FANUC SP9081 means that the position at which the spindle motor sensor generates its one-rotation signal is incorrect. The CNC displays SP9081, while the spindle amplifier displays alarm 81.
This definition is important because alarm 81 is sometimes described incorrectly as “no one-rotation signal.” FANUC assigns that condition to alarm 82 / SP9082. With SP9081, the drive is receiving or evaluating a one-rotation reference, but its occurrence does not agree with the expected motor-to-spindle relationship.
FANUC Maintenance Manual B-65285EN/04 divides the official checks into two configurations: systems using an external one-rotation signal, normally from a proximity switch, and other systems using an αiMZ or αiBZ motor sensor. The correct diagnostic path depends on which configuration the machine actually uses.
| Alarm item | Display or condition | Meaning |
|---|---|---|
| CNC alarm | SP9081 | Motor-sensor one-rotation signal position error |
| Spindle amplifier | 81 | The one-rotation signal occurred at an incorrect position |
| External-signal configuration | Proximity switch and mechanical transmission | Check gear-ratio data and mechanical slip |
| Motor-sensor configuration | αiMZ or αiBZ sensor | Check sensor parameters, adjustment, cable shielding, routing, and hardware |
For the surrounding serial spindle alarm numbers, see the FANUC Spindle Alarm Codes SP9034–SP9092.
What Does FANUC SP9081 Mean?
A one-rotation signal provides a reference once during each mechanical revolution. The spindle control compares that reference with the motor-sensor feedback and the configured relationship between the motor and spindle. If the reference appears at a position that does not match the expected relationship, the amplifier generates alarm 81 and the CNC reports SP9081.
The alarm can therefore be caused by different mechanisms:
- Incorrect motor-sensor or gear-ratio parameters
- Mechanical slippage between the motor and spindle
- Incorrect adjustment of an αiMZ or αiBZ sensor
- A damaged or poorly connected sensor cable
- Inadequate feedback-cable shielding
- Sensor wiring routed together with a motor power lead
- A faulty spindle amplifier or control printed-circuit board
Do not begin by replacing the amplifier. FANUC lists configuration, mechanical transmission, sensor adjustment, and cable conditions before drive replacement.
SP9081 Is Not the Same as SP9082
The distinction between alarms 81 and 82 determines the correct diagnosis.
| CNC alarm | Amplifier display | FANUC description | Diagnostic meaning |
|---|---|---|---|
| SP9081 | 81 | The position where the motor-sensor one-rotation signal is generated is incorrect | A one-rotation reference occurs, but its position is inconsistent |
| SP9082 | 82 | The motor-sensor one-rotation signal is not generated | The required reference signal is missing |
If the amplifier display is 81, diagnose signal position, ratio, slip, adjustment, shielding, and routing. If it displays 82, follow the missing-signal procedure instead. Do not combine the two alarm descriptions simply because both concern the one-rotation reference.
1. Confirm the Exact Alarm and Machine Configuration
Before changing a parameter or moving a sensor, confirm:
- The CNC alarm screen shows SP9081.
- The spindle amplifier status display shows 81.
- The fault is associated with the spindle motor sensor rather than the separate spindle sensor.
- Whether the machine uses an external one-rotation proximity switch.
- Whether the machine uses an αiMZ or αiBZ motor sensor for the relevant feedback.
- Whether the motor and spindle are directly connected, gear-driven, or connected by a timing belt.
- Whether any motor, sensor, belt, gear, spindle, amplifier, cable, or parameter work occurred before the alarm began.
Record the complete CNC model, spindle amplifier model and suffix, spindle motor model, sensor type, software series, and existing parameter values. Parameter numbers and bit assignments can vary by CNC generation, so use the applicable FANUC spindle parameter manual and machine-builder documentation.
2. If the External One-Rotation Signal Is Used
In an external-signal configuration, a proximity switch supplies the once-per-revolution reference. The spindle amplifier compares this external reference with the motor-sensor feedback and the configured motor-to-spindle ratio.
Check the Gear-Ratio Data
FANUC B-65285EN/04 identifies parameters 4171 and 4173 as denominator data and 4172 and 4174 as numerator data for the gear ratio between the motor sensor and spindle on the listed 0i and 30i systems. Earlier CNC generations can use corresponding 3171–3174 parameters.
Compare the saved values with the machine builder's specification. Do not calculate and enter a new ratio from appearance alone. The correct values depend on the actual pulley, gear, motor-sensor, spindle, and control configuration.
Other detector-selection parameters shown in a machine diagram, including settings in the 4000, 4002, 4004, 4010, 4011, and 4056–4059 ranges, are configuration-dependent. Treat the values in the accompanying diagram as a configuration example, not universal settings for every machine.
