Fanuc spindle alarm 24 means the serial communication data between the CNC and the spindle amplifier contained an error — the serial spindle link corrupted or dropped data. Before anything else, know the note from Fanuc's own manual: this alarm is also issued simply when the CNC power is off — that is not a failure. An AL-24 frozen on the amplifier display after the control was switched off means nothing; only AL-24 with the CNC running is a fault. When it is real, the cause is nearly always noise or cabling, and only rarely the electronics — Fanuc's five-step order reflects that.
Cause 1 — Electrical noise on the link
The CNC-to-amplifier serial cable is a data line living in a cabinet full of contactors and drives:
- Check the cable's routing and length against the connection rules in the αi Descriptions manual (B-65282EN) — over-length or non-spec cable margins vanish in a noisy cabinet.
- Suspect timing: does AL-24 coincide with a big contactor operating, a coolant pump starting, welding nearby? Noise faults cluster around switching events.
- Verify the cable's shield is intact and properly terminated — a shield cut during a previous repair converts the cable into an antenna.
Cause 2 — Communication cable bundled with power leads (the classic)
Fanuc gives this its own step because it is so common: the serial cable cable-tied neatly alongside the spindle motor power lead — usually after maintenance, when everything got re-loomed tidily into one bundle. The motor lead carries hard-switched PWM; the data cable next to it picks it all up. Separate them physically — different cable routes, crossings at 90° where unavoidable. Tidiness is not a virtue in cabinet wiring; separation is.
Cause 3 — The cable itself is faulty
- Reseat both ends (JA7B on the amplifier side / the serial spindle connector on the CNC side); inspect for bent pins and corrosion.
- Intermittent AL-24 with cabinet vibration → broken strands — wiggle-test, then replace; do not repair a data cable.
- If an optical I/O link adapter is used (long runs / separate cabinets): the optical adapter or its fibre may be at fault — same optical hygiene as FSSB alarm U: clean faces, bend radius, seated latches.
Cause 4 & 5 — The electronics (last, not first)
- Spindle amplifier side: the serial interface on the control PCB has failed — board-level repair or exchange. Suspect it when AL-24 persists with a new, correctly-routed cable.
- CNC side: the serial-spindle board/module in the control — rarest of all. A swap-test with a known-good amplifier (or watching whether a second serial spindle on the same line also errors) localizes which end before buying anything.
Diagnosis checklist (in order)
- Is the CNC actually powered? Displayed AL-24 with the control off is normal.
- Reseat both cable ends; inspect connectors.
- Walk the cable route: bundled with motor power leads? Separate them.
- Check shield integrity and cable spec/length; replace a suspect cable outright.
- Optical link adapter fitted? Clean/inspect fibre and adapter.
- Persists → amplifier control board (repairable) → then CNC serial spindle module.
Related guides
- Fanuc alarm U (FSSB optical communication error)
- Fanuc spindle alarms 13 & 14 (board self-test faults)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained