FANUC SP9087 Alarm 87: Spindle Sensor Signal Error and Troubleshooting

FANUC SP9087 means that the spindle amplifier detected an abnormal spindle-sensor feedback signal. The CNC displays SP9087, while the spindle amplifier displays alarm 87.

FANUC Maintenance Manual B-65285EN/04 states that alarm 87 must be troubleshot by referring to alarm 83. In that procedure, the spindle amplifier checks the number of phase-A and phase-B feedback pulses each time the one-rotation signal occurs. The alarm is generated when the pulse count falls outside the specified range.

SP9087 is therefore not simply a disconnected-sensor alarm; it identifies an abnormal relationship between the A/B pulses and the one-rotation reference.

For the surrounding serial spindle alarms, see the FANUC Spindle Alarm Codes SP9034–SP9092.

What Does “Spindle Sensor Signal Error” Mean?

A typical incremental spindle detector produces repeating phase-A and phase-B signals plus a reference signal that occurs once per revolution. At each one-rotation reference, the spindle amplifier checks whether the accumulated A/B pulse count is within the range expected for the selected detector and parameter configuration.

SP9087 is issued when that count is not plausible. The possible explanations include:

• Incorrect spindle-sensor or detector parameters
• An intermittent conductor in the spindle feedback cable
• Coolant or oil contamination inside a connector
• Loose, recessed, damaged, or oxidized connector contacts
• An incorrectly adjusted spindle sensor
• Excessive, nonuniform, or incorrect sensor-to-ring clearance
• A damaged, contaminated, loose, or eccentric detection ring
• A defective phase-A, phase-B, or one-rotation sensing element
• Faulty cable shielding or shield termination
• Feedback wiring bundled with spindle or servo motor power leads
• Noise entering the feedback circuit
• An unsuitable replacement cable or incorrect pin assignment
• A defective spindle-amplifier input circuit

The alarm therefore does not prove that the complete sensor signal has disappeared. A signal may still be present but contain missing pulses, extra pulses, distortion, intermittent transitions, or an incorrect pulse count between reference markers.

How SP9087 Differs From SP9084, SP9085, and SP9086

These four spindle-sensor alarms describe different detection conditions:

Do not diagnose all four alarms as the same cable problem. A broken conductor can cause more than one alarm depending on which signal is affected and whether the break is permanent or intermittent, but the alarm number identifies the test that failed.

Does SP9087 Always Mean Connector JYA4 Is Faulty?

No. JYA4 is used for the spindle-side or second-sensor channel on certain FANUC spindle-amplifier configurations. It is reasonable to inspect JYA4 when the applicable machine wiring diagram identifies it as the spindle-sensor connector, but the alarm number alone does not prove that every SP9087 machine uses the same connector or pin assignment.

Connector use depends on the spindle-amplifier family, hardware specification, detector type, spindle option, and machine builder. Before unplugging or measuring a feedback circuit, record the complete amplifier model and suffix and confirm the machine electrical drawing and applicable FANUC connection manual.

The term encoder is also too broad for selecting a replacement. The installed device may be an αiMZ, αiBZ, MZi, αiCZ, position coder, or another supported spindle detector. Identify the exact sensor and ring combination before adjustment or replacement.

1. Confirm SP9087 and Record When It Occurs

Before changing parameters or replacing parts, confirm:

• The CNC alarm screen shows SP9087.
• The spindle amplifier display shows 87.
• The alarm is associated with the spindle-sensor channel rather than the motor-sensor channel.
• The detector type and connector match the machine documentation.
• Whether the alarm occurs immediately, only when the spindle rotates, during orientation, or at a particular speed.
• Whether moving the spindle head, feedback cable, or cable carrier triggers the alarm.
• Whether the problem started after parameter loss, spindle repair, sensor adjustment, cable work, or amplifier replacement.
• Whether SP9084, SP9085, SP9086, or another feedback alarm is also present.

Record the CNC series, complete spindle-amplifier model and suffix, spindle motor, detector model, sensor parameters, connector location, alarm timing, commanded speed, and machine state. This information determines which manual, signal path, and replacement part apply.

2. If Cable Movement Triggers the Alarm, Inspect the Cable First

FANUC gives cable movement its own diagnostic branch. If alarm 87 appears when the feedback cable moves, the manual identifies a possible broken conductor and instructs technicians to replace the cable. It also instructs them to clean the connector if coolant has entered it.

An intermittent conductor may make contact while stationary and open when the cable bends. This explains why a machine may start normally and then alarm when the spindle head, cable carrier, or related mechanism changes position.

