Powertrain Control Module (PCM) Overview

Class II Serial Data

The U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the Class II communications system. Each bit of information can have one of two lengths: long or short. This allows the vehicle wiring to be reduced by the transmission and reception of multiple signals over a single wire. The messages carried on the Class II data streams are also prioritized. In other words, if two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that the regulation provides the scan tool manufacturers with the capability of accessing the data from any make or model vehicle that is sold in the United States.

The Diagnostic Executive

The Diagnostic Executive is a unique segment of software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are:

On Board Diagnostic Tests

A diagnostic test is a series of steps which has a beginning and an end. The result of which is a pass or a fail which is reported to the Diagnostic Executive. When a diagnostic test reports a pass result, the Diagnostic Executive records the following data:

When a diagnostic test reports a fail result, the Diagnostic Executive records the following data:

Trip

The ability for a diagnostic test to run depends largely upon whether or not a Trip has been completed. A Trip for a particular diagnostic is defined as a key ON and a key OFF cycle in which all of the enabling criteria for a given diagnostics has been met, allowing the diagnostic to run the vehicle operation, followed by an engine OFF period of duration and driving mode such that any particular diagnostic test has had sufficient time to complete at least once. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some diagnostic tests run only once per trip (e.g. catalyst monitor) while others run continuously (e.g. misfire and fuel system monitors). If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.

Warm Up Cycle

A Warm-up cycle consists of an engine start-up and a vehicle operation of such that the coolant temperature has risen greater than 40°F from the start-up temperature and reached a minimum engine coolant temperature of 160°F. If this condition is not met during the ignition cycle, the diagnostic may not run.

Diagnostic Information

The diagnostic Tables and functional checks are designed to locate a poor circuit or component through a process of logical decisions. The Tables are prepared with the assumption that the vehicle functioned correctly at the time of assembly and that there are no multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code will set and the Malfunction Indicator Lamp (MIL) Malfunction Indicator Lamp will illuminate on some applications.

Malfunction Indicator Lamp (MIL)

The MIL (Malfunction Indicator Lamp) is on the instrument panel. The MIL has the following functions:

When the MIL remains on while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform an On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults which the technician may not detect if other diagnostics are performed first.

Intermittent Malfunction Indicator Lamp

In the case of an intermittent malfunction, the MIL (Malfunction Indicator Lamp) may illuminate and then after 3 trips turn off. However, the corresponding diagnostic trouble code will store in the memory. When the unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

Data Link Connector (DLC)

The provision for communicating with the control module is a Data Link Connector (DLC). The DLC is usually located under the instrument panel. The DLC is used in order to connect to a scan tool. Some common uses of the scan tool are listed below:

Reprogramming (Flashing) The Control Module

Some vehicles allow the reprogramming of the control module without any removal from the vehicle. This provides a flexible and a cost-effective method of making changes in the software and the calibrations.

Refer to the latest Techline information on the reprogramming or the flashing procedures.

Verifying Vehicle Repair

Verification of the vehicle repair will be more comprehensive for the vehicles with the OBD II system diagnostics. Following a repair, the technician should perform the following steps:

1. Review the fail records and the freeze frame data for the DTC which was diagnosed. Record the fail records or freeze frame data. The freeze frame data will only store for an A or B type diagnostic and only if the MIL has illuminated.
2. Clear the DTCs.
3. Operate the vehicle within the conditions noted in the fail records or the freeze frame data.
4. Monitor the DTC status information for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.

Following these steps is very important in verifying the repairs on the OBD II systems. Failure to follow these steps could result in unnecessary repair.

Reading Diagnostic Trouble Codes

The procedure for reading the diagnostic trouble codes is to use a diagnostic scan tool. When reading DTCs follow these steps could result in unnecessary repairs.

DTC Modes

On the OBD II vehicles there are three options available in the scan tool DTC mode to display the enhanced information available. A description of the new modes, DTC Info and Specific DTC, follows. After selecting DTC, the following menu appears:

The following is a brief description of each of the sub menus in DTC Info and Specific DTC. The order in which they appear here is alphabetical and not necessarily the way they will appear on the scan tool.

DTC Info Mode

Use the DTC Info mode in order to search for a specific type of stored DTC information. There are seven choices. The electronic service information may instruct the technician to test for the DTCs in a certain manner. Always follow the published service procedures.

In order to retrieve a complete description of any status, press the Enter key before pressing the desired F key. For example, pressing enter, then an F key will display a definition of the abbreviated scan tool status.

DTC Status

This selection displays any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. The DTC tests which run and pass removes that DTC number from the scan tool screen.

Fail This Ign. (Fail This Ignition)

This selection displays all of the DTCs that have failed during the present ignition cycle.

History

This selection displays only the DTCs that are stored to the control module history memory. The history memory will not display the type B DTCs that have not requested the MIL. The history memory will display all of the type A and type B DTCs that have the MIL and have failed within the last 40 warm-up cycles. In addition, the history memory will display all of the type C DTCs that have failed within the last 40 warm-up cycles.

