Accessing vehicle data through the CANbus opens up a world of possibilities for customization and tuning. While parameters like ethanol content, octane adjustments, and speed limiters are relatively straightforward to decode using VCDS (Vag-Com Diagnostic System), accessing date and time information can prove more challenging. This article delves into the intricacies of decoding date and time data from Module 17 (Instruments) within VCDS, specifically focusing on the complexities of interpreting the date values transmitted on the CANbus.
Understanding the date and time information available on the CANbus eliminates the need for external hardware like a real-time clock module. Within VCDS, Module 17 provides both date and time data. While decoding the time value is relatively simple, the date presents a more complex puzzle. Through extensive experimentation involving adjusting the date via the MMI (Multi-Media Interface) and recording the corresponding raw CANbus data, clear request and response patterns have been identified. However, deciphering the specific encoding of the date within these messages remains elusive.
The typical request and response sequence for accessing date information involves consistent, unchanging data bytes (highlighted in yellow in the accompanying image). While the initial hypothesis suggested a three-value date representation corresponding to the three response lines, the constant nature of two of these lines disproves this theory. Further analysis focused on the variable data byte (highlighted in orange) across different sampled dates.
Converting the variable byte to decimal values reveals a potential correlation between byte 8 and the year. However, the relationship between byte 7 and the month and day remains unclear. The table below presents the decimal values of the variable byte for various sampled dates, including an “nth day” column to explore potential correlations. Byte 1 appears to function as a real-time clock within the module, independent of the date itself.
While a definitive solution for decoding the VCDS date data remains elusive, this exploration highlights the complexity of accessing seemingly simple information from the CANbus. Further investigation into the encoding scheme utilized by Module 17 is necessary to fully unlock the potential of this data for accurate data logging and other applications reliant on precise time stamping. This ongoing research underscores the importance of understanding the intricacies of CANbus communication for advanced vehicle diagnostics and modifications.