With power isolated and the required discharge time observed, inspect:

• Flex points near the spindle head and cable carrier
• Cable clamps and points where the jacket is crushed or sharply bent
• Connector backshells and strain relief
• Pulled, recessed, bent, or corroded contacts
• Oil, coolant, condensation, or conductive contamination
• Unapproved splices, extensions, and field repairs
• Pin-to-pin continuity while the cable is carefully flexed
• Insulation between signal conductors, shield, and frame ground

Do not rely only on a stationary continuity measurement. A cable can pass while lying still and fail in the operating position. Reproduce cable movement only under a safe, controlled maintenance procedure.

Replace the cable if it is intermittent, incorrectly pinned, contaminated beyond service, or outside the specified electrical construction. Use the exact compatible feedback cable rather than a visually similar substitute.

3. Back Up and Verify the Spindle-Sensor Parameters

For alarms not specifically tied to cable movement, FANUC first lists an incorrect sensor parameter as a possible cause. Compare the installed settings with the applicable FANUC AC Spindle Motor Parameter Manual B-65280EN and the machine builder's original parameter record.

Verify as applicable:

• Spindle-sensor or detector type
• Number of detector teeth or feedback pulses
• Position-feedback source selection
• Use of the spindle-side or second-sensor channel
• Rotation direction and detector mounting direction
• A/B phase and one-rotation signal configuration
• Motor-to-spindle gear relationship
• Machine-specific orientation and position-control options

Parameter selection must match the physical detector. If the amplifier expects a different pulse count from the sensor's actual output, it can judge the A/B count as abnormal even when the cable and sensor are functional.

Parameter errors are especially likely after memory loss, restoration from another machine, control-board replacement, spindle-amplifier replacement, detector conversion, or uncontrolled initialization.

Do not initialize the complete spindle parameter set merely to test the alarm. A full initialization can erase motor, detector, orientation, gear, and machine-specific data. Preserve a verified backup and change only values supported by the exact documentation.

4. Inspect and Adjust the Spindle Sensor

FANUC's alarm-83 procedure identifies incorrect sensor adjustment as another cause. The official direction is to adjust the sensor and, if the signal cannot be adjusted correctly or cannot be observed, replace the connection cable and sensor.

The correct adjustment method depends on the detector. Check as applicable:

• Sensor-head mounting, alignment, and fastener security
• Sensor-to-ring or sensor-to-target clearance
• Gap uniformity over one complete spindle revolution
• Detection-ring concentricity and runout
• Damage, wear, scratches, impact marks, or metal debris
• Oil or coolant on the sensing surfaces
• A loose, shifted, cracked, or incorrectly installed ring
• Correct sensor head and detection-ring combination
• Phase-A, phase-B, and one-rotation waveform quality

Do not apply a generic clearance value to every detector. The required gap and adjustment procedure vary among αiMZ, αiBZ, MZi, αiCZ, and position-coder arrangements. Use the correct gauge and the specification for the installed device.

Prevent the sensor head from contacting the detection ring during adjustment or spindle rotation. Contact can damage the sensor and turn an adjustment problem into a permanent hardware failure.

If waveform observation is required, use the FANUC check board, check terminals, measuring method, spindle speed, and limits specified for the installed amplifier. Only qualified personnel should probe an energized spindle-feedback circuit.

5. Check the Feedback Cable and Every Connector

Even when cable movement does not immediately reproduce the alarm, inspect the entire feedback path between the spindle sensor and the spindle amplifier.

Check for:

• Correct connector engagement and locking hardware
• Correct pin assignment for A, B, and one-rotation signals
• Bent, recessed, loose, oxidized, or contaminated contacts
• Broken drain wires or shield connections
• Damaged cable jacket and crushed internal pairs
• Excessive bend radius or poor strain relief
• Oil or coolant migration through the cable
• Incorrect intermediate terminal blocks or relay connectors
• Nonstandard extensions or replacement cables

On machines where the electrical drawing assigns the spindle sensor to JYA4, inspect that connector and its complete cable path. Do not transfer JYA4 pin assumptions from a different amplifier model without checking the applicable documentation.

Replacing only the sensor while leaving an intermittent or incorrectly constructed cable in service can allow SP9087 to return. Conversely, replacing the cable will not correct a wrong parameter or damaged detection ring. Use observations from the failure condition to isolate the defective element.

6. Verify Shielding, Grounding, and Cable Routing

FANUC lists faulty shielding and feedback wiring bundled with motor power leads as separate causes. These checks are essential when SP9087 occurs only while the spindle or another axis is powered, at particular speeds, or when nearby contactors and drives operate.