Last Test Fail

This selection displays only the DTCs that have failed during the last time the test ran. The last test may have ran during a previous ignition cycle if the a type A or B DTC is displayed. For type C DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.

MIL Request

This selection displays only the DTCs that are requesting the MIL. The type C DTCs cannot be displayed by using this option. This selection will report any type B DTCs only after the MIL has been requested.

Not Run SCC (Not Run Since Code Clear)

This option displays up to 33 DTCs that have not run since DTCs were last cleared. Since any displayed DTCs have not run, their condition (passing or failing) is unknown.

Test Fail SCC (Test Failed Since Code Clear)

This selection displays all active and history DTCs that have reported a test failure since the last time DTCs were cleared. The DTCs that last failed over 40 warm-up cycles before this option is selected will not be displayed

Specific DTC Mode

This mode is used in order to check the status of individual diagnostic tests by DTC number. This selection can be accessed if a DTC has passed or failed. Many OBD II DTC mode descriptions are possible because of the extensive amount of information that the Diagnostic Executive monitors regarding each test. Some of the many possible descriptions follow with a brief explanation.

This selection only allows the entry of DTC numbers that are supported by the vehicle being tested. If an attempt is made to enter DTC numbers for tests which the diagnostic executive does not recognize, the requested information will not be displayed correctly and the scan tool may display an error message. The same applies to using DTC trigger option in the Snapshot mode. If an invalid DTC is entered, the scan tool will not trigger.

Failed Last Test

For type A and B DTCs, this message will display during the subsequent ignition cycles until the test passes or the DTCs are cleared. For type C DTCs, this message clears whenever the ignition is cycled.

Failed Since Clear

This message displayed indicates that the diagnostic test failed at least once within the last 40 warm-up cycles since the last time the control module cleared the DTCs.

Failed This Ign. (Failed This Ignition)

This message displayed indicates that the diagnostic test has failed at least once during the current ignition cycle. This message will clear when the DTCs are cleared or the ignition is cycled.

History DTC

This message displayed indicates that the DTC has stored to memory as a valid fault. A DTC displayed as a History fault does not necessarily mean that the fault is no longer present. The history description means that all the conditions necessary for reporting a fault have met.

MIL Requested

This message displayed indicates that the DTC is currently causing the MIL to turn on. Remember that only type A and B DTCs can request the MIL. The MIL request cannot determine if the DTC fault conditions are currently being experienced. This is because the diagnostic executive requires up to 3 trips during which the diagnostic test passes to turn off the MIL.

Not Run Since Cl (Not Run Since Cleared)

This message displayed indicates that the selected diagnostic test has not run since the last time the DTCs were cleared. Therefore, the diagnostic test status (passing or failing) is unknown. After the DTCs are cleared, this message continues to be displayed until the diagnostic test runs.

Not Run This Ign. (Not Run This Ignition)

This message displayed indicates that the selected diagnostic test has not run this ignition cycle.

Test Ran and Passed

This message displayed indicates that the selected diagnostic test has the following items:

Whenever the indicated status of the vehicle is Test Ran and Passed after a repair verification, the vehicle is ready to be released to the customer.

If the indicated status of the vehicle is Failed This Ignition after a repair verification, then the repair is incomplete. A further diagnosis is required.

Prior to repairing a vehicle, use the status information in order to evaluate the state of the diagnostic test and to help identify an intermittent problem. The technician can conclude that although the MIL is illuminated, the fault condition that caused the code to set is not present. An intermittent condition must be the cause.

Powertrain Control Module (PCM) Control Module Communication

The control module system has a computer, Powertrain Control Module (PCM) to control fuel delivery, timing, and some emission control systems. The control module system, monitors a number of engine and vehicle functions and controls the following operations:

The diesel Powertrain Control Module (PCM) is located in the passenger compartment and is the control center of the control module system. The PCM used on the electronic fuel injected 6.5L diesel is referred to as PCM.

The PCM constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. The PCM performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the MIL (Service Engine Soon), and store one or more DTCs which identify the problem areas to aid the technician in making repairs. See diagnosis Section for more information.

The PCM is designed to process the various input information and then sends the necessary electrical responses to control fuel delivery, timing and other emission control systems. The input information has an interrelation to more than one output, therefore, if the one input failed it could effect more than one systems operation.

Aftermarket (Add-On) Electrical And Vacuum Equipment

Aftermarket (Add-On) Electrical and Vacuum Equipment is defined as any equipment installed on a vehicle after leaving the factory that connects to the vehicle's electrical or vacuum systems. No allowances have been made in the vehicle design for this type of equipment.

Notice: Do not attach add-on vacuum operated equipment to this vehicle. The use of add-on vacuum equipment may result in damage to vehicle components or systems.

Notice: Connect any add-on electrically operated equipment to the vehicle's electrical system at the battery (power and ground) in order to prevent damage to the vehicle.

Add-On electrical equipment, even when installed to these strict guidelines, may still cause the Powertrain system to malfunction. This may also include equipment not connected to the vehicle's electrical system such as portable telephones and radios. Therefore, the first step in diagnosing any Powertrain problem is to eliminate all Aftermarket electrical equipment from the vehicle. After this is done, if the problem still exists, diagnose the problem in the normal manner.

