Fanuc spindle alarm 47 means the count of αi position coder signal pulses is abnormal. The mechanism is elegant self-checking: the coder's A and B phases deliver 4096 pulses per spindle revolution, and the amplifier counts them between one-rotation (Z) signals — every single rev. If the count between two Z pulses is outside the specified range, pulses were lost or gained somewhere — and AL-47 trips. Lost pulses = broken conductor, noise, or contamination; the coder is being miscounted, which corrupts threading and orientation silently before it ever alarms.
Case 1 — The alarm appears when the cable moves (Fanuc's own special case)
If AL-47 correlates with machine motion — a travelling spindle head, a moving column — Fanuc is blunt: a conductor in the cable is broken; replace the cable. Flexing opens and closes the break, dropping pulses each time. And: if coolant has penetrated the connector, clean it — coolant bridging pins corrupts the differential signals the same way. In practice:
- Wiggle-test the cable while watching the alarm/diagnosis screen — correlation is confirmation.
- Open both connector shells and inspect for coolant, corrosion and swollen inserts; clean with IPA, dry thoroughly — but treat a coolant-flooded connector as a warning: the cable jacket has likely wicked coolant along its length, and replacement is the durable fix.
Case 2 — Other situations
- Sensor parameters incorrect: check against B-65280EN — wrong sensor type/resolution settings change the expected count, so a healthy coder ‘fails’ the check. After amplifier swaps or restores, this comes first.
- Cable shielding faulty: shield continuity end to end and frame-grounded via the cable clamp per the B-65282EN connection diagram — an open shield lets switching noise inject phantom pulses (count too high) as readily as a break loses them (count too low).
- Signal cable bundled with servo power leads: separate them — the recurring routing rule across this alarm family (AL-41, AL-46): PWM noise from a power lead cable-tied to the sensor cable is a pulse-count corrupter.
- The spindle amplifier: last — the counting/input stage on the control PCB; board-level repair or exchange once parameters, cable, shielding and routing are cleared.
Diagnosis checklist (in order)
- Correlate: does the alarm follow machine motion, cutting load, coolant events — or nothing?
- Motion-correlated → wiggle-test → replace the cable; clean/inspect connectors for coolant.
- Recent work → verify sensor parameters (B-65280EN).
- Check shield continuity + frame-ground clamp; separate the cable from power leads.
- All clear → coder (disc/drive) vs amplifier counting stage — scope or swap-test to split them.
- Verify with M19 orientation repeatability and a multi-pass test thread before production.
Related guides
- Fanuc spindle alarm 41 (Z signal position incorrect)
- Fanuc spindle alarm 42 (Z signal not generated)
- Fanuc spindle alarm 46 (Z signal error during threading)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained