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Power down the machine and LOTO. Visually inspect the spindle force sensor cable (often labeled FB or FBS) from the motor to the drive for damage, wear, or loose connections.

Ensure proper lockout/tagout procedures before accessing electrical connections.

Check connector pins at both the spindle motor and drive amplifier ends for bent pins, corrosion, or debris. Clean contacts with electrical contact cleaner if necessary.

Using a multimeter, check continuity of each wire in the force sensor cable. Check for shorts between wires and shorts to ground/shield.

If cable damage is found, replace the spindle force sensor cable with the correct Fanuc part number. Ensure proper routing away from power cables and secure with appropriate strain relief.

Access the spindle drive diagnostics screen. Check for spindle load values and force sensor output voltage. Normal readings are typically 0-10V proportional to load.

Manually rotate the spindle (with power off and LOTO) and listen/feel for rough bearing operation, grinding, or abnormal resistance that could cause force spikes.

Ensure spindle brake is released and power is locked out before manual rotation.

Check spindle parameter settings for force sensor scaling and limits (parameters 4220-4229 series). Compare against machine documentation to verify correct configuration.

If sensor readings are erratic or out of range with good cables, the spindle motor force sensor has likely failed and requires spindle motor replacement or repair by Fanuc authorized service.

Access the spindle amplifier diagnostics. Check for any additional fault codes or LED indicators on the drive that point to internal amplifier issues.

Verify all DC bus voltages and control power voltages to the spindle drive are within specification (check service manual for voltage ranges).

Use appropriate PPE and voltage-rated test equipment when measuring high voltage DC bus.

If available, swap the spindle amplifier with a known good spare or test unit to isolate whether the fault is in the drive electronics.

Follow proper ESD procedures when handling amplifier modules. Document all parameter and configuration settings before replacement.

If drive replacement resolves the alarm, the original amplifier requires repair or replacement. Contact Fanuc or authorized distributor for amplifier exchange or repair service.