| Basic notes on self-diagnosis |
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| The control unit of a system with self-diagnosis function detects any faults or malfunctions during vehicle operation and stores them in a fault memory. This information will remain available even if the system voltage fails. |
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| The self-diagnosis function enables faults to be traced quickly and efficiently. The self-diagnosis functions available can be implemented via vehicle diagnostic, testing and information system -VAS 5051 A- or vehicle diagnosis and service information system -VAS 5052-. |
Note | This Workshop Manual describes how to perform self-diagnosis using vehicle diagnostic, testing and information system -VAS 5051 A-. |
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| The respective control unit and the vehicle diagnostic, testing and information system -VAS 5051 A- must be able to exchange data in order to communicate with each other. The diagnostic connector is the interface for communication between the control unit and vehicle diagnostic, testing and information system -VAS 5051 A-. |
| Fault detection by ABS with EDL control unit -J104- |
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| The control unit detects any faults which may occur during vehicle operation and stores them in a fault memory. |
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| To commence fault finding, initiate self-diagnosis and interrogate the stored faults. It is necessary to interrogate the fault memories of all on-board systems with self-diagnosis capability, since the ABS with EDL control unit -J104- exchanges data with the engine control unit as well as the steering angle sender -G85-, the four-wheel drive control unit -J492- and the automatic gearbox control unit -J217- (if fitted). This can be done by selecting the function “00 - Interrogate fault memory - Complete system” → Anchor. If faults are stored in any other systems, check whether the stored faults affect the ABS/EDL/TCS/ESP system by referring to the respective fault tables. |
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| After evaluating all received data, the ABS with EDL control unit -J104- distinguishes between sporadic faults (occurring intermittently) and static faults (permanently present for a defined period), and stores these faults in an internal memory. |
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| If a fault occurs, it is initially stored as being static. If the fault does not reoccur over a certain period of time or while the vehicle is driven a certain distance, it is then stored as sporadic fault. |
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| When interrogating the fault memory, sporadic faults are marked as “sporadic” on the display. |
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| A fault is also stored as sporadic if the ignition is switched off and on again while the fault memory is being interrogated, or if the fault memory is not erased after performing repairs. |
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| Sporadic faults are erased automatically if they do not occur again within the next 50 vehicle starts and driving off sequences (road speed greater than 20 km/h). |
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| The ABS with EDL control unit -J104- is able to detect whether any of the required signals are not being transmitted via the CAN bus. However, defective CAN bus wires (e.g. open circuit) cannot be detected directly. The defective CAN bus wires can only be traced after interrogating the fault memories of all control units → Chapter. |
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| Sporadic faults affecting the CAN bus are erased automatically if they do not occur again within the next 15 vehicle starts and driving off sequences (vehicle speed greater than 20 km/h). |
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