Fanuc spindle alarm 78 occurs only with Dual Check Safety, and means the spindle amplifier's safety parameter check does not match the CNC's safety parameter check — the two redundant channels each verified their copy of the safety parameters and got different answers. It sits between two neighbours: AL-71 (one side's safety parameters are corrupt) and AL-72 (the live speed computations disagree). AL-78 says: the two sides are not working from the same safety data — and a safety system whose channels disagree about their own limits cannot be trusted, so the machine stops.
Fanuc's remedy — and the checks before it
The manual: replace the CPU card in the CNC or the Spindle Amplifier control PCB. Before shipping boards:
- Was safety parameter work done recently? An interrupted or partial safety-parameter setting procedure leaves the two channels out of step — re-run the complete safety parameter setting/confirmation procedure with the builder's documented values (the same discipline as AL-71: from the machine's verified record, never from memory or a sister machine).
- After board or software swaps: a transplanted board carrying another configuration's safety data mismatches immediately — verify board pedigree and software series/edition (the AL-14/AL-70/AL-77 lesson, safety edition).
- One clean power cycle and the alarm history: companion codes name the failing side — amplifier self-tests (AL-74/75/76) → amplifier board; CNC-side system alarms → CPU card.
The repair
Re-set fails to hold, or no configuration work explains it → one channel's parameter storage is failing: board-level repair or exact-spec exchange of the guilty side, accepted with a documented safety-function test — the standing rule for every Dual Check Safety repair.
Related guides
- Fanuc spindle alarm 71 (safety parameter error)
- Fanuc spindle alarm 72 (speed check mismatch)
- Fanuc αi drive alarms — PSM, SVM & SPM codes