Electrostatic Discharge Damage

Notice: In order to prevent possible Electrostatic Discharge damage to the PCM, Do Not touch the connector pins or the soldered components on the circuit board.

Electronic components used in the control systems are often designed in order to carry very low voltage. Electronic components are susceptible to damage caused by electrostatic discharge. Less than 100 volts of static electricity can cause damage to some electronic components. There are several ways for a person to become statically charged. The most common methods of charging are by friction and by induction. An example of charging by friction is a person sliding across a car seat. Charging by induction occurs when a person with well insulated shoes stands near a highly charged object and momentarily touches ground. Charges of the same polarity are drained off leaving the person highly charged with the opposite polarity. Static charges can cause damage; therefore, it is important to use care when handling and testing electronic components.

Engine Controls Information

The driveability and emissions information describes the function and operation of the control module. The emphasis is placed on the diagnosis and repair of problems related to the system.

Engine Components, Wiring Diagrams, and Diagnostic Tables (DTCs):

The Component System includes the following items:

The DTCs also contain diagnostic support information containing circuit diagrams, circuit or system information, and helpful diagnostic information.

Maintenance Schedule

Refer to the General Motors Maintenance Schedule in Section 0B of the appropriate service manual for the maintenance that the owner or the technician should perform in order to retain the emission control performance.

Visual and Physical Underhood Inspection

Perform a careful visual and physical underhood inspection when performing any diagnostic procedure or diagnosing the cause of an emission test failure. This can often lead to repairing a problem without further steps. Use the following guidelines when performing a visual and physical inspection:

This visual and physical inspection is very important. Preform the inspection carefully and thoroughly.

Basic Knowledge Of Tools Required

Notice: Lack of basic knowledge of this powertrain when performing diagnostic procedures could result in incorrect diagnostic performance or damage to powertrain components. Do not attempt to diagnose a powertrain problem without this basic knowledge.

A basic understanding of hand tools is necessary in order to effectively use this information.

Wiring Harness Service

The control module harness electrically connects the control module to the various solenoids, switches, and sensors in the vehicle engine room and the passenger compartment.

Replace the wire harnesses with the proper part number replacement. When splicing the signal wires into a harness, use the wiring that has high temperature insulation.

Consider the low amperage and voltage levels that are utilized in the Powertrain control systems. Make the best possible bond at all of the splices. Use rosin-core solder in these areas.

Molded-on connectors require a complete replacement of the connector. Splice a new connector into the harness. Replacement connectors and terminals are listed in Group 8.965 in the Standard Parts Catalog.

For a wiring repair, refer to Wiring Repairs .

Connectors and Terminals

In order to prevent a shorting between the opposite terminals, use care when probing a connector and when the replacing terminals. Damage to the components could result.

Always use jumper wires between connectors for circuit checking.

Never probe through Weather-Pack seals.

Use the tachometer adapter J 35812, or the equivalent, which provides a convenient connection to the tachometer lead. The connector test adapter kit J 35616-A , or the equivalent, contains an assortment of flexible connectors that are used in order to probe the terminals during diagnosis. The fuse remover and the test tool BT-8616, or the equivalent, is used for removing a fuse and to adapt the fuse holder to a meter for a diagnosis.

Open circuits are often difficult to locate by sight because of oxidation or terminal misalignment which are hidden by the connectors. Merely wiggling a connector on a sensor, or in the wiring harness may temporarily correct the open circuit. Oxidized or loose connections may cause intermittent problems.

Be certain of the type of connector and the terminal before making any connector or terminal repair. Weather-Pack and Com-Pack III terminals look similar, but are serviced differently.

Powertrain Control Module (PCM) General Description

Powertrain Control Module

The PCM processes the various input information. The PCM sends the necessary electrical responses to the control fuel delivery, the spark timing, and the other emission control systems.

The input information has an interrelation to more than one output. One failed input can affect more than one operation of a system.

On-Board Diagnostic System Check

The On-Board Diagnostic (OBD) System Check is an organized approach that is used in order to identify a problem which is caused by an electronic engine control system malfunction. The OBD must be the starting point for any driveability complaint diagnosis. The OBD directs the technician to the next logical step in diagnosing the complaint.

Use the Engine Scan Tool Data Definitions list for a comparison after fulfilling the following items:

The Engine Scan Tool Definitions are an average of values which are recorded during normal vehicle operation. The Definitions are intended to represent what a normal functioning system should display.

Important: Do not use a scan tool that displays malfunctioning data. Report the problem to the manufacturer. Using a malfunctioning scan tool can result in misdiagnosis and unnecessary parts replacement.

Only the parameters listed in the Engine Scan Tool Definitions are used for diagnosis. When a scan tool reads other parameters, the values are not recommended by General Motors for use in diagnosis.

For more description on the values and the use of the scan tool to diagnose the PCM inputs, refer to the applicable diagnostic file.

When all of the values are within the range which is illustrated in the Engine Scan Tool Data Definitions, refer to Driveability Symptoms.