FANUC SP9084 Alarm 84: Spindle Sensor Signal Disconnected and Troubleshooting

FANUC SP9084 means that the spindle amplifier detected a disconnected or abnormal spindle-sensor feedback signal. The CNC displays SP9084, while the spindle amplifier displays alarm 84.

FANUC Maintenance Manual B-65285EN/04 describes alarm 84 as a disconnected spindle-sensor signal and instructs technicians to use the alarm-73 troubleshooting procedure. This does not make SP9084 identical to SP9073: alarm 73 concerns the motor sensor, while alarm 84 concerns the separate spindle sensor. The diagnostic logic is shared, but the actual detector, feedback channel, connector, and cable must match the installed machine.

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

What Does FANUC SP9084 Mean?

The spindle sensor supplies speed or position feedback from the spindle side of the mechanical system. Depending on the machine and amplifier specification, it can be a separate detector from the sensor mounted on or associated with the spindle motor. If the amplifier cannot receive or correctly interpret the required spindle-sensor signal, it generates alarm 84.

“Disconnected” does not always mean that a plug is visibly unplugged. The alarm can also result from:

• An open or intermittent conductor in the feedback cable

• A loose, contaminated, or damaged connector

• Incorrect detector parameters

• An incorrectly adjusted or defective spindle sensor

• Inadequate grounding or feedback-cable shielding

• Feedback wiring routed beside motor power leads

• An incorrect cable specification

• A failed spindle-amplifier input circuit or control board

The operating condition at the moment of the alarm is essential. FANUC's referenced alarm-73 procedure separates faults that occur while the drive is inactive, while the cable moves, and while the spindle rotates.

SP9084 Is a Spindle-Sensor Alarm, Not a Motor-Sensor Alarm

The terms motor sensor and spindle sensor are not interchangeable.

FANUC tells technicians to use alarm 73 as the troubleshooting model for alarm 84 because the electrical failure modes are similar. Apply the procedure to the spindle-sensor circuit, not automatically to the motor-sensor cable.

On a two-channel system, the motor sensor can remain healthy while the separate spindle-sensor circuit fails.

Is the SP9084 Sensor Connected to JYA4?

JYA4 is relevant on some, but not all, configurations. The check-terminal tables in B-65285EN/04 and B-65325EN/02 identify a second feedback channel with phase signals PA2, PB2, and PS2 entering through JYA4 on the documented hardware. The αi Series table marks this channel for Type B arrangements, and the listed sensors can include an αiBZ sensor, analog αiCZ sensor, or position coder depending on the specification.

In those configurations, JYA4 can be the correct connector to inspect for a spindle-sensor alarm. However:

• Connector assignments vary by amplifier family and configuration.

• JYA4 can support other functions in other FANUC applications.

• The sensor type and pin assignment must come from the applicable connection manual and machine wiring diagram.

• Do not assume that every SP9084 machine uses JYA4 solely from the alarm number.

Confirm the full spindle-amplifier model, hardware type, sensor selection, and machine-builder drawing before disconnecting or testing JYA4.

1. Confirm the Exact Alarm and Operating Condition

Before replacing any part, confirm:

• The CNC screen shows SP9084.

• The spindle amplifier display shows 84.

• The machine actually uses a separate spindle sensor.

• The applicable connector and feedback channel for that sensor.

• Whether the alarm occurs before any motion, when another axis or the spindle head moves, or only while the spindle rotates.

• Whether the fault began after sensor, spindle, cable, amplifier, parameter, grounding, or mechanical work.

• Whether moving the suspected feedback cable changes the alarm.

Record the CNC series, amplifier model and suffix, sensor