A2
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Self-diagnosis
General notes on self-diagnosis
Systems with self-diagnosis capability feature a control unit which detects faults whilst driving and stores them in a fault memory, where the information is retained even in the absence of battery voltage. The self-diagnosis relates to the electrical/electronic section of a system, i.e. faults are only recognised if they affect electrical signals. Self-diagnosis provides assistance with specific fault-finding. The self-diagnosis functions can be implemented using fault reader V.A.G 1551/1552 or vehicle system tester VAS 5051. This involves an exchange of data between the fault reader and the appropriate control unit. In other words, communication must be possible between the two. The diagnostic connector is the interface for communication between the control unit and fault reader. Start the fault-finding procedure by connecting the fault reader to the diagnostic connector. Then establish the communication link with the control unit of the system concerned . When the communication link has been established, the fault-finding functions provided by the system being tested can then be selected. The functions take the form of program sequences specified within the framework of self-diagnosis. Their applications are described in the appropriate Sections of this Workshop Manual. After first checking the control unit version, start with function 02 "Interrogating fault memory" . The faults stored are indicated on the display of the fault reader. The fault information can be used to consult fault tables indicating possible causes and appropriate remedies. These fault tables are contained in the relevant Workshop Manual Sections. The self-diagnosis function distinguishes between "static" and "sporadic" faults. If a fault which has been stored in the memory as a "static" fault is no longer present when the ignition is switched on, it is converted into a "sporadic" fault. Sporadic faults are marked as such by "/SP" on the right of the tester display. A sporadic fault is erased automatically if it no longer occurs over a certain number of driving cycles. Static faults remain stored in the fault memory until the memory is erased with the fault reader. If a control unit has to be replaced, first interrogate the control unit version. This appears on the display together with the code when communication has been established between control unit and fault reader. |
