Engine Control Module: Description and Operation

Powertrain Control Module (PCM):

PCM MONITORING
The PCM is responsible for maintaining proper spark and fuel injection timing for all driving conditions. To provide optimum driveability and emissions, the PCM monitors input signals from the following components in calculating Ignition Control (IC) spark timing:

^ Ignitions Control module (ICM).
^ Engine Coolant Temperature (ECT) sensor.
^ Intake Air Temperature (IAT) sensor.
^ Mass Air Flow (MAF) sensor.
^ Trans Range inputs from Transaxle Range switch.
^ Throttle Position (TP) sensor.
^ Vehicle Speed Sensor (VSS).

PCM CONTROL
The Powertrain Control Module (PCM) controls the following items:

^ Fuel metering system.
^ Transaxle shifting.
^ Ignition timing.
^ On-board diagnostics for Powertrain functions.

DESCRIPTION
The Powertrain Control Module (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, alert the driver through the MIL (Service Engine Soon), and store diagnostic trouble codes which identify the problem areas to aid the technician in making repairs. The PCM used on the 3100 engines is referred to as PCM 32U.

PCM FUNCTION
The PCM supplies either 5 or 12 volts to power various sensors or switches. This is done through resistances in the PCM which are so high in value that a test light will not light 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 with at least 10 megohms input impedance is required to ensure accurate voltage readings. Tool J 39200 meets this requirement. The PCM controls output circuits such as the injectors, IAC, cooling fan relays, etc. by controlling the ground or power feed circuit through transistors or a device called a Driver.

EEPROM
The Electrically Erasable Programmable Read Only Memory (EEPROM) is a permanent memory that is physically soldered within the PCM. The EEPROM contains program and calibration information that the PCM needs to control powertrain operation. Unlike the PROM used in certain past applications, the EEPROM is not replaceable. If the PCM is replaced, the new PCM will need to be programmed. Techline equipment (Tech-2) containing the correct program and calibration for the vehicle is required to program the PCM.

For a description of Ignition Module to PCM circuits refer to Ignition System / Description and Operation / Spark Control System. Description and Operation