Smart Forfour Trans Slipping & TCM Warning: Fix Guide

As someone with some auto repair experience, I had a similar issue with my Smart Fortwo from around the same period. The symptoms were nearly identical, rough shifting, transfer case warning, and traction control problems. The fault codes initially pointed to the transfer case, but it turned out the root cause was actually the ABS control module sending incorrect signals. This triggered several consequential faults across multiple systems. The good news, no parts needed replacement. The workshop simply needed to reset and recalibrate the gear selector and control module systems. Since this was identified as a known issue under warranty, the repair cost was 0€. The fix took about 2 hours and has been stable since. To help diagnose your specific case, could you share: Exact build year and mileage, Any recent maintenance or repairs, Complete fault codes from diagnostics, Whether issues occur more in specific conditions (cold starts, highway speeds, etc), If the check engine light is also on This additional information would help determine if your case matches the known ABS-related pattern or points to a different root cause.

Last service was done at 180000km and included transmission fluid change. Looking at responses, I should clarify that my powertrain malfunction started appearing intermittently before the transmission issues became constant. Diagnostic trouble codes from my basic scanner show P0730 and P0841, which seem to confirm transmission-related faults rather than just sensor issues. My symptoms differ slightly as the rough shifting mainly occurs during cold starts and uphill acceleration. The limp mode engages more frequently now, especially after driving for about 15 minutes. Control module data suggests transmission pressure problems, though I will need professional diagnosis to confirm the exact cause.

Thanks for those additional details about the fault codes and symptoms. Having dealt with similar issues on my Smart Forfour, I should revise my initial assessment. Your P0730 and P0841 codes, combined with the cold start and uphill acceleration problems, point more toward a transmission pressure issue rather than the ABS module fault I originally experienced. The transmission pressure problems often trigger limp mode as a protective measure, especially when the system detects irregular shift patterns. The fact that your issues worsen after 15 minutes of driving suggests the transmission fluid might be breaking down under heat, despite the recent fluid change. Based on my experience, I recommend getting: A transmission pressure test performed, The solenoid pack checked for wear, The transmission filter inspected for debris My repair ended up requiring a solenoid replacement at 450€ plus labor. The traction control and transfer case warnings were secondary symptoms triggered by the transmission fault. After the repair, all systems returned to normal operation. These symptoms left untreated could lead to more severe transmission damage, so professional diagnosis would be wise despite the cost.

Thanks for the detailed responses and suggestions. The symptoms definitely seem more serious than I initially thought. Given the fault codes and how the issues worsen with heat and driving time, I will schedule a professional diagnosis this week. The transmission pressure test seems essential based on the feedback. The torque converter could also be contributing to the rough shifting during cold starts, so I will ask them to inspect that as well. While 450€ plus labor for a solenoid replacement is not cheap, it beats having to replace the entire transmission if left unchecked. I appreciate learning that the traction control and transfer case warnings are likely secondary symptoms. Will update once I get the professional diagnosis and know the full scope of repairs needed. Hoping the fix is just the solenoid pack rather than more extensive transmission work.