Powertrain Control Module (PCM)

PCM:

DESCRIPTION
The Powertrain Control Module (PCM) is designed to maintain exhaust emission levels while maintaining excellent driveability and fuel efficiency. The PCM controls the following operations:

^ Fuel control
^ Ignition Control (IC)
^ Knock Sensor (KS) system
^ Automatic transmission shift functions
^ Manual transmission
^ Cruise Control Enable (if so equipped)
^ Generator (if so equipped)
^ Evaporative Emission (EVAP) Purge
^ A/C Clutch Control
^ Cooling Fan Control
^ Secondary Air Injection (AIR) (if so equipped)
^ Exhaust Gas Recirculation (EGR) (if so equipped)

PCM FUNCTION
The PCM supplies a buffered voltage to various sensors and switches. The PCM controls most components with electronic switches which complete a ground circuit when turned ON. The Powertrain Control Module (PCM) is located in the engine compartment and is the control center of the vehicle. It controls the following:

^ Fuel metering system.
^ Transmission shifting.
^ Ignition timing.
^ On-board diagnostics for powertrain functions.

The PCM 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. The PCM also alerts the driver through the MIL (Malfunction Indicator Lamp). When the PCM detects a malfunction, it stores a diagnostic trouble code (DTC). A DTC stored, will identify the problem areas. This will aid the technician in making repairs.

The PCM supplies either 5.0 or 12.0 volts to power various sensors or switches. This is done through resistances in the PCM. The resistance is so high in value that a test lamp will not illuminate when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low. Therefore, a digital voltmeter (J 39200) with at least 10 megaohms input impedance is required to ensure accurate voltage readings.

The PCM controls output circuits such as the injectors, IAC, cooling fan relays, etc. by controlling the ground or the power feed circuit through transistors or a device called an Output Driver Module.

TORQUE MANAGEMENT
Torque Management is a function of the PCM that reduces engine power under certain conditions. Torque Management is performed for the following reasons:

1. To prevent overstress of the powertrain components.
2. To reduce engine power during certain Throttle Actuator Control system faults.
3. To limit the engine power when the brakes are applied more than approximately 40%.
4. To prevent damage to the vehicle during certain abusive maneuvers.

ENGINE OUTPUT TORQUE
The PCM monitors the following sensors and engine parameters to calculate engine output torque:

^ Air/Fuel ratio
^ Mass Air Flow
^ Manifold Absolute Pressure
^ Intake Air Temperature
^ Spark Advance
^ Engine Speed
^ Engine Coolant Temperature
^ A/C Clutch Status

ENGINE POWER REDUCTION
The PCM monitors the torque converter status, the transmission gear ratio, and the extended brake switch input in order to determine if torque reduction is required. The PCM retards the spark as appropriate to reduce engine torque output if torque reduction is required. The PCM also shuts off the fuel to certain injectors to reduce the engine power in the case of an abusive maneuver.

The following are instances when engine power reduction is likely to be experienced:

^ During transmission upshifts and downshifts.
^ Heavy acceleration from a standing start.
^ The brakes are applied with moderate to heavy throttle (with the traction system active).
^ When the driver is performing stress-inducing (abusive) maneuvers such as shifting into gear at high throttle angles or shifting the transmission from reverse to drive to create a rocking motion.

The driver is unlikely to notice the torque management actions in the first two instances. The engine power output will be moderate at full throttle in the other two cases. The PCM calculates the amount of spark retard necessary to reduce the engine power by the desired amount. The PCM disables the fuel injectors for cylinders 1, 4, 6, and 7 in the case of an abusive maneuver.