Changing wheel size on a Volkswagen vehicle requires more than just bolting on new rims and tires. Your car’s computer needs to know about the change to accurately calculate speed, distance, and other critical driving data. This is where VCDS (Vag-Com Diagnostic System) comes in, allowing you to adjust the coding in your car’s control modules. This article discusses a common issue encountered when attempting to revert to a smaller wheel size using VCDS and offers potential solutions.
Understanding Wheel Size Coding with VCDS
In older Volkswagen models, adjusting the speedometer for different wheel sizes was done via “K-Number” coding in module 17 (Instrument Cluster). This process was relatively straightforward. However, in newer models like the T6.1, this coding has been replaced with a numerical “Variant” setting. While increasing the “Variant” to accommodate larger wheels is usually possible, reverting to a lower value for smaller wheels can be problematic.
This article details a real-world scenario where a T6.1 owner upgraded to 20″ wheels and then wanted to return to the original size. Using VCDS, the “Variant” was adjusted, but attempts to revert back resulted in a “Coding Rejected Error 22: Conditions Not correct or Request Sequence Error.” Even using ODIS online with genuine VAS6154 and GeKo access failed to correct the issue.
Troubleshooting “Coding Rejected” Errors
The “Coding Rejected” error suggests that certain preconditions aren’t met, preventing VCDS from accepting the change. Several troubleshooting steps were attempted:
- Battery Disconnect: Disconnecting the battery to reset the system did not resolve the coding issue and introduced new errors, including a malfunctioning ACC (Adaptive Cruise Control) in gears 5 and 6. This points to a potential conflict between the “Variant” coding and the gear selection logic, a known issue in some T5.1 and T6 models.
- ODIS Online: Utilizing the official ODIS diagnostic tool also failed to accept the coding change, further emphasizing the complexity of the problem.
Figure 1: Example of VCDS Coding Screen for Instrument Cluster
Potential Solutions and Considerations
While a definitive solution wasn’t found in this specific case, here are potential avenues to explore when facing similar challenges:
- Component Protection: Certain control modules may have component protection enabled, requiring a specific procedure to unlock before coding changes can be made.
- Software Version: Ensure that all relevant control modules, especially the instrument cluster and ABS, have the latest software updates installed. Outdated software can lead to compatibility issues.
Figure 2: Example of Auto-Scan Results in VCDS Showing Fault Codes
- Coding Dependencies: The “Variant” coding might be dependent on other settings in related modules like the ABS or Engine control unit. A thorough review of these modules and their coding might reveal interconnected settings that need adjustment.
- Factory Reset: A full factory reset of the relevant control modules might be necessary, although this should be considered a last resort.
- Dealer Intervention: If all else fails, consulting with a Volkswagen dealer equipped with the latest diagnostic tools and access to factory information might be required.
Conclusion: Proceed with Caution
Changing wheel size with VCDS requires careful consideration and a thorough understanding of the coding process. While increasing the “Variant” is usually straightforward, reverting to a lower value can trigger unexpected errors and system malfunctions. Always perform a complete Auto-Scan before and after any coding changes to document existing faults and identify any new issues. If encountering difficulties, exhaust all troubleshooting options before resorting to drastic measures like factory resets. When in doubt, seek professional assistance from a qualified Volkswagen technician.
