FANUC SP9073 Alarm 73: Motor Sensor Disconnected and Troubleshooting

FANUC SP9073 means that the spindle amplifier cannot detect a valid spindle-motor sensor signal. The CNC displays SP9073, while the spindle amplifier displays alarm 73. On the systems covered by the relevant FANUC alarm lists, the motor-sensor feedback enters the spindle amplifier through connector JYA2.

The most efficient diagnosis depends on exactly when the alarm occurs:

• If alarm 73 appears while the motor is deactivated, check the motor-sensor parameters, feedback cable, connector, and sensor adjustment.

• If it appears when the spindle head or feedback cable moves, suspect an intermittent cable conductor, loose connection, or coolant contamination.

• If it appears only while the motor rotates, inspect grounding, shielding, cable construction, and separation from motor power leads.

Replace the spindle amplifier or its control printed-circuit board only after the applicable parameter, cable, connector, and motor-sensor checks have been completed.

For the surrounding alarm numbers, see the FANUC Spindle Alarm Codes SP9034–SP9092.

What Does FANUC SP9073 Mean?

FANUC uses slightly different wording across amplifier generations. The αi Series maintenance manual describes alarm 73 as a disconnected motor-sensor signal, while the αi-B Series manual describes the signal as abnormal. These descriptions are consistent in practice: the spindle amplifier is not receiving a motor-sensor signal that it can accept as valid.

“Disconnected” is therefore a diagnostic result, not proof that one conductor is physically open. A broken cable is a common cause, but the same alarm can result from:

• An incorrect motor-sensor or detector parameter

• A loose, contaminated, or damaged feedback connector

• An intermittent conductor inside a moving cable

• An incorrectly adjusted αiMZ or αiBZ motor sensor

• Improper shield termination or frame grounding

• Feedback cable routing too close to motor power leads

• Incorrect feedback cable construction or specification

• A defective motor sensor

• A defective spindle amplifier or spindle-amplifier control PCB

The correct English component name is motor sensor, not “electrode sensor.” Alarm 73 also is not identified by FANUC as a Dual Check Safety-only alarm. It is a spindle motor-feedback alarm and should be diagnosed from the actual feedback circuit.

1. Confirm SP9073 and Record When It Occurs

Before disconnecting hardware, confirm that:

• The CNC alarm screen shows SP9073 or a corresponding SSPA:73 message.

• The spindle amplifier display shows 73.

• The alarm belongs to the motor-sensor circuit, not a spindle-sensor circuit or an unrelated encoder alarm.

Record the following information:

• Complete CNC series and model

• Full spindle-amplifier model and every suffix

• Exact spindle motor model

• Installed motor-sensor type, when identifiable

• Connector designation used by the applicable machine documentation

• Whether the alarm occurs at power-up, spindle enable, cable movement, or motor rotation

• Whether a motor, sensor, amplifier, cable, or connector was recently replaced or disturbed

The occurrence condition is not a minor detail. FANUC organizes the official alarm 73 procedure around it.

2. If SP9073 Appears While the Motor Is Deactivated

When alarm 73 is present before the motor rotates, follow this order.

Check the motor-sensor parameters. Compare the installed motor and sensor with the applicable FANUC spindle motor parameter manual and the machine builder's parameter backup. A detector setting intended for another motor or sensor can cause the amplifier to reject the feedback signal. Do not initialize the complete spindle parameter set merely to test this alarm; verify only the applicable settings against an authoritative record.

Inspect the feedback connection. With machine power safely isolated and the DC link discharged according to the applicable maintenance procedure, inspect JYA2 where it is the specified motor-sensor connector. Confirm that the connector is fully seated and latched, pins are not bent or recessed, and there is no corrosion, coolant, oil, or damaged strain relief.

Check or substitute the feedback cable. FANUC's stated action for a disconnected feedback cable is replacement. If electrical testing is permitted by the machine documentation, disconnect both ends before checking conductor continuity, shorts between conductors, and unintended connections to the shield. Do not apply an insulation tester or megohmmeter through the motor sensor or spindle-amplifier electronics.

Check motor-sensor adjustment. For systems using an αiMZ or αiBZ sensor, adjust and evaluate the motor-sensor signal by the procedure for the exact motor, sensor, amplifier, and spindle software. Do not set a generic air gap or move the sensor by trial and error. If the signal cannot be adjusted into specification or no signal can be observed, FANUC directs technicians to replace the connection cable and motor sensor.

