Fanuc Spindle Alarm 31 (AL-31, Motor Lock / Speed Detection Fault) — Causes, Diagnosis & Fixes

Fanuc spindle alarm 31 (AL-31) means the motor could not follow the speed command — the amplifier commanded rotation but the speed feedback stayed near zero (or wildly wrong), so the drive concluded the motor is locked or the speed detection signal is faulty. You will see 31 on the spindle amplifier's 7-segment display and typically SP9031 / AL-31 ‘MOTOR LOCK / SPEED DETECT FAULT’ on the CNC screen. Unlike the hard-short alarms (AL-12), AL-31 is a disagreement between command and feedback — so the cause is either mechanical (the spindle genuinely cannot turn) or informational (it turns, but the drive cannot see it).

First question: does the spindle actually try to move?

Command a low speed (S100 M03) and watch/listen at the moment of the alarm:

• The motor hums, jerks, or creeps then alarms → the drive is pushing current but the shaft cannot rotate (mechanical lock) or the rotor position/speed signal is wrong (sensor/commutation). Go to Section A.
• The spindle clearly spins up, then alarms anyway → the motor turns fine but the drive cannot see the speed — a feedback path problem. Go to Section B.
• Nothing at all happens, then the alarm → could be either; do the hand-rotation check first.

Section A — The shaft cannot turn (mechanical / commutation)

1. Hand-rotate the spindle (power off, E-stop in): it should turn smoothly by hand. A spindle you cannot turn — seized bearings, a jammed gear stage, a stuck tool clamp, chips packed in the labyrinth, or on lathes a engaged spindle brake/lock — explains the alarm completely. Fix the mechanics; the drive is doing its job.
2. Gear/belt stage: on geared heads, a half-engaged gear shift (failed shift cylinder or switch) locks the train. Verify the gear-change solenoids and confirm the in-position switches on the diagnosis screen.
3. Brake/clamp signals: C-axis clamp or spindle brake still applied when rotation is commanded — a PMC sequence fault after maintenance. Check the clamp output state.
4. Commutation-side: if the shaft is free but the motor only jerks and growls, the rotor-position feedback is wrong — sensor gap, damaged sensor ring/gear, or wrong parameters after a motor swap (verify parameter 4133 motor code and sensor-type parameters 4000-range against the actual motor).

Section B — The shaft turns but the drive cannot see it (feedback path)

1. Sensor and cable: the speed signal comes from the motor sensor (older: pulse generator; αi: MZi/BZi/CZi sensor) via the feedback cable to the amplifier's JYA2/JYA4 connector. Inspect the cable along its whole run — oil-rotted insulation near the motor and broken strands at the connector are the classic finds — and reseat both ends.
2. Sensor gap and ring: on gap-type sensors, check the air gap against spec (typically ~0.15–0.3 mm) and inspect the sensor ring/gear teeth for damage or swarf stuck to them. A single damaged tooth gives speed ripple and intermittent AL-31 at specific speeds.
3. Check the observed speed: on the CNC's spindle monitor/diagnosis screen, compare commanded vs feedback speed while running low RPM. Feedback stuck at 0 with the shaft visibly turning confirms the feedback path.
4. Amplifier input stage: if a known-good sensor and cable still read zero, the amplifier's feedback receiver circuit is faulty — a board-level repair we do routinely on SPM and αi SP units.

Common root causes ranked (what we actually find)

1. Feedback cable damage (oil, chips, drag-chain fatigue) — by far the most frequent.
2. Sensor failure or wrong gap after motor/bearing work.
3. Mechanical: seized bearings, jammed gear shift, brake left engaged.
4. Amplifier feedback-receiver or control-board fault.
5. Parameter mismatch after a motor or amplifier swap.

Can you keep running?

If the cause is feedback-side and intermittent, the risk is not just downtime: wrong speed feedback in a closed loop can command full torque unexpectedly. Treat repeating AL-31 as a stop-and-fix, especially before rigid tapping or heavy cuts.

Related guides

• Fanuc spindle alarm 12 (DC link overcurrent)
• Fanuc αi drive alarms — PSM, SVM & SPM codes
• Identifying and replacing a faulty Fanuc encoder