Vehicle:  2005 Mercedes Benz S 500, 4MATIC (220.184), V8-5.0L (113.966)

Mileage: 262,744

Problem: The customer reported that the car’s battery went dead overnight. While he was recharging the battery, the fog lights began flashing continuously. With the battery fully charged, he noticed the fog lights would flash on and off whether the engine was running or not. If he disconnected the fog lamps, the battery discharging stopped.

Case Details: Not having a lot of Mercedes Benz experience, the technician put a call into ALLDATA Tech-Assist for some diagnostic assistance. The consultant advised the technician that it’s common for the fog lights to flash on these models as a warning that the alarm siren module batteries are low, or the siren module has failed. The siren module is located inside the right front wheel well. To access it, the technician would need to remove the inner fender.

Confirmed Repair: The technician found that the siren module was corroded. He replaced the siren module and the fog lights worked perfectly. Problem solved!