Generator Exercise Runs Fine No Output At Panel Decision Tree

Why this matters

The weekly exerciser fires the engine, it runs its programmed minutes, and the homeowner assumes they are covered. Then a real outage hits and not a single light in the house comes on, even though the generator is running. The reason this is a distinct and common failure is that the standard exercise on most residential sets is a no-transfer exercise: the engine runs but the ATS never switches the house to generator power, so a downstream output fault is invisible until the day it matters. "Runs fine, no output at panel" splits into two big questions: is the alternator actually producing voltage, and is that voltage getting through the transfer switch and the generator main breaker to the house. Working it in order separates a true output failure from a transfer-path failure.

Symptom presentation

The generator starts and runs smoothly on exercise or on an outage, but no powered circuits in the house come alive. The controller may show no fault, or it may show that it transferred while the house stays dark. Measuring at house receptacles reads zero. Some units run the engine but the ATS contactor never pulls in; others transfer but the generator main breaker is open or the output is dead.

Quick checks

• Measure generator output voltage at the generator's output terminals (line-to-line and line-to-neutral) while running. A 120/240 V single-phase set should read about 240 V line-to-line and 120 V each leg-to-neutral. Zero output here is an alternator/excitation problem; correct voltage here pushes you downstream.
• Check the generator main circuit breaker on the unit. A tripped or open generator main blocks all output even with the alternator making voltage.
• Confirm the ATS actually transferred: watch or listen for the contactor and check which source the switch is on. A running engine with the ATS still on the (dead) utility side means no transfer.
• Verify this is a real load test, not just a no-transfer exercise. Force a transfer (simulate outage) to see real behavior.

Isolation tree

Branch 1 - No voltage at the generator output terminals. The alternator is not producing. On a brushless self-excited or AVR-controlled set this is an excitation failure: a failed AVR, lost residual magnetism (flashing the field may be required per OEM), an open exciter winding, or a blown excitation fuse. On sets with a separate excitation breaker or fuse, check it first. Measure field/exciter per the OEM procedure. No output at the terminals means the house can never see power regardless of the transfer path.

Branch 2 - Voltage present at the output terminals but generator main breaker is open/tripped. The alternator is fine; the on-unit main is blocking it. Find why it tripped before resetting - an overload, a downstream fault, or a nuisance trip. Reset and re-test under load. If it trips again on transfer, you have a fault on the load side or a sized-wrong main.

Branch 3 - Output and main are good, but the ATS never transfers. The engine runs but the switch stays on utility. The controller may be issuing a transfer command the ATS does not act on, or the ATS may not be getting the start/transfer signal. Confirm the transfer-control wiring between controller and ATS, the ATS control power, and the transfer coil/contactor. A mechanically jammed transfer mechanism or a failed transfer relay leaves the house tied to dead utility while the generator idles unloaded.

Branch 4 - ATS transfers to generator but house is still dead. Voltage makes it to the ATS generator terminals but not to the panel. Check the conductors between the ATS and the panel, the panel main, and - critically - the neutral. On a switched-neutral ATS, a neutral that did not transfer leaves you with no return path and no usable voltage at loads even though the legs are hot. Verify continuity and that the neutral pole transferred with the legs.

Branch 5 - Partial output (one leg dead). If one 120 V leg is present and the other is zero, suspect a lost leg: an open output lead, a single blown line fuse, or one pole of the main/ATS not making. 240 V loads will not run and half the 120 V circuits stay dark. Trace the dead leg from alternator output through the main and ATS to the panel.

Confirming diagnosis

You are done when a simulated outage transfers the ATS to generator, the generator main holds closed, full voltage (about 240 V L-L, 120 V each leg-to-neutral) reaches the panel, and real loads run. Verify both legs and the neutral. Then confirm retransfer back to utility and cooldown. Tell the customer the plain-exercise does not prove output - schedule or perform a transfer/load test, because the no-transfer exercise is exactly what hid this failure.

Remediation

For lost excitation, repair/replace the AVR or flash the field per OEM and replace any blown excitation fuse. For a tripped generator main, find and clear the cause before reset. For a non-transferring ATS, repair the control signal, coil, or mechanism. For a dead path to the panel, repair the conductor or the un-transferred neutral. For a lost leg, trace and repair the open. Always finish with a real transfer under load, not a no-transfer exercise.

References

• Generac Air-Cooled Standby Generator Service Manual, alternator excitation, AVR testing, field flashing, and exercise vs transfer behavior.
• Kohler Generator Service Manual, voltage regulation, exciter, and output troubleshooting.
• Cummins/Onan Service Manual, AVR and excitation diagnostics.
• ATS manufacturer Operation and Installation Manual, transfer-control wiring and switched-neutral configuration.
• NFPA 70 (NEC) Article 702 and 250, transfer-equipment and neutral/grounding requirements for optional standby systems.