Generator will not transfer load; controller displays F09 Low AC Voltage.

Safety first: Isolate the generator from the electrical system by opening the generator output breaker and ATS. Verify with a meter that no voltage is present before touching terminals.

• Measure output voltage: With the generator running in manual mode and no load, measure line-to-line and line-to-neutral voltages at the generator terminals. Compare to the nameplate rating.
• Inspect AVR connections: Locate the Siemens automatic voltage regulator inside the generator control compartment. Check all plug connectors and terminal screws for tightness and signs of overheating. Reseat connectors.
• Check excitation circuit: Inspect the excitation leads from the AVR to the rotor or exciter field. Look for broken or burnt wires. If brushes and slip rings are used, inspect for excessive wear, pitting, or contamination. Clean slip rings and replace worn brushes.
• AVR replacement: If excitation voltage from the AVR is absent or unstable while input sensing is correct, replace the AVR with the Siemens-specified part for your alternator model.
• Stator inspection (technician): If voltage remains low with a known-good AVR, the stator windings may be damaged. This requires insulation testing and possible rewinding by a generator specialist.