Kohler Locked Rotor Alarm

Locked Rotor — Shutdown Alarm The Locked Rotor alarm means the controller sent a start command but detected no engine rotation within the preset time window (typically 2–3 seconds). To protect the starter motor from damage, the controller halts all crank attempts and locks out the unit until the fault is cleared. Despite the alarming name, the engine itself is often not physically seized — the most common cause is a weak or dead battery or a failed speed sensor. Dead or weak 12V battery (most common — 90% of cases) Failed magnetic pickup / speed sensor (no rotation signal) Faulty or stuck starter motor or solenoid Loose, corroded, or unseated wiring harness connectors Physically seized engine (debris, corrosion, extended storage) Worn or misaligned brushes / slip rings (generator end) Faulty controller / outdated controller firmware Faulty battery charger — not maintaining battery charge

This is the #1 cause of Locked Rotor alarms. A battery below ~11V cannot deliver enough current to crank the engine, and the controller immediately flags it as a Locked Rotor fault. Use a multimeter to test battery voltage.

Battery voltage guide: 12.6V or higher — Battery is healthy. Move to Step 2. 12.0–12.5V — Battery is weak. Charge fully and retest. Below 11.5V — Battery is dead or failing. Replace it. Battery older than 4–5 years — Replace regardless of voltage reading. Also check: Inspect battery terminals for corrosion (white/blue buildup). Clean with a wire brush and reconnect firmly. Verify the battery charger circuit breaker inside the generator cabinet is ON and that the charger has 120V AC available.

Before any electrical diagnostics, confirm the engine is not physically seized. This is rare but occurs after extended storage, water intrusion, or pest damage.

Manual rotation test: With the battery disconnected and fuel shut off, attempt to rotate the engine manually by hand using the crankshaft pulley or fan blade. It should turn with moderate resistance — smooth and free. If it will not rotate at all or feels seized, stop immediately and contact A&A Power Generators. Do not attempt to force-start a seized engine.

Loose or oxidized harness connector pins are a surprisingly common cause of intermittent Locked Rotor faults — even when the engine and battery are both healthy. Vibration and moisture can push pins back in their housings over time.

Inspection steps: Locate the main wiring harness connectors between the controller and the engine/alternator end Unplug each multi-pin connector, inspect pins for corrosion, browning, or pins pushed back in housing Use needle-nose pliers to gently reseat any backed-out pins Reconnect firmly — you should hear or feel a click/latch Check the Emergency Stop pushbutton wiring for continuity if present (DPDT contacts)

Once battery and connection checks are complete, clear the fault from the controller before attempting a restart. The procedure varies by controller model.

DC2 / RES Controller: Press the OFF/Reset button on the controller panel. The fault lamp should extinguish. Decision-Maker® 3000 / APM402: Press OFF/Reset → navigate to the Alarm screen using the Select button → press Enter to clear active faults. Alternative reset: Pull the controller fuses on the control panel, wait 10 seconds, then reinstall. This performs a hard reset and clears all fault codes. ⚠ Do not clear the alarm repeatedly without correcting the root cause.

If the battery is confirmed good but the engine still will not crank, test the starter circuit. The controller sends a crank signal — if this signal doesn't reach the starter, or the starter doesn't respond, the fault appears.

Voltage test procedure: Command a start attempt and measure voltage at the starter motor terminals If 12V+ present at starter but engine won't crank → starter motor is faulty If no voltage at starter → check the starter solenoid and contactor If no voltage at solenoid signal terminal → fault is in wiring or controller output As a bench test: carefully jump 12V directly to the starter solenoid small terminal — engine should crank (will not start — no fuel/ignition signal)

Older Kohler RES models (and some newer units) use a magnetic pickup sensor (MPU) or crankshaft position sensor (CPS) to detect engine rotation. If this sensor fails or loses its signal, the controller sees no rotation and triggers Locked Rotor — even though the engine may be cranking normally.

Location: Near the generator head end of the shaft (flywheel ring gear area). Test: With the controller in start mode, measure the AC voltage output of the MPU sensor leads — you should see a small AC signal (approximately 1–3V AC) during cranking. No signal indicates a failed sensor. Also check: Air gap between sensor tip and ring gear teeth (should be 0.030–0.050 inches). A sensor that is too far from the ring gear may not generate a sufficient signal. Note: Newer RESV models receive rotation feedback directly from the rotor/controller back-end and do not use an external MPU — if this is your model, skip to Step 7.

After completing all checks, attempt a restart by pressing AUTO on the controller. If the unit starts and runs normally, return it to AUTO standby mode and monitor through the next exercise cycle. If the alarm returns, escalation is required.

Escalation — contact A&A Power Generators if: Locked Rotor alarm returns after clearing and battery replacement Engine cranks but immediately faults — possible controller or firmware issue No voltage output from starter circuit despite battery and wiring checks Speed sensor replacement did not resolve the fault Brushes, slip rings, or rotor inspection / replacement needed Engine is physically seized or has not been serviced in 2+ years Controller displays additional fault codes alongside Locked Rotor 📞 888-991-4500 · ✉ [email protected]

Kohler Locked Rotor Alarm

What LOCKEDROTOR means

Follow the diagnostic and reset sequence

Check Battery Voltage & Condition

Inspect for Physical Engine Seizure

Inspect & Reseat All Wiring Harness Connectors

Clear the Locked Rotor Fault Code

Test the Starter Motor & Solenoid Circuit

Inspect Speed Sensor / Magnetic Pickup (MPU)

Attempt Restart & Evaluate — Call for Service if Needed

Did the code clear?

Yes — code cleared

No — code returned

Move from troubleshooting into the right support path.

Best practices and next checks

Replace Battery Every 3–5 Years

Verify Battery Charger Monthly

Get a Maintenance Plan

Run Weekly Exercise Cycles

Annual Professional Service

Enable Kohler OnCue Alerts

A&A Power can take it from here.

Safety Information

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