Only after these checks should the spindle amplifier or its control PCB be considered faulty.

3. If the Alarm Changes When the Cable or Spindle Moves

An alarm that appears when another axis moves, the spindle head travels, or the feedback cable is flexed strongly suggests an intermittent connection. FANUC specifically identifies a broken conductor as a likely cause when the alarm follows cable movement.

Inspect the complete moving cable path for:

• Tight bend radii or repeated flexing at the same point

• Crushing, abrasion, or contact with sheet metal

• Loose backshells or strain-relief clamps

• Pulled or recessed connector contacts

• Previous splices that interrupt a twisted pair or shield

• Coolant or cutting-fluid entry at either connector

If coolant has entered a connector, clean and dry it using the connector and machine builder's approved procedure, then inspect for corrosion or loss of contact tension. Cleaning cannot repair a fatigued conductor or damaged contact; replace the feedback cable when the fault is intermittent or physical damage is found.

A static continuity test may miss a conductor that opens only during bending. Any controlled flex test must be performed with the cable disconnected and the machine in a safe, de-energized condition. Avoid probing or moving feedback wiring near energized power circuits or moving machinery.

4. If SP9073 Appears Only While the Motor Rotates

When the signal is accepted at rest but alarm 73 occurs during rotation, electrical noise and signal integrity become more likely than a fully open cable.

Inspect frame grounding. The αi-B maintenance procedure calls for low-impedance grounding of the equipment flange, cable shield, and applicable components. Ground connections must contact the specified unpainted conductive surface and follow the applicable FANUC and machine builder installation instructions.

Inspect the feedback-cable shield. Confirm that the shield is continuous through any relay or intermediate connection and terminated exactly as specified for the installed sensor and amplifier. Keep unshielded conductor tails as short as the applicable connection manual requires. Do not assume that connecting the shield at an arbitrary point is equivalent to the FANUC wiring arrangement.

Separate feedback and power wiring. A motor power lead can induce noise into a parallel or bundled sensor cable. Route the motor-sensor feedback cable separately from spindle and servo motor power leads, especially over long runs. Preserve the machine builder's required spacing, ducting, crossings, and grounding arrangement.

Verify the cable specification. The αi-B procedure specifies shielded feedback cable and appropriate twisted-pair construction for the sensor and thermistor signals. A cable may pass a simple continuity test yet still be unsuitable because its pairing, shielding, capacitance, or pin assignment is wrong.

If the alarm began after a cable was fabricated or replaced, compare the entire pinout and cable construction with the manual for that exact amplifier and sensor before replacing electronic hardware.

5. Inspect and Adjust the Motor Sensor Correctly

Alarm 73 concerns the motor sensor, which may be an αiMZ, αiBZ, or another detector specified for the motor. It should not automatically be treated as a generic encoder fault.

Use the applicable maintenance and parameter manuals to confirm:

• Correct sensor type for the spindle motor

• Correct detector and motor parameters

• Sensor mounting and connector orientation

• Specified signal-check method

• Signal amplitude, offset, or waveform criteria supported by that control and software

• Applicable adjustment procedure

If the sensor cannot produce a valid signal with a verified cable and correct settings, replace the sensor according to the motor procedure.

Physical sensor wear is not listed as the first generic action in FANUC's alarm 73 procedure. However, if inspection reveals contact damage or repeated sensor wear, do not simply install another sensor. Determine why contact occurred—such as a loose mounting component, abnormal shaft movement, or another mechanical condition—and correct that cause before returning the spindle to service. This is a conditional mechanical inspection, not a substitute for the official electrical checks.

6. Replace the Spindle Amplifier Only After the Signal Path Is Verified

FANUC lists the spindle amplifier or spindle-amplifier control printed-circuit board after the parameter, cable, connector, and sensor checks. Replacing the amplifier first can leave the original wiring fault in place and may expose the replacement unit to the same contamination or grounding problem.

Before replacement, verify:

• The correct detector parameters are loaded

• The feedback cable and connector have been inspected or replaced

• The motor sensor is correctly adjusted and can produce the specified signal

• Shielding, grounding, and routing comply with the applicable installation manual

• The alarm still occurs under a repeatable condition

For a replacement unit, match the complete FANUC part number and suffix, hardware series, connector arrangement, spindle software support, motor, and machine configuration. Browse FANUC servo, spindle, and power drives or send the full model information to REACO CNC for a compatibility check.

