Fanuc spindle alarm 27 means the signal of the αi position coder is disconnected — the spindle's position coder (the separate encoder that tracks spindle position for threading, rigid tapping and orientation, usually belt- or direct-driven from the spindle itself rather than built into the motor) is not delivering a valid signal to the amplifier. Note the distinction from AL-73: 73 is the motor sensor missing; 27 is the position coder — plain speed control may still work, but threading, tapping and orientation are dead. Fanuc's checklist for the motor-deactivated case has three steps, in cost order.
Cause 1 — Sensor parameter setting incorrect (check first, it's free)
The drive must be told which position sensor exists and where. Wrong sensor settings make it look for a coder that is not connected — or ignore the one that is.
- Check the sensor-setting parameters against the FANUC AC Spindle Motor αi Parameter Manual (B-65280EN) — sensor type and presence bits in the spindle parameter area (400x range).
- Prime suspect when: the alarm appeared after a parameter restore, an amplifier swap, or commissioning work on a retrofit. A machine that threaded happily for years and suddenly alarms did not change its own parameters — skip to the cable.
Cause 2 — The cable (the usual physical culprit)
- Power OFF and check the connection path: position coder → its cable → the amplifier's JYA3 connector (typical position-coder port on αi SP units). Reseat both ends; inspect for bent pins, coolant in the shells, corrosion.
- Inspect the run — position coder cables often route along the headstock where oil and swarf live; look for crush, rub-through and oil-swollen jacket.
- Wiggle-test while watching the display; meter the conductors end-to-end, flexing at suspect points.
- Connection correct and cable healthy-looking but the alarm persists → Fanuc says replace the cable anyway before condemning electronics — broken strands hide inside intact insulation, and a cable is cheap insurance.
Cause 3 — The position coder itself, then the amplifier
- The coder: check the physical drive to it — a snapped or slipping belt (the coder reads zero while the spindle turns), a sheared coupling, a loose mounting bracket. Then the unit itself: bearing rumble, oil ingress at the cable gland. A coder that spins freely by hand but outputs nothing is dead — replace it (standard αi position coders are stock items).
- The amplifier: if a known-good coder and cable still read disconnected, the JYA3 receiver input on the control PCB has failed — board-level repair or exchange, per Fanuc's final step.
Tip: The failure pattern narrows it fast. AL-27 constantly, spindle won't even orient → hard break: cable, connector, dead coder. AL-27 only during threading/tapping at speed → marginal signal: belt slip, contaminated optical disc, or noise on a damaged cable. After headstock maintenance → the coder belt or connector was disturbed — nine times out of ten. And if AL-27 arrived together with a new amplifier or restored parameters → it's the sensor parameters, not hardware.
After the repair — verify before cutting threads
- Orient the spindle (M19) — smooth, repeatable stop at position.
- Check the position coder reading on the spindle diagnosis screen while rotating slowly — counts should track rotation smoothly, no jumps.
- Air-cut a test thread or rigid tap in air before touching a workpiece — position feedback faults scrap parts and break taps first.
Related guides
- Fanuc spindle alarm 73 (motor sensor disconnected) — the motor-side sibling
- Fanuc spindle alarm 21 (position sensor polarity)
- Identifying and replacing a faulty Fanuc encoder
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained
AL-27 stopping your threading or rigid tapping? JJ Automation stocks αi position coders and feedback cables, repairs amplifier receiver circuits at component level, and can diagnose on site in NCR — usually same-day for Gurugram/Manesar. WhatsApp us or call +91 88022 12394.