Fanuc Spindle Alarm 01 (AL-01, Motor Overheat) — Causes, Diagnosis & Fixes

Fanuc spindle alarm 01 (AL-01 / SP9001) means the spindle motor's internal temperature has exceeded its limit — the thermostat/thermal sensor built into the motor windings has tripped. The amplifier shows 01 on its 7-segment display and stops the spindle. Fanuc's own troubleshooting splits this alarm three ways, and the split is the whole diagnosis: was the motor actually hot when it tripped? Hot under heavy cutting, hot under light load, or stone cold — each points somewhere completely different.

Case 1 — Alarm during heavy cutting (motor genuinely hot)

The motor is overheating for the honest reason: heat in exceeds heat out.

1. Check the motor's cooling fan first. Spindle motors carry their own cooling fan on the tail — if it has stopped, check its power supply (a separate single/3-phase feed via the machine's transformer, NOT from the amplifier); if powered but dead, replace the fan. A dust-choked or dead tail fan is the single most common cause of AL-01 in Indian shops — same dust story as the amplifier fan alarms.
2. Liquid-cooled motors: verify coolant flow, pump, and a blocked heat exchanger.
3. Ambient temperature: a cabinet-adjacent motor in a 45°C summer shop with the fan blowing pre-heated air is running out of headroom — improve ventilation around the motor.
4. Recheck the cutting conditions: if cooling is healthy, the process is simply demanding more than the motor's continuous rating — heavier cuts, dull tooling raising spindle load, or a new job outside the machine's envelope. Watch the spindle load meter: sustained running near/over 100% is the answer.

Case 2 — Alarm under light load (motor hot without a hard process)

1. Acceleration/deceleration duty too high: frequent spindle start/stop or orient cycles (bar work, rigid-tap-heavy programs) heat the motor even though each cut is light — acceleration current is many times continuous rating. Fanuc's guidance: arrange the duty so the average output including accel/decel stays within the continuous rating — in practice, reduce start/stop frequency, lengthen accel time, or combine operations per spindle start.
2. Motor-specific parameters wrong: incorrect spindle parameters make the drive magnetize/load the motor wrongly and it runs hot at idle. Check the motor-specific parameters against the FANUC AC Spindle Motor αi Parameter Manual (B-65280EN) — especially after any amplifier swap or parameter restore from another machine.

Case 3 — Alarm with the motor cold (the false alarm path)

Touch the motor. If AL-01 trips with the motor at ambient temperature, the motor is not overheating — the temperature signal path is lying. Fanuc's checklist, in order:

1. Feedback cable: the thermal sensor signal travels in the spindle feedback cable — a damaged conductor or connector reads as ‘overheat’. Inspect and replace the cable (the same cable checks as in our AL-73 guide).
2. Motor-specific parameters: the overheat-detection setting depends on the motor type — parameter 4134 (FS16i/0i-B/C and FS30i/0i-D; 3134 on FS15i) and, for α series motors, a 0 setting per the manual. A wrong motor code after board/amplifier work makes a healthy motor read hot.
3. Control PCB / amplifier: the sensing input on the spindle amplifier's control board has failed — board-level repair or exchange.
4. The motor's thermostat itself: the embedded sensor has failed shorted — confirmed by measuring the thermal sensor circuit at the motor connector against spec; if truly failed, the motor goes for rewind/sensor replacement.

Diagnosis checklist (in order)

1. Touch-test the motor at the moment of alarm: hot, warm or cold?
2. Hot → tail fan spinning? Fan power? Fins clean? Then load meter / cutting conditions.
3. Light-load hot → start/stop frequency and accel duty; verify motor parameters.
4. Cold → feedback cable → parameter 4134 (3134 on 15i) → amplifier control board → motor thermostat, in that order.
5. After ANY overheat event, check motor insulation warm — repeated overheating degrades winding insulation and becomes tomorrow's AL-12.

Related guides

• Fanuc spindle alarm 12 (DC link overcurrent)
• Fanuc spindle alarm 73 (motor sensor disconnected)
• Fanuc SVM alarm F (cooling fan failure)
• Fanuc αi drive alarms — PSM, SVM & SPM codes explained