Smart Forfour's Electric Nightmare: Battery vs ECU Battle

Having similar issues with my Smart Fortwo 2016. As someone with moderate car repair experience, I was getting comparable symptoms, system failures and ECU voltage errors. After diagnostic tests, the workshop found the voltage regulator in the alternator was failing, causing electrical issues across multiple systems. This created an unstable power supply, sometimes too low for proper startup, other times producing dangerous high voltage spikes affecting the car electronics. The repair involved: 1. Complete battery test and replacement 2. Alternator inspection and voltage regulator replacement 3. Full system diagnostic reset Total cost was 290€ and resolved all electrical issues. The car ECU stopped showing voltage-related errors after the fix. To provide more specific guidance for your case, could you share: Current battery age, Any recent electrical upgrades or modifications, Whether problems occur more during cold starts or after the engine is warm, If you notice dimming headlights or fluctuating dashboard lights, Scanner data showing actual voltage readings if available This information would help determine if your issues match what I experienced with my Smart.

Thanks for sharing your experience. My car just had its last service at 81982KM a few weeks ago where they did a full inspection. The alternator failure symptoms you described match perfectly. My battery is 3 years old and voltage readings fluctuate between 11.8V and 15.2V during operation. The circuit protection system keeps triggering warnings, especially after the engine warms up. No electrical modifications were done, but I notice dashboard lights flickering during acceleration. The power surge seems most severe when electrical accessories like AC and headlights are running simultaneously. Will schedule an alternator diagnostic to confirm if the voltage regulator is the culprit. The repair cost you mentioned gives me a good reference point.

Thanks for those extra details. Your voltage readings definitely confirm my suspicion of alternator failure, those fluctuations between 11.8V and 15.2V are nearly identical to what my Smart was showing before repair. The flickering dashboard during acceleration is a classic sign of voltage regulator problems. When the engine RPMs increase, the failing regulator cant properly control the alternator output, causing power surge issues across vehicle systems. One thing I noticed before getting mine fixed, the electrical issues got progressively worse over about 2-3 weeks. The car electronics started acting more erratic, especially when multiple systems were drawing power. Your description of problems with AC and headlights matches this pattern exactly. Since your battery is 3 years old, consider replacing it along with the alternator repair. In my case, the constant high voltage spikes had damaged the battery cells, even though it wasnt that old. Quick tip: while waiting for repair, try minimizing use of heavy electrical loads like AC and high beams when possible. This can help reduce stress on the failing alternator and prevent potential damage to other electrical components.

The voltage readings and symptoms you described match my situation perfectly. After getting my alternator checked, I learned it was indeed the voltage regulator causing these issues. The high voltage spikes were affecting multiple systems just as you experienced. The service center confirmed my 3-year-old battery shows signs of damage from power surge events. The voltage fluctuations between 11.8V and 15.2V during normal operation indicate the alternator is failing to regulate power properly. My car ECU keeps logging these overvoltage events, particularly when electrical loads increase. I have scheduled the alternator replacement for next week and will have them replace the battery too. Following your advice about minimizing electrical loads until then, keeping AC usage low and avoiding high beams when possible. The 290€ repair cost reference helps with budgeting. Really appreciate the detailed explanation of how the voltage regulator failure progresses. This matches exactly what I've noticed with increasing electrical system instability over recent weeks. Will update once repairs are complete to confirm if this resolves all the electrical faults.