Check for Mechanical Slip
FANUC next instructs technicians to check for slippage between the spindle and motor. Inspect the applicable mechanical transmission for:
- A loose timing belt or insufficient tension
- Worn or damaged timing-belt teeth
- A loose pulley, coupling, key, taper, or locking element
- Gear wear, backlash, or movement on a shaft
- Evidence that the motor-to-spindle phase relationship changed after mechanical service
FANUC specifically states that the external one-rotation signal is not applicable to a V-belt connection. A conventional V-belt can slip and cannot maintain the fixed positional relationship required for this comparison. If the actual machine configuration conflicts with the documented detector method, verify the design and parameters with the machine builder.
Check the External Sensor Installation
The maintenance manual's alarm-81 procedure focuses on ratio and slip, but the physical external reference must also remain mechanically stable. With machine power safely isolated, confirm that the proximity switch, bracket, target, connectors, and cable have not moved or been damaged. Verify the detection gap and mounting only against the applicable machine documentation.
Do not reposition the switch merely to suppress the alarm. Changing the physical reference can affect spindle orientation and other position-dependent functions.
3. If the Machine Uses an αiMZ or αiBZ Motor Sensor
For configurations that do not use the external one-rotation reference described above, FANUC directs technicians to check the detector settings, motor-sensor adjustment, cable shielding, signal-cable routing, and spindle amplifier.
Verify the Motor-Sensor Parameters
Compare the installed motor and sensor with the standard parameter data in the applicable FANUC AC Spindle Motor Parameter Manual B-65280EN. Confirm the sensor type, number-of-teeth setting, feedback selection, rotation direction, and transmission ratio applicable to the machine.
Do not copy parameters from a similar machine without verifying that the motor model, sensor, amplifier, mechanical ratio, software, and machine-builder configuration are identical. An incorrect detector selection or tooth-count setting changes the control's interpretation of the feedback pulses and one-rotation reference.
Check and Adjust the Motor Sensor
FANUC identifies incorrect adjustment of the αiMZ or αiBZ sensor as a possible cause. Sensor adjustment should be performed only by qualified personnel using the motor-specific procedure and the correct measuring equipment.
If the sensor signal cannot be adjusted correctly, or if the required signal cannot be observed, FANUC instructs technicians to replace the connection cable and sensor. Before replacement, record the original mounting position and confirm the exact sensor and cable part numbers.
Do not treat “adjust the encoder” as a generic instruction. The covered component is the motor-sensor assembly used by the installed spindle motor, and its adjustment method depends on the precise motor and detector specification.
Inspect the Feedback Cable and JYA2 Connection
On the configurations represented in the supplied diagram, the motor-sensor feedback connects to JYA2. Confirm the exact connector designation in the amplifier connection manual before disconnecting anything.
With power isolated and the required discharge time observed, inspect:
- Connector seating and locking hardware
- Bent, recessed, contaminated, or corroded contacts
- Cable damage near moving sections and cabinet entry points
- Coolant or oil contamination
- Shield termination and continuity
- Repairs, extensions, or substitutions that do not match the FANUC cable specification
A cable may appear intact externally while its conductor or shield is damaged. If movement changes the alarm behavior, cable failure becomes more likely.
Check Cable Shielding and Routing
FANUC B-65285EN/04 specifically lists inadequate shielding between the sensor and spindle amplifier as a cause. Check the shield termination using the applicable FANUC Servo Amplifier Descriptions manual B-65282EN and the machine builder's wiring documentation.
The manual also instructs technicians to separate the sensor signal cable from the motor power lead if they are bundled together. High-current motor wiring can introduce electrical noise into low-level feedback signals. Maintain the specified separation, grounding, clamping, and cabinet-routing practices rather than adding an improvised ground connection.
4. Replace the Spindle Amplifier Only After the External Checks
If the applicable parameters, mechanical transmission, sensor adjustment, cable condition, shielding, and routing are correct, FANUC identifies the spindle amplifier as the remaining cause.
For the αi Series covered by B-65285EN/04, the stated action is to replace the spindle amplifier or spindle-amplifier control printed-circuit board. The αi-B manual B-65515EN/01 provides the same diagnostic branches and should be followed for the exact installed hardware. Newer or different amplifier families may specify replacement at a different assembly level.
Before choosing replacement hardware, verify:
- Complete FANUC part number and all suffixes
- Amplifier series and hardware revision
- CNC and spindle software compatibility
- Motor and detector compatibility
- Connector arrangement
- Machine-specific option and parameter requirements
If a replacement unit is required, browse FANUC servo, spindle, and power drives or send the complete model and suffix to REACO CNC for a compatibility check.