7. Retest Under the Original Alarm Condition

After correcting the identified fault, retest in a controlled sequence:

1. Confirm that the connectors are fully seated and all covers and grounding hardware are restored.

2. Power up according to the machine builder's procedure and verify that SP9073 and alarm 73 do not appear with the motor deactivated.

3. Where applicable, move the spindle head or cable carrier through its normal range and watch for an intermittent alarm.

4. Run the spindle using the machine builder's prescribed low-risk test and confirm stable feedback during rotation.

5. Increase speed and load only through the approved commissioning sequence.

6. If a different alarm appears, stop and diagnose the number actually displayed.

Do not consider the repair complete merely because the alarm clears once after a connector is disturbed. An intermittent cable fault must be proven absent under the condition that originally triggered it.

Difference Between SP9073, SP9081, SP9082, SP9083, and SP9084

These alarms all concern spindle feedback, but they identify different detected conditions.

On systems that use the standard designations, the motor sensor is associated with JYA2, while the spindle sensor is associated with JYA4. Always confirm the connector designation in the documentation for the exact amplifier and machine.

FANUC SP9073 Diagnostic Table

What Not to Do

• Do not interpret “disconnected” as proof of a physically open conductor without testing the complete signal path.

• Do not call this an electrode-sensor alarm; FANUC identifies the component as the motor sensor.

• Do not assume SP9073 is restricted to Dual Check Safety systems.

• Do not initialize all spindle parameters without a verified backup and the correct motor data.

• Do not change sensor spacing or position by trial and error.

• Do not bundle the replacement feedback cable with motor power leads.

• Do not bypass or arbitrarily reterminate the cable shield.

• Do not use a megohmmeter through a connected sensor or amplifier input.

• Do not replace the spindle amplifier before checking the parameter, cable, connector, sensor, shielding, and routing evidence.

Recommended Troubleshooting Order

1. Confirm SP9073 on the CNC and 73 on the spindle amplifier.

2. Record the complete CNC, amplifier, motor, sensor, and spindle software information.

3. Determine whether the alarm occurs while deactivated, during cable movement, or during motor rotation.

4. Verify the applicable motor-sensor parameters against the exact motor data and machine backup.

5. Isolate power safely and inspect JYA2 where applicable, the motor-end connection, contacts, strain relief, and contamination.

6. Check or replace the feedback cable; pay particular attention to flex points if movement triggers the alarm.

7. If rotation triggers the alarm, inspect frame grounding, shield continuity and termination, cable specification, and separation from power leads.

8. Adjust and evaluate the motor-sensor signal using the exact FANUC and machine builder procedure.

9. If a valid signal cannot be obtained, replace the specified connection cable and motor sensor.

10. If the verified signal path is good and the alarm remains, replace the compatible spindle-amplifier control PCB or spindle amplifier.

11. Retest under the original trigger condition before returning the machine to production.

Technical References

• FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Alarm Code 73

• FANUC AC Servo Motor αi-B Series, AC Spindle Motor αi-B/βi-B Series, Servo Amplifier αi-B Series Maintenance Manual, B-65515EN/01, Alarm Code 73

• FANUC AC Spindle Motor αi/βi Series Parameter Manual, B-65280EN, motor-sensor and detector parameters

• FANUC Servo Amplifier αi Series Descriptions, B-65282EN, sensor cable connection and shielding

• FANUC Servo Amplifier αi-B Series Descriptions, B-65412EN, feedback cable connection, grounding, shielding, and installation

Use the manual edition that applies to the exact CNC, spindle amplifier, spindle motor, detector, spindle software, and machine. Connector assignments and supported diagnostic displays can differ by generation.

Need Help With FANUC SP9073 Alarm 73?

REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers, motor sensors, and feedback components. Send the CNC alarm screen, spindle-amplifier display, complete amplifier model and suffix, spindle motor model, sensor type, and a description of exactly when the alarm occurs.

Visit the FANUC Technical Support Center or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The applicable procedure depends on the exact CNC, amplifier series, motor, sensor, software, wiring, and machine configuration.