The battery voltage is unlikely to be the source of issues with the wheel speed sensor. I recently encountered the message "ABS Inoperative, consult owner's manual" on my SLK. A code reader indicated a malfunction with the rear nearside wheel speed sensor, which I replaced, and after deleting the code, the ABS warning on the dashboard has ceased. However, you need a diagnostic tool that, like to MB's Xentry system, provides comprehensive information on the defect and its location. It took me less than half an hour to detach the rear wheel, remove the old sensor, install the new, reattach the wheel, and use his code reader to clear the error code and verify appropriate functionality.