Verify that:

• The feedback cable uses the specified shielded construction.
• The shield is continuous through the connector and any approved intermediate connection.
• Shield clamps make secure, low-impedance contact where specified.
• The spindle, sensor housing, and amplifier cabinet have the documented frame-ground connection.
• Ground surfaces are clean and free of paint where required.
• Unshielded conductor lengths are kept as short as the connection specification requires.
• The feedback cable is separated from spindle-motor and servo-motor power leads.
• Excess cable is not coiled beside power devices, contactors, or braking components.
• Replacement cable pairs and shield termination match the FANUC specification.

Separating a feedback cable from power wiring is not merely a cosmetic routing improvement. Induced noise can create extra transitions or corrupt legitimate pulses, causing the counted A/B total to fall outside the expected range.

Do not add improvised ground connections without checking the machine wiring standard. Incorrect shield termination can create a ground loop or make noise susceptibility worse.

7. Replace the Spindle Amplifier Only After External Checks

If the parameters, cable, connectors, sensor adjustment, detection ring, shielding, grounding, and routing are verified, the spindle-amplifier feedback input or processing circuit may be defective.

For the αi hardware covered by B-65285EN/04, FANUC permits replacement of the spindle amplifier or spindle-amplifier control printed-circuit board. The βi SVPM manual gives the corresponding direction for the supported SVPM or control PCB.

Before replacing hardware, verify:

• Complete FANUC part number and every suffix
• Amplifier family and hardware specification
• Supported spindle-sensor interface
• Spindle software compatibility
• Connector arrangement
• Machine-specific parameter and commissioning requirements

Do not select a replacement solely by physical appearance or base model. A unit with a different suffix or unsupported detector interface can introduce another alarm.

If replacement hardware is required, browse FANUC servo, spindle, and power drives or send the complete amplifier model to REACO CNC for a compatibility check.

FANUC SP9087 Diagnostic Table

What Not to Do With Alarm 87

• Do not describe SP9087 only as a disconnected spindle sensor.
• Do not assume the entire sensor has failed because the pulse count is abnormal.
• Do not assume every alarm-87 installation uses JYA4.
• Do not call every supported detector an encoder or use one adjustment method for all types.
• Do not initialize all spindle parameters without a verified backup.
• Do not apply one sensor-gap value to every detector.
• Do not ignore coolant contamination inside a connector.
• Do not test only stationary cable continuity when movement triggers the alarm.
• Do not bundle the spindle-feedback cable with motor power wiring.
• Do not replace the spindle amplifier before the external signal path is verified.

Recommended Troubleshooting Order

1. Confirm SP9087 on the CNC and alarm 87 on the spindle amplifier.
2. Record the complete amplifier model, detector type, connector, parameters, alarm timing, and operating state.
3. Confirm that the machine drawing identifies the affected spindle-sensor channel and connector.
4. Determine whether moving the feedback cable or spindle mechanism triggers the alarm.
5. If movement triggers it, inspect for an intermittent conductor, connector damage, and coolant entry; replace the faulty cable.
6. Back up and verify the spindle-sensor parameters against the applicable FANUC and machine-builder documentation.
7. Inspect the sensor head, detection ring, mounting, clearance, concentricity, contamination, and damage.
8. Adjust the sensor with the detector-specific procedure and observe the specified signals when required.
9. Inspect the complete feedback cable, pin assignment, connector contacts, and intermediate connections.
10. Verify shield continuity, frame grounding, cable construction, and separation from spindle and servo power leads.
11. Replace the correct cable and sensor if the signal cannot be adjusted or observed correctly.
12. Replace the supported spindle-amplifier control PCB or complete amplifier only after external causes are eliminated.
13. Retest spindle rotation, orientation, positioning, and affected machine functions under the machine-builder 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 83 and 87
• FANUC AC Servo Motor βi Series, AC Spindle Motor βi Series, Servo Amplifier βi SVPM Maintenance Manual, B-65325EN/01, Alarm Codes 83 and 87
• FANUC AC Spindle Motor αi/βi Series Parameter Manual, B-65280EN, spindle-sensor settings
• FANUC Servo Amplifier αi Series Descriptions, B-65282EN, detector connections, shielding, grounding, and cable routing

Always use the manual edition that applies to the exact CNC, spindle amplifier, spindle software, detector, connector arrangement, and machine.

Need Help With FANUC SP9087 Alarm?

REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers, spindle sensors, feedback cables, control boards, and related hardware. Send us the CNC alarm screen, amplifier display, complete CNC and amplifier models, spindle-sensor model, parameter backup, connector photos, sensor installation photos, and details of when SP9087 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 detector type, parameter configuration, signal cable, connector assignment, amplifier series, and machine-builder design.