Fanuc Spindle Alarm 76 (Spindle Safety Function Not Executed — Dual Check Safety) — Causes & Fixes

Dual Check SafetyJJ Automation

Fanuc spindle alarm 76 occurs only with Dual Check Safety, and means the spindle safety function has not been executed — the safety monitoring that must run on the amplifier's control board did not run. The supervisory logic noticed the watchdog it depends on simply never did its job — and a safety function that silently fails to execute is exactly what a dual-channel architecture is built to catch, so the machine stops.

Fanuc's remedy

One line, one suspect: replace the Spindle Amplifier control printed-circuit board. Notably — unlike AL-72/77 — the CNC-side CPU card is not on the list: this failure lives on the amplifier's board, where the spindle safety task executes.

The sensible preamble (10 minutes)

  1. One clean power cycle — as with every safety self-test code, a single occurrence after a power event may be transient; a repeat is a diagnosis.
  2. Reseat the control-board faceplate — a half-seated board-to-board connection can prevent the safety task's environment from coming up correctly.
  3. History check: AL-76 in company with AL-74/AL-75 confirms the board's safety processor/storage environment is failing — one repair addresses the family.
  4. After board or software work: a control board with the wrong software series/edition for a Dual Check Safety configuration may never start the safety task — verify compatibility before condemning hardware.

The repair, done properly

Board-level repair or an exact-specification replacement control PCB — correct model, correct software series/edition for a safety-configured machine. Acceptance: a documented safety-function test proving the spindle safety monitoring actually executes and intervenes (safe-speed check with guard open) — the one test that specifically verifies the failure AL-76 reported.

Tip: Of the Dual Check Safety alarm family, AL-76 is the quiet one that deserves the most respect: it is not ‘the check disagreed’ but ‘the check never ran’. Never bridge, bypass or parameter-around it — and be wary of any repaired unit returned without a safety-function test report; ‘it boots without alarms’ does not prove the safety task runs.

Related guides

AL-76 on a safety-configured machine? JJ Automation repairs amplifier control boards at component level and returns every safety-chain repair with a documented safety-function test. Exact-spec exchange units in stock, on-site diagnosis in NCR. WhatsApp us or call +91 88022 12394.
Tags: Dual check safety, Fanuc, Safety function, Spindle alarm 76, Troubleshooting