Why Your Mercedes S500’s Alternator Charges Only Once - Then Quits

A curious electrical problem is stumping owners of the Mercedes S500 with the M273 engine, and it’s one that doesn’t fit the usual pattern. The alternator works perfectly on the first start of the day, charging the battery without issue during a drive. But the moment you shut the car off and restart it, the alternator stops charging entirely.

The fix? Disconnecting the negative battery terminal and reconnecting it, then everything works again until the next restart.

This is not a simple bad alternator scenario. Diagnostics on the vehicle return a “generator fault” code, pointing to a communication or control issue rather than a mechanical failure. Both batteries are new, so the problem isn’t weak cells or poor connections. The behavior suggests something in the car’s electrical management system is losing its reference or failing to wake the alternator after a warm restart.

The fact that a battery reset temporarily restores function hints at a glitch in the control module logic.

Could the engine control unit (ECU) be at fault? It’s possible. The ECU commands the alternator’s voltage regulator, and if it’s sending the wrong signal after a restart, or none at all, the alternator stays idle. However, a faulty alternator with an intermittent internal regulator can also produce these symptoms.

Given the mileage (200,000 km), both are suspects. A professional shop should test the alternator’s field coil and the ECU’s LIN bus signal to the regulator. Trust the software to guide the diagnosis, but don’t rule out a hardware swap. Either way, this S500 needs a methodical check of the charge circuit, not just a quick part replacement.