input shaft of the transaxle through the torque converter. This reduces the slippage losses in the converter, increasing fuel economy. The PCM uses vehicle speed information along with other inputs such as Throttle Position sensor, RPM and air flow to determine the correct shift times for the transaxle.

EMISSION SYSTEMS

Evaporative Emission (EVAP) Control System
The PCM operates a normally closed solenoid valve, which controls vacuum to the purge valve in the charcoal canister. The PCM turns ON the PWM signal to the solenoid valve to control purge when all conditions for allowing purge are met.

Exhaust Gas Recirculation (EGR) System
The main element of the system is the linear EGR valve. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber. With the fuel/air mixture thus diluted, combustion temperatures are reduced.
The valve controls EGR flow from the exhaust to the intake manifold through an orifice with a PCM controlled pintle. During operation, the PCM controls pintle position by monitoring the pintle position feedback signal. If a problem with the EGR system will not allow the PCM to control pintle position properly, DTC P1406 should set. The PCM also tests for EGR flow; if incorrect flow is detected, DTC P0401 should set.

IGNITION TIMING
The Electronic Ignition (EI) System does not use the conventional distributor and coil. The ignition system consists of three ignition coils, an electronic ignition module, a dual Hall-effect crankshaft position sensor, an engine crankshaft balancer with interrupter rings attached to the rear, related connecting wires, and the Ignition Control (IC) and fuel metering portion of the PCM. To provide optimum engine performance, fuel economy, and control of exhaust emissions, the PCM controls spark advance of the ignition system.

A/C CLUTCH CONTROL
The A/C compressor operation is controlled by the PCM in order to improve idle quality, wide open throttle performance, and to protect the compressor in the event of incorrect refrigerant pressures.

COOLING FAN CONTROL
The PCM controls the operation of the high and low speed cooling fans in order to regulate the engine and A/C refrigerant temperatures. When certain DTC's are present, the PCM may command the cooling fans to run all the time.