The PCM processes the various input information. The PCM sends the necessary electrical responses to control fuel delivery, spark timing, and other emission control systems.
The input information has an interrelation to more than one output. One failed input can affect more than one system's operation.
On-Board Diagnostic System Check
The On-Board Diagnostic (OBD) System Check is an organized approach to identify a problem 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:
| • | Completes the OBD System Check |
| • | The on-board diagnostics are functioning properly |
| • | Displays no diagnostic trouble codes |
The Engine Scan Tool Definitions are an average of display values recorded from normal operating vehicles. The Definitions intend to represent what a normal functioning system should display.
Important: Do not use a scan tool that displays faulty data. Report the problem to the manufacturer. Using a malfunctioning scan tool can result in misdiagnosis and unnecessary parts replacement.
Use only the parameters listed in the Engine Scan Tool Definitions for diagnosis. When a scan tool reads other parameters, General Motors recommends not using the values 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 the values are within the range illustrated in the Engine Scan Tool Data Definitions, refer to Driveability Symptoms.Control Module Communications
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:
| • | Fuel control. |
| • | Fuel injection timing. |
| • | Exhaust gas recirculation. |
| • | Transmission shift and shift quality functions. |
| • | Specific transmission control diagnostics are covered in Transmissions. |
The diesel Powertrain Control Module (PCM), located in the passenger compartment, is the control center of the control module system. The Powertrain Control Module used on the electronic fuel injected 6.5L diesel is referred to as a 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 the specific diagnosis procedure for more information.
The PCM processes the various input information and 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 system's operation.
Aftermarket (Add-On) Electrical And Vacuum Equipment
Any equipment installed on a vehicle after leaving the factory that connects to the electrical or vacuum systems of the vehicle defines Aftermarket (Add-On) Electrical and Vacuum Equipment. The vehicle design makes no allowances 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.
The Add-On electrical equipment, even when installed to these strict guidelines, may still cause the Powertrain system to malfunction. This may also include any equipment which is not connected to the electrical system of the vehicle such as portable telephones and radios. Therefore, the first step in diagnosing any Powertrain problem is to eliminate all of the Aftermarket electrical equipment from the vehicle. After this is done, if the problem still exists, diagnose the problem in the normal manner.
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 Locations |
| • | The Wiring Diagrams |
| • | The Control Module Terminal End View and Terminal Definitions |
| • | The On-Board Diagnostic (OBD) System Check |
| • | The Diagnostic Trouble Code Tables (DTCs) |
The Component System includes the following items:
| • | The Component and circuit description |
| • | The On-vehicle service for each subsystem |
| • | The Functional checks with the Diagnostic Tables |
| • | How To Use Electrical Systems Diagnostic Information |
The DTCs also contain the diagnostic support information containing the circuit diagrams, the circuit or the system information, and helpful diagnostic information.
Wiring Harness Service
The control module harness electrically connects the control module to the various solenoids, switches, and sensors in the vehicle engine compartment and passenger compartment.
Replace the wire harnesses with the proper part number replacement. When splicing signal wires into a harness, use the wiring that has high temperature insulation.
Consider the low amperage and voltage levels utilized in the Powertrain control systems. Make the best possible bond at all splices. Use rosin-core solder in these areas.
Molded-on connectors require 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 wiring repair, refer to Wiring Repairs.
Connectors and Terminals
In order to prevent shorting between opposite terminals, use care when probing a connector and when replacing terminals. Damage to the components could result.
Always use jumper wires between connectors for circuit checking.
Never probe through the Weather-Pack seals.
The connector test adapter kit J 35616 , or the equivalent, contains an assortment of flexible connectors used in order to probe the terminals during the 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 diagnosis.
Open circuits are often difficult to locate by sight because oxidation or terminal misalignment 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 the type of connector and terminal before making any connector or terminal repair. Weather-Pack and Com-Pack III terminals look similar, but are serviced differently.
