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Power down the machine and lock out main power following safety procedures.

Always follow LOTO procedures before accessing electrical components.

Inspect the encoder cable from spindle motor to drive unit for physical damage, cuts, pinch points, or coolant contamination. Check cable routing for proper strain relief.

Disconnect and reconnect all encoder cable connectors at both motor and drive ends. Check connector pins for bent, corroded, or pushed-back pins. Clean connectors with electrical contact cleaner if necessary.

If damage is found, replace the encoder cable with OEM equivalent. Verify proper shielding continuity and grounding after replacement. Clear alarm and test spindle operation.

Access spindle drive diagnostics or use ladder/PMC to monitor encoder signals. Check for presence of A, B, and Z phase pulses during manual spindle rotation (drive disabled, power off).

Manually rotate spindle only when drive power is confirmed OFF and motor brake is released if applicable.

Measure encoder power supply voltage at encoder connector (typically +5VDC). Verify voltage is within specification (±5% tolerance). Low voltage indicates power supply or cable issue.

If no encoder pulses are detected or signals are erratic, remove encoder cover and inspect for contamination, moisture, bearing play, or visible damage to encoder disk or sensor head.

Replace spindle encoder if faulty. Verify encoder specification matches (pulses per revolution). After installation, verify coupling alignment, perform encoder reset procedure, and calibrate spindle orientation if equipped.

Verify spindle encoder parameters in the drive: check parameter 3736 (encoder pulses per revolution) matches actual encoder specification. Common values are 1024, 2048, or 2500 PPR.

Check spindle control parameters 3740-3745 for speed detection settings and thresholds. Compare against machine manufacturer specifications or backup parameters if available.

Monitor spindle amplifier LED diagnostics during alarm condition. Check for additional sub-alarms or error codes displayed on drive panel that indicate specific amplifier faults.

If parameters are correct but alarm persists, suspect drive encoder input circuit failure. Swap spindle drive with known good unit if available, or contact Fanuc for drive module diagnostics and repair.

Ensure replacement drive has identical part number and software version to prevent compatibility issues.