Fanuc Spindle Alarm 42 (Position Coder One-Rotation Signal Not Generated) — Causes, Diagnosis & Fixes

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Fanuc spindle alarm 42 means the one-rotation signal (Z signal) of the αi position coder is not being generated at all — the once-per-revolution reference pulse never arrives. It completes a family of three: AL-27 = the whole coder signal missing, AL-41 = the Z pulse arrives in the wrong place, AL-42 = the Z pulse never arrives — while the A/B phase train may be perfectly fine, so the spindle can show speed yet cannot orient, thread or tap. Fanuc gives three causes in cost order.

Cause 1 — Sensor parameter setting incorrect

Check the sensor-setting parameters against the AC Spindle Motor αi Parameter Manual (B-65280EN): a configuration that doesn't match the installed coder can make the drive ignore or never expect the Z channel. As always — prime suspect after amplifier swaps, restores or commissioning; unlikely on a machine that threaded happily yesterday.

Cause 2 — The coder or its cable (Fanuc's PSD check-pin test)

Fanuc's diagnostic: observe check pin PSD on the spindle check board while rotating the spindle slowly — if no pulse appears per rotation, replace the connection cable and position coder. Practical notes:

  1. No check board? An oscilloscope on the Z-channel pair at the amplifier's JYA3 connector answers the same question.
  2. Before condemning the coder, split the path: check the Z-pair conductors in the cable for continuity (a single broken conductor kills only the Z channel while A/B keep working — which is exactly the AL-42 signature), and inspect both connectors for a bent pin on the Z pair.
  3. Then the coder itself: contaminated or damaged disc at the index track, or failing output driver. Verify its mechanical drive (belt/coupling) is intact while you are there.

Cause 3 — The spindle amplifier

If a scope shows a clean Z pulse arriving at the amplifier connector and AL-42 persists, the coder-input stage on the control PCB has failed — board-level repair or exchange, the last suspect for good reason.

Tip: The tell for AL-42 vs its siblings: the spindle runs and shows correct speed (A/B intact) but orientation hunts forever or threading refuses to start (no Z reference). One broken conductor — out of the whole cable — does this. That is why Fanuc says replace cable AND coder when the PSD pin is silent: a cable-only swap that leaves a dying coder in place gives you AL-42 again in a month.

Diagnosis checklist (in order)

  1. Recent parameter/hardware work? → verify sensor settings (B-65280EN).
  2. Observe PSD on the check board (or scope the Z pair at JYA3) while hand-rotating.
  3. No pulse → check Z-pair continuity and connector pins → replace cable and/or coder.
  4. Pulse present at the amplifier → coder-input stage: board repair or exchange.
  5. Verify with M19 orientation and an air-cut thread before production.

Related guides

AL-42 blocking orientation or threading? JJ Automation stocks αi position coders and cables, scopes feedback signals on site in NCR, and repairs amplifier coder-input stages at component level. WhatsApp us or call +91 88022 12394.
Tags: Fanuc, One-rotation signal, Position coder, Psd check pin, Spindle alarm 42, Troubleshooting