Fanuc spindle alarm 81 means the one-rotation signal of the MOTOR sensor is generated at an incorrect position — the motor-side counterpart of the position-coder alarm AL-41. The αiMZ/αiBZ sensor on the motor produces a once-per-revolution reference; when it appears in the wrong place relative to the A/B pulse train, position-referenced spindle functions cannot be trusted. Fanuc splits the troubleshooting by whether the machine uses an external one-rotation signal (a proximity switch on the spindle itself, used when motor and spindle are geared) — and that split is the first thing to establish.
Case 1 — Machines using the external one-rotation signal
- Gear ratio parameters wrong: the control converts motor-sensor rotation to spindle rotation through the gear ratio between motor sensor and spindle: denominator in parameters 4171/4173, numerator in 4172/4174 (per gear stage; 15i: 3171–3174). If this data doesn't match the machine's real gearing, the external signal and the computed position disagree — verify against the machine specification.
- Slippage between spindle and motor: any slip breaks the fixed relationship the conversion depends on. Check the transmission — and note Fanuc's important restriction: the external one-rotation signal is not applicable to V-belt connections, precisely because V-belts creep. A machine converted or retrofitted onto V-belt drive while still configured for the external signal will alarm chronically — the configuration, not a component, is the fault.
Case 2 — All other machines
- Sensor parameters: verify the sensor-setting parameters per B-65280EN — first suspect after amplifier swaps or restores.
- Sensor (αiMZ / αiBZ) not adjusted correctly: these sensors have an adjustable signal (gap and amplitude). Adjust per the sensor's procedure; if the signal cannot be brought into spec — or is absent — replace the connection cable and sensor together (Fanuc's own instruction; a marginal sensor and a degraded cable hide each other's faults).
- Cable shielding: shield continuity end to end, grounded via the cable clamp per B-65282EN — note the connection detail from Fanuc's diagram: the shield lands on the specified pin at the sensor end and pin 10 on the SP (amplifier) side. A shield ‘terminated’ anywhere else invites exactly this alarm.
- Routing: sensor cable bundled with a servo motor power lead → separate them — the recurring rule of the signal-quality family.
- The amplifier: last — the sensor input stage on the control PCB; board-level repair or exchange.
Tip: AL-81's siblings tell you which link broke: AL-73 = motor sensor signal absent; AL-81 = present but the reference lands wrongly. After motor work, the two usual self-inflicted causes are a disturbed sensor gap and a sensor ring shifted on the shaft — both ‘adjustment’ problems, not dead parts. And on geared machines with the external signal, always re-check 4171–4174 after ANY transmission work — a changed pulley pair silently invalidates the ratio.
Diagnosis checklist (in order)
- External one-rotation signal in use? → verify gear ratio 4171–4174 (15i: 3171–3174) against the real gearing; check for slip; confirm the drive is not V-belt.
- Otherwise: verify sensor parameters (B-65280EN).
- Check/adjust the αiMZ/αiBZ signal; unable to adjust or absent → replace cable AND sensor.
- Verify shield continuity and its correct termination (pin 10 SP side); separate the cable from power leads.
- All clear → amplifier input stage: bench repair or exchange.
- Verify orientation repeatability and rigid tapping in air before production.
Related guides
- Fanuc spindle alarm 73 (motor sensor disconnected)
- Fanuc spindle alarm 41 (position coder Z signal incorrect)
- Fanuc spindle alarm 61 (dual feedback difference / slippage)
- Fanuc αi drive alarms — PSM, SVM & SPM codes
AL-81 after motor or transmission work? JJ Automation adjusts and replaces αiMZ/αiBZ sensors, stocks matched sensor-and-cable sets, verifies gear-ratio configurations, and repairs amplifier input stages at component level. On-site service in NCR. WhatsApp us or call +91 88022 12394.