Powertrain Management: Description and Operation

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

- Fuel metering system.
- Transaxle shifting.
- Emission Systems.
- 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 Sensors (H02S) located in the exhaust system between the Catalytic converters and the exhaust manifolds. 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

Automatic (4L60E)
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.

Manual (ZF S6-40)
The PCM uses engine speed, vehicle speed, throttle position and engine coolant temperature sensors to determine if the vehicle is within the parameters to shift directly to 4th gear while it is in 1st gear. The PCM provides a path to ground which energizes the relay when the vehicle should be shifted from 1st gear to 4th gear.

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 EGR valve mounted on the intake manifold and operated by vacuum. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber.
The EGR valve is opened by vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. If too much exhaust gas enters, combustion will not occur. For this reason, very little exhaust gas is allowed to pass through the EGR valve. The EGR valve is usually open during warm engine operation when the vehicle is above idle speed.

Secondary Air Injection (AIR) System
The AIR pump with integral stop valve is controlled by the PCM. The PCM will turn "ON" the AIR pump by providing the ground to complete the circuit which energizes the AIR pump relay. When air to the exhaust ports is desired, the PCM turns "ON" both the integral stop valve and AIR pump.

IGNITION TIMING
The distributor ignition system is a net build distributor (no timing adjustment) that provides angle based timing information to the Powertrain Control Module (PCM) for individual cylinder spark timing. The ignition module is capable of providing both 4X and 18OX timing pulses each crankshaft revolution. Using these timing pulses, the PCM processes ignition spark timing and sends an ignition control (IC) signal to the ignition coil module to activate the secondary ignition system.