The Powertrain Control Module (PCM) controls most all engine and transmission related operations. Its ability to receive information from various sensors, perform rapid calculations and output commands to the ignition and fuel injection allow the engine to achieve low exhaust emissions while maintaining surprising engine performance. The PCM is a complex, sophisticated on-board solid state computer which receives signals from many sources and sensors; it uses these data to make judgements about operating conditions and then control output signals to the fuel and emission systems to match the current requirements. It is factory programmed with the vehicle's Vehicle Identification Number (VIN) to match its computing ability to a specific platform/powertrain/option combination.
Inputs are received from many sources to form a complete picture of Powertrain operating conditions. Some inputs are simply Yes or No messages, such as that from the Park/Neutral switch; the vehicle is either in gear or in Park/Neutral; there are no other choices. Other data is sent in quantitative input, such as engine RPM or coolant temperature. The PCM is programmed to recognize acceptable ranges or combinations of signals and control the outputs to control emissions while providing good driveability and economy. The PCM also monitors some output circuits, making sure that the components function as commanded. For proper engine operation, it is essential that all input and output components function properly and communicate properly with the PCM.
Since the PCM is programmed to recognize the presence and value of electrical inputs, it will also note the lack of a signal or a radical change in values. It will, for example, react to the loss of signal from the vehicle speed sensor or note that engine coolant temperature has risen beyond acceptable (programmed) limits. Once a fault is recognized, a Diagnostic Trouble Code (DTC) is assigned and held in memory. The dashboard Malfunction Indicator Lamp (MIL)-CHECK ENGINE or SERVICE ENGINE SOON-will illuminate to advise the operator that the system has detected a fault.
More than one DTC may be stored. Although not every engine uses every code, there is a large range of codes. Additionally, the same code may carry different meanings relative to each engine or engine family.
In the event of a PCM failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although likely at a reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is almost always affected when the PCM enters this mode.
The PCM can compensate for minor variations within the fuel system through the block learn and fuel integrator systems. The fuel integrator monitors the oxygen sensor output voltage, adding or subtracting fuel to drive the mixture rich or lean as needed to reach the ideal air fuel ratio of 14.7:1. The integrator values may be read with a scan tool; the display will range from 0-255 and should center on 128 if the oxygen sensor is seeing a 14.7:1 mixture.
The temporary nature of the integrator's control is expanded by the block learn function. The name is derived from the fact that the entire engine operating range (load vs. rpm) is divided into 16 sections or blocks. Within each memory block is stored the correct fuel delivery value for that combination of load and engine speed. Once the operating range enters a certain block, that stored value controls the fuel delivery unless the integrator steps in to change it. If changes are made by the integrator, the new value is memorized and stored within the block. As the block learn makes the correction, the integrator correction will be reduced until the integrator returns to 128; the block learn then controls the fuel delivery with the new value.
The next time the engine operates within the block's range, the new value will be used. The block learn data can also be read by a scan tool; the range is the same as the integrator and should also center on 128. In this way, the systems can compensate for engine wear, small air or vacuum leaks or reduced combustion.
Any time the battery is disconnected, the block learn values are lost and must be relearned by the PCM. This loss of corrected values may be noticed as a significant change in driveability. To re-teach the system, make certain the engine is fully warmed up. Drive the vehicle at part throttle using moderate acceleration and idle until normal performance is felt.
DASHBOARD WARNING LAMP
The primary function of the dash warning lamp (properly called the Malfunction Indicator Lamp, or MIL) is to advise the operator and that a fault has been detected, and, in most cases, a code stored. Under normal conditions, the dash warning lamp will illuminate when the ignition is turned ON . Once the engine is started and running, the PCM will perform a system check and extinguish the warning lamp if no fault is found.
On the vehicles covered by this guide, the DTCs can only be read using a scan tool, such as GM's Tech 1® or Tech 2®, or equivalent. These tools are so-called OBD-II Compliant. This means they are compatible with the federal-mandated On-Board Diagnostic, Second Generation systems. Almost from the beginning of computer controlled vehicles, some sort of on-board diagnostic ability has been built into the vehicle's computer to assist the technician. At one time, the vehicle's instrument panel Malfunction Indicator Lamp could be used to read some codes. Even a vehicle owner could read the flashing MIL lamp and interpret the trouble code. Scan tools became available to the profession technician. A scan tool allows any stored codes to be read from the PCM memory. The tool also allows the technician to view the data being sent to the PCM while the engine is running. While the scan tool makes collecting information easier, the data must be correctly interpreted by a technician familiar with the system.
As emission regulations get more stringent and vehicles get more complex, the vehicle's on-board computers have evolved into powerful diagnostic tools. The vehicles covered by this guide require the use of OBD-II compliant Scan Tools for efficient troubleshooting, reading DTCs and reading the PCM's data stream. These vehicles use what is called a Class II Serial Data circuit to a Diagnostic Link Connector which allows bi-directional communication between the PCM and the scan tool.
An example of the usefulness of the scan tool may be seen in the case of a temperature sensor which has changed its electrical characteristics. The PCM is reacting to an apparently warmer engine (causing a driveability problem), but the sensor's voltage has not changed enough to set a fault code. Connecting the scan tool, the voltage signal being sent to the PCM may be viewed. Comparison to either factory published information of normal values or a known good vehicle may reveal the problem quickly.