Fanuc spindle alarm 84 means the SPINDLE sensor signal was disconnected — the sensor mounted on the spindle side (αiCZ or spindle-mounted MZ/BZ sensor, connected to the amplifier's JYA4 port on most configurations) stopped delivering. Fanuc's manual routes the troubleshooting to alarm code 73 — the identical fault on the motor sensor — because the procedure is the same signal-path hunt: parameters → cable → sensor → amplifier input.
The AL-73 procedure, aimed at the spindle sensor
- Sensor parameters (B-65280EN): a configuration that doesn't declare the spindle sensor correctly — first suspect after amplifier swaps or restores.
- The cable: reseat both ends (JYA4 side at the amplifier); inspect and wiggle-test the run. Spindle-sensor cables live closer to the business end of the machine than motor cables — coolant, swarf and headstock maintenance traffic make the cable the leading physical cause.
- The sensor: gap/adjustment (disturbed during spindle bearing or belt work), damaged sensor ring, failed head — if the signal cannot be adjusted into spec or is absent, replace cable and sensor together.
- The amplifier input — last: board-level repair or exchange.
Tip: Which sensor a machine has where decides which alarm you get: motor sensor missing → AL-73; spindle sensor missing → AL-84. If AL-84 appeared right after spindle-side mechanical work (bearings, belts, encoder ring), the disturbance is nine-tenths of the diagnosis — check the sensor gap and connector before anything else.
Related guides
- Fanuc spindle alarm 73 (motor sensor disconnected) — the full procedure
- Fanuc spindle alarm 27 (position coder disconnected)
- Fanuc αi drive alarms — PSM, SVM & SPM codes
AL-84 on your spindle sensor? JJ Automation stocks spindle sensors and cables, sets sensor gaps correctly after mechanical work, and repairs amplifier inputs at component level. On-site service in NCR. WhatsApp us or call +91 88022 12394.