- Description of On-Board Diagnostics: Description And Operation
COMPONENTSThis vehicle is equipped with an extensive on-board diagnostic system which is capable of detecting and compensating for faults, alerting the operator of a fault and recording pertinent data during the failure. It is also capable of displaying information about current vehicle performance and recorded vehicle performance when a fault occurred. You must understand how to use this on-board diagnostic system in order to properly diagnose the powertrain. Details on the on-board diagnostic system are contained under this heading, Vehicle On-Board Diagnostics.
INSTRUMENT PANEL CLUSTER
At the heart of the computer system is the Instrument Panel Cluster (IPC). It has an internal microprocessor which is the center for communication with all the other components in the system. System sensors and switches are monitored by the IPC or one of the other major components that complete the computer system. These components are:
1. Powertrain Control Module (PCM)
2. Air Conditioning Programmer (ACP) - HVAC
3. Diagnostic Energy Reserve Module (DERM) - Supplemental Inflatable Restraint System
4. Electronic Brake Control Module (EBCM) or (EBTCM) with traction.
5. Road Sensing Suspension Module (RSS)
A combination of inputs from these major components and the other sensors and switches communicate with the IPC, either as individual inputs, or on the common communications link, called the data line (CKT 800). The various inputs to the IPC combine with program instructions within the system memory to provide accurate control over the many subsystems involved.
COMMUNICATION SYSTEM OVERVIEW
Shared Information
Between the IPC and the other major components of the computer system, a communication process has been incorporated, which allows the devices to share information. In a method similar to a telegraph system, the IPC's internal circuitry rapidly switches a circuit between 0 and 5 volts like a telegraph key. This process is used to convert information into a series of pulses which represents coded data messages understood by other components.
Component Recognition Code
Also, much like a telegraph system, each major component has its own recognition code (address). When a message is sent out on the data line, only the component that matches the assigned recognition code will pay attention, and the rest of the components will ignore it.
Computer Module Circuit:
Data Line
The communication link is a single circuit (CKT 800) which is referred to as the Data Line or Serial Data Line. Circuit 800 is displayed in the accompanying image. This circuit is looped between all components for redundancy and circuit protection. Since only one connection is necessary at each component, a single open in the circuit will not cause a loss of communication. However, if multiple opens occur, the components separated from the IPC (the communication master) will not be able to receive or transmit information. If a short to ground or voltage occurs anywhere along the data line, the line will be held at a constant voltage and no communication will be possible. When loss of communications occurs a diagnostic trouble code (defined later) will be logged.
Shared Information
The use of the communication line allows for the separate computers to share information. Examples of this may be gage information sent from the PCM to the IPC, A/C compressor requests sent from the ACP to the PCM, or Climate Control information sent between the IPC and ACP. This shared information architecture also provides for the on-board self-diagnostic capabilities discussed later.
SELF DIAGNOSTIC FEATURES
Segment Check
Segment check causes a total illumination of all segments and some bulbs on the IPC (turn signals will not light). This is helpful in determining if any bulbs or segments of the Vacuum Fluorescent Display (VFD) are out or always on.
Trouble Codes
In the process of controlling the various subsystems, the PCM, IPC, ACP, DERM, TCS, and RSS continually monitor operating conditions for possible system malfunctions. By comparing system conditions against standard operating limits, certain circuit and component malfunctions can be detected. A four digit alphanumeric 'Trouble Code' is stored in computer memory when a problem is detected by this self-diagnostic system. These 'Trouble Codes' can later be displayed by the service technician as an aid in system repair.
Status Lights
While in Service Diagnostics, the mode indicators on the Climate Control Center (CCC) are used to indicate the status of certain operating modes. The different modes of operation are indicated by the status light being turned "ON" or turned "OFF."
SYSTEM TESTS
System tests allow for the display of values as actually seen or commanded by the various Systems (i.e PCM, IPC, ACP, DERM, TCS, or RSS).
The test types available are dependent on the system selected, but may include the following:
Data (Analog Inputs)
Displays analog values as seen by the system.
Inputs (Digital Inputs)
Displays digital values as seen by the system and provides an indication of whether the input has cycled.
Outputs (Digital Outputs)
Cycles digital outputs controlled by the system off/on for 3 seconds each cycle.
Overrides (Analog Outputs)
Allows for analog outputs of the system to be set at a desired value.
Clear Codes
Will erase all codes stored for the component currently selected (Note: if the fault is present, the code may immediately reset).
Snapshot
Stores all inputs (Analog and Digital) to a component at a particular moment (either when a code is detected or when manually triggered). These values may then be reviewed.
How to access and utilize each of test types is explained in Vehicle On-Board Diagnostics. The exact content of each system's diagnostic capabilities is covered in the particular system's diagnostic section.