Fanuc spindle alarm 87 means a SPINDLE sensor signal is abnormal — the signal exists but is out of specification: amplitude wrong, waveform distorted, or counts inconsistent (Fanuc's manual references the motor-sensor equivalent, alarm 83, for the procedure). Where AL-84 is ‘nothing arrives’ and AL-86 is ‘the reference never comes’, AL-87 is ‘what arrives cannot be trusted’ — which points at signal quality: adjustment, contamination, cable degradation, or noise.
The diagnosis — signal quality in order of likelihood
- Sensor adjustment and gap: spindle-mounted MZ/BZ/CZ-type sensors produce an analog signal whose amplitude depends on the air gap. A gap disturbed during bearing/belt/ring work gives a weak or clipped signal — adjust per the sensor procedure; unadjustable → replace cable and sensor together.
- Contamination: swarf stuck to the sensor ring teeth or coolant film on the sensor face modulates the signal — clean and re-check. A single damaged tooth shows as a repeating glitch at the same shaft angle.
- Cable degradation: oil-swollen insulation and broken strands distort differential signals before they kill them — wiggle-test, inspect the run, and replace rather than repair.
- Shielding and routing: shield continuity, frame-ground clamp, separation from power leads — noise-corrupted counts are indistinguishable from a failing sensor until the routing is fixed (method per AL-47).
- Sensor parameters (B-65280EN) — wrong sensor type declared makes a healthy signal read ‘abnormal’; prime suspect after configuration work.
- The amplifier input — last, once the signal is verified good at the connector.
Tip: Correlation narrows it fast: abnormal at a repeating shaft angle → the ring (damaged/dirty tooth). Worse when drives work hard → noise/shielding. After spindle mechanical work → gap. Intermittent with machine motion → cable. The same map as every signal-quality alarm in this family — and the reason we scope the signal before condemning any part.
Related guides
- Fanuc spindle alarm 84 (spindle sensor disconnected)
- Fanuc spindle alarm 86 (no spindle sensor Z signal)
- Fanuc spindle alarm 47 (pulse count / noise methodology)
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