Fanuc spindle alarm 09 (AL-09) means the heat sink of the spindle amplifier's main circuit has overheated — not the motor this time (that's AL-01), but the amplifier's own power-stage cooling. A model note from Fanuc's manual worth knowing: AL-09 is issued on αi SP22, αi SP30HV and larger units; the smaller αi SP2.2–SP15 and SP5.5HV–SP15HV raise Alarm 12 for the same condition — so if you searched AL-12 on a small amplifier and found overcurrent articles that don't fit, an overheating heat sink may be your real problem. As with the motor-side alarm, the diagnosis splits on one question: is the heat sink genuinely hot?
Case 1 — Alarm during cutting, heat sink genuinely hot
Check the spindle load meter first — it decides which branch you are in:
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Load meter BELOW the amplifier's continuous rating → the amplifier shouldn't be hot; its cooling has failed:
- Radiator fan stopped: check the fan and its power supply at the CX1A/CX1B connectors (the 200 V control/fan feed daisy-chained along the amplifier row). Power present but fan dead → replace the fan unit; Fanuc says ‘replace the amplifier’, but the fan is a serviceable part — the same fan logic (sense wire, genuine spec) as our SVM fan alarm guide.
- Dirty heat sink: the finned sink on the back of the amplifier lives in the cabinet's rear duct, collecting oil-laden dust and sometimes direct coolant spray. Power down and blow it clean — a felted heat sink can lose most of its cooling capacity while both fans run perfectly. Fanuc's own advice: if coolant reaches the sink directly, add a baffle/structure so it cannot.
- Ambient too high: a 45°C summer cabinet-rear duct leaves no thermal headroom — improve duct ventilation.
- Load meter ABOVE the continuous rating → the amplifier is being asked for more than it can sustain — ‘improve the use method’: lighter cuts, sharper tooling (dull tools push spindle load up dramatically), or accept that the job needs a bigger spindle package.
Case 2 — Alarm under light load
- Accel/decel frequency too high: rapid spindle start/stop or orient cycles heat the power stage even when each cut is trivial — same duty-cycle physics as the motor-side alarm. Arrange the cycle so average output including accel/decel stays within continuous rating: fewer spindle stops, longer accel time constants.
- Motor-specific parameters wrong: incorrect settings (check against the AC Spindle Motor αi Parameter Manual, B-65280EN) can over-excite the motor and load the amplifier at idle — prime suspect after any amplifier swap or parameter restore.
Case 3 — The half-seated control board
Fanuc's recurring dark horse, same as on the PSM alarm A page: push the faceplate (control PCB) fully home. A detached board-to-board connector between control and power PCBs corrupts the temperature sensing and raises a phantom AL-09. Power OFF, lock out, wait for the charge LED, press the faceplate as far as it will go.
Case 4 — Heat sink cold, alarm anyway
Touch the heat sink (carefully) right after the trip. Cold sink + AL-09 = the thermal sensing chain on the amplifier is lying — thermistor or sensing input failed. Fanuc's remedy is replace the amplifier; the economical route is board-level repair of the sensing circuit, a routine bench job.
Diagnosis checklist (in order)
- Note the load meter reading at the moment of the alarm.
- Sink hot + load low → fan (CX1A/B power), then clean the sink and duct, then ambient.
- Sink hot + load high → cutting conditions / tooling / duty cycle; verify motor parameters if recently changed.
- Reseat the control-board faceplate.
- Sink cold → sensing circuit → bench repair or exchange unit.
- Chronic borderline trips in summer only → fix the duct airflow and clean every sink in the row — heat ages the capacitors of the whole amplifier rack, not just this one.
Related guides
- Fanuc spindle alarm 01 (motor overheat)
- Fanuc spindle alarm 12 (DC link overcurrent — and heat-sink overheat on small αi SP units)
- Fanuc SVM alarm F (cooling fan failure)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained