Engine Control Module: Description and Operation

PCM Connectors:

PURPOSE
The Powertrain Control Module (PCM) is the control center of the fuel injection system. It constantly looks at the information from various sensors and controls the systems that affect vehicle performance.

The PCM also performs a diagnostic function check of the system. It can recognize operational problems and alert the driver through the Malfunction Indicator Lamp (MIL) "Service Engine Soon" and store Diagnostic Trouble Code(s) (DTC) which identify the problem areas to aid the technicians making repairs.

OPERATION
The PCM supplies 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, the use of a 10 megohm input impedance digital voltmeter (J 39200) is required to assure accurate voltage readings.

MEMORY TYPES
There are three types of memory storage within the PCM: Read Only Memory (ROM), Random Access Memory (RAM) and Electrically Erasable Programmable Read Only Memory (EEPROM).

ROM
Read Only Memory (ROM) is a permanent memory that is physically soldered to the circuit boards within the PCM. The ROM contains the overall control programs. Once the ROM is programmed, it cannot be changed. The ROM memory is non-erasable, and does not need power to be retained.

RAM
Random Access Memory (RAM) is the microprocessor "scratch pad." The processor can write into, or read from this memory as needed. This memory is erasable and needs a constant supply of voltage to be retained. If the voltage is lost, the memory is lost.

EEPROM
Electrically Erasable Programmable Read Only Memory (EEPROM) is a permanent memory that is physically soldered to the circuit boards within the PCM. The EEPROM contains the overall control algorithms. The EEPROM can be reprogrammed by using the Tech 1 scan tool or other Tecline terminal/equipment.