ATS Transfers But Genset Will Not Start Decision Tree

Why this matters

A customer calls after a power outage saying the lights stayed off, the transfer switch clicked over to the generator side, but the genset never cranked. This is a classic symptom split between three subsystems: the two-wire start signal from the ATS to the controller, the controller itself, and the engine cranking circuit. Each subsystem has its own fault tree and the right one cuts the diagnostic call from two hours of guessing to a confirmed isolation in 20 minutes. Misreading the symptom leads to controller replacements that resolve nothing because the actual fault is a corroded 14 AWG wire run from the ATS terminal block.

Symptom presentation

Confirm the ATS actually transferred. Standby ATS units have a mechanical position indicator visible through the cover; a unit that the homeowner reports as transferred may have only attempted to transfer and stalled. Read the position indicator and the panel light. Pull the controller event log on the genset; the controller should show an attempted crank, a failed crank, an overcrank trip, or no event at all. The event log is the diagnostic anchor.

Confirm utility voltage at the ATS line side and the load side. A unit that the homeowner says is transferred but is reading utility voltage on the load side has not actually transferred; the ATS contactor is stuck or the homeowner is misreading the panel.

Quick checks at the controller

Pull the genset's two-wire start terminals at the controller and bridge them with a jumper. A healthy controller responds to the bridged terminals with a crank attempt. If the controller cranks on the jumper, the fault is upstream in the ATS-to-genset signal path. If the controller does not crank on the jumper, the fault is at the controller, the battery, or the engine.

Read battery voltage at rest, during cranking, and at the controller's battery input terminals. A battery that reads 12.6 volts at rest, holds 11 volts at the starter under cranking, but drops to 8 volts at the controller battery input is showing voltage drop across a corroded battery cable; the controller sees a brownout and locks out the crank.

Isolation tree

Start with the jumper-at-controller test result.

Bridged terminals crank the engine: the fault is in the ATS-to-genset two-wire signal. Confirm the ATS auxiliary contact closes when the ATS transfers; the auxiliary contact is the dry contact that pulls the genset's two-wire start to ground or sources the start signal depending on the ATS design. Read continuity through the auxiliary contact in the transferred position. If continuity is good at the ATS terminals but the controller does not see the signal, walk the wire run looking for damage; underground burial runs in conduit are common and rodent damage at the genset end is a known cause. If continuity fails at the auxiliary contact in the transferred position, the contact is worn or burned; replace the auxiliary block per the ATS service manual.

Bridged terminals do not crank the engine: the fault is at the controller, the battery, or the engine starting circuit. Check battery voltage during the crank attempt at the starter post and at the controller input. A battery that holds 11 volts at the starter but drops below 9 volts at the controller is showing cable drop; replace the cables, clean the terminals, and apply dielectric grease. A battery that drops to 8 volts at the starter is the battery; load-test and replace if the load test fails.

If battery and cables pass, read the controller's crank output to the starter solenoid. A controller that pulls the starter solenoid wire to ground during the crank attempt but the starter does not engage points to the starter or the solenoid; replace per the OEM. A controller that does not pull the wire to ground during the crank attempt points to the controller; verify firmware, then replace the controller per the OEM.

Confirming the call

Document the jumper test result, the battery voltage at three points during cranking, the ATS auxiliary contact continuity in transferred and standby positions, and the controller event log entries from the failed start. The defensible diagnosis is the four data points. Schedule a full power-loss simulation after any fix; isolate the utility feed with the main breaker, confirm the ATS transfers, confirm the genset starts within the OEM-rated time window, and confirm the load transfers and carries.

Remediation by branch

ATS auxiliary contact worn: replace the auxiliary contact block per the ATS OEM service procedure. Confirm with continuity test in both positions before closing the cover.

ATS-to-genset wire run damaged: repair the damaged section with weatherproof splices or replace the run end-to-end. Document the burial path for the next service.

Battery cable voltage drop: replace cables, clean terminals to copper, apply dielectric grease, torque to the OEM spec, and re-test cranking voltage at the controller input.

Battery failed: replace with the OEM-spec battery for the model. AGM and flooded each have specific charger profiles; verify the controller charge profile matches the new battery chemistry.

Starter solenoid failed: replace per the OEM service procedure.

Controller failed: verify firmware first against the OEM bulletin list. If firmware is current and the controller does not pull the crank signal on the jumper, replace per the OEM. Document the model and firmware revision in the customer record.

References

1. NFPA 110, Standard for Emergency and Standby Power Systems, 2022 edition.
2. NEC NFPA 70 Article 702, Optional Standby Systems, 2023 edition.
3. UL 1008, Standard for Transfer Switch Equipment, current revision.
4. Generac Power Systems, RTS Transfer Switch Service Manual, current revision.
5. Kohler Power, RXT and Model 200 Transfer Switch Installation and Service, current revision.