Engine Control Module: Description and Operation

Powertrain Control Module:

PURPOSE
The Powertrain Control Module (PCM), is located under hood between the right shock tower and the bulkhead. It is the control center of the fuel injection system and constantly monitors 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 (MIL) "Service Engine Soon," and store a Diagnostic Trouble Code (DTC) which identifies the problem areas to aid the technician in making repairs. A scan tool must be used to retrieve DTCs from the PCM memory. DTCs can no longer be retrieved by grounding terminals "A" to "B" at the DLC. This also eliminates the PCM function of flashing Code 12.
The PCM used on this engine is referred to as SFI-66. For service, this PCM consists of only two parts: a dual processor controller and Knock Sensor (KS) module.

OPERATION
The PCM contains a power supply which regulates the 12 volt vehicle supply input to 5 and 12 volts, and these voltages are used for various internal and external functions.

The PCM supplies a buffered 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 analog 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 or a Tech 1 scan tool is necessary to assure accurate voltage readings.
The Input/Output (I/O) devices in the PCM include analog to digital converters, signal buffers, counters and special drivers. The PCM controls output circuits such as the fuel injectors, Idle Air Control (IAC) Valve, cooling fan relay, etc. by controlling the ground circuit through transistors or a device called a "quad-driver" in the PCM.

MEMORY
There are two types of memory storage within the PCM: EEPROM and RAM.

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 Techline terminal/equipment.

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