Difference Between SP9081, SP9083, and SP9085
Several nearby spindle alarms involve feedback signals but identify different detection conditions.
| CNC alarm | Amplifier display | Signal identified by FANUC | Main distinction |
|---|---|---|---|
| SP9081 | 81 | Motor-sensor one-rotation signal | The reference occurs at an incorrect position |
| SP9082 | 82 | Motor-sensor one-rotation signal | The reference is not generated |
| SP9083 | 83 | Motor-sensor phase A/B feedback | Pulse count is outside the specified range |
| SP9084 | 84 | Spindle-sensor signal | The spindle-sensor signal is disconnected or abnormal |
| SP9085 | 85 | Spindle-sensor one-rotation signal | The spindle-sensor reference occurs at an incorrect position |
| SP9086 | 86 | Spindle-sensor one-rotation signal | The spindle-sensor reference is not generated |
The words motor sensor and spindle sensor are not interchangeable in these alarm descriptions. Confirm the exact number before selecting a cable, sensor, or parameter procedure.
FANUC SP9081 Diagnostic Table
| Finding | Interpretation | Recommended action |
|---|---|---|
| CNC shows SP9081 and amplifier shows 81 | The motor-sensor one-rotation reference occurs at the wrong position | Identify the installed feedback configuration and follow the matching branch |
| External proximity switch is used | Ratio data or motor-to-spindle slip can shift the detected reference | Verify parameters 4171–4174 or the applicable equivalents, then inspect the mechanical transmission |
| Machine uses a V-belt with an external one-rotation reference | The configuration conflicts with FANUC's stated application limitation | Confirm the detector design and parameter setup with the machine builder |
| αiMZ or αiBZ sensor is used | Sensor parameters, adjustment, cable, or noise may be involved | Verify detector settings, sensor adjustment, JYA2 connection, shielding, and cable routing |
| Alarm changes when the feedback cable moves | An intermittent conductor, connector, or shield fault is likely | Inspect and replace the exact sensor cable if defective |
| Signal cable is bundled with motor power wiring | Electrical noise may corrupt feedback timing | Separate the cables and restore the specified shielding and grounding arrangement |
| All external causes have been eliminated | The spindle amplifier or its control board may be faulty | Replace the supported assembly specified for the installed series |
| Amplifier displays 82 instead of 81 | The one-rotation signal is missing, not merely misplaced | Follow the SP9082 procedure |
What Not to Do With Alarm 81
- Do not describe SP9081 as “no one-rotation signal”; that is SP9082.
- Do not enter parameter values from the example image without matching them to the exact machine.
- Do not reposition an external proximity switch without the machine-builder setup procedure.
- Do not use an external one-rotation reference with a V-belt transmission unless the applicable machine documentation explicitly provides a valid arrangement.
- Do not bundle the feedback cable with spindle or servo motor power leads.
- Do not perform insulation or dielectric testing through a connected sensor circuit.
- Do not replace the spindle amplifier before checking the documented ratio, slip, sensor, shielding, and routing causes.
Recommended Troubleshooting Order
- Confirm SP9081 on the CNC and alarm 81 on the spindle amplifier.
- Record the CNC, spindle amplifier, motor, sensor, software, and parameter information.
- Determine whether the system uses an external one-rotation proximity switch or an αiMZ/αiBZ motor sensor configuration.
- For an external signal, verify the applicable gear-ratio data and inspect for slip between the motor and spindle.
- Confirm that the external-signal configuration does not rely on a slipping V-belt relationship.
- For the motor-sensor configuration, verify the detector parameters against B-65280EN and the machine data.
- Check and correctly adjust the αiMZ or αiBZ sensor using the specified procedure.
- Inspect the sensor cable, JYA2 connection, shield termination, grounding, and cable routing.
- Separate the feedback cable from motor power wiring if they are bundled together.
- Replace the correct sensor cable or sensor if adjustment or signal verification fails.
- Replace the supported spindle-amplifier control board or complete amplifier only after the external causes are eliminated.
- Retest spindle operation and all affected position-dependent functions under the machine builder's procedure.
Technical References
- FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Alarm Codes 81 and 82
- FANUC AC Servo Motor αi-B Series, AC Spindle Motor αi-B Series, Servo Amplifier αi-B Series Maintenance Manual, B-65515EN/01, Alarm Code 81 (SP9081)
- FANUC AC Spindle Motor αi/βi Series Parameter Manual, B-65280EN, detector and spindle parameter settings
- FANUC Servo Amplifier αi Series Descriptions, B-65282EN, sensor-cable connection and shielding requirements
Always use the manual edition that applies to the exact CNC, spindle amplifier, motor, detector, software, transmission, and machine.
Need Help With FANUC SP9081 Alarm?
REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers, spindle motors, motor sensors, feedback cables, and control boards. Send us the CNC alarm screen, amplifier display, complete CNC model, spindle amplifier model and suffix, spindle motor model, sensor type, parameter backup, and details of when the alarm occurs.
Visit the FANUC Technical Support Center for more FANUC alarm guides, or contact REACO CNC for assistance.
Reference Source: FANUC technical documentation. The correct procedure depends on the exact hardware, feedback configuration, parameters, transmission method, and machine-builder design.
