Engine Control Module: Description and Operation

PCM Connectors:

PCM With Knock Sensor Module:

DESCRIPTION
Vehicle Control Center
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.

Diagnostic Function
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 a Diagnostic Trouble Code (DTC) or DTCs which identify the problem areas to aid the technician in making repairs. For more information on using the diagnostic function of the PCM refer to System Diagnosis/Procedures/Vehicle On-Board Diagnostics.

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
Three Memory Types
There are three types of memory storage within the PCM: ROM, RAM and EEPROM. They are each described below.

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 he 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 Techline terminal/equipment.

SHORT TERM FUEL TRIM
Injector Pulse Width Control
The short term fuel trim is a PCM erasable memory register that will contain a number between 0 and 255. The neutral value for the short term fuel trim is 128; any deviation from this value indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 128. The short term fuel trim changes the pulse width by varying the "Closed Loop" factor of the base pulse width equation. As the PCM monitors the oxygen sensor's input, it is constantly varying the short term fuel trim value.

Short Term Fuel Trim Updates
The value is updated very quickly, therefore, the short term fuel trim only corrects for short term mixture trends. The correction of long term mixture trends is the function of long term fuel trim.

LONG TERM FUEL TRIM
RPM and MAP Sensor Registers
The long term fuel trim is a matrix of cells arranged by RPM and Manifold Absolute Pressure (MAP). Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the PCM will switch from cell to cell to determine what "long term fuel trim" factor to use in the base pulse width equation.

Long Term Fuel Trim Updates
While in any given cell, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from 128, the PCM will change the long term fuel trim value. Once the long term fuel trim value is changed, it should force the short term fuel trim back toward 128.

Long Term Fuel Trim Corrections
If the mixture is still not correct, as judged by the Heated Oxygen Sensor (HO2S), the short term fuel trim will continue to have a large deviation from the ideal 128. In this case, the long term fuel trim value will continue to change until the short term fuel trim becomes balanced.

FUEL TRIM CONTROL LIMITS
Both the short term fuel trim and long term fuel trim have limits which vary by calibration. If the mixture is off enough so that long term fuel trim reaches the limit of its control and still cannot correct the condition, the short term fuel trim would also go to its limit of control in the same direction. If the mixture is still not corrected by both short term fuel trim and long term fuel trim at their extreme values, an HO2S Diagnostic Trouble Code (DTC) will likely result. Under the conditions of power enrichment, the PCM sets the short term fuel trim to 128 and freezes it there until power enrichment is no longer in effect. This is done so the "Closed Loop" factor and the long term fuel trim will not try to correct for the commanded richness of power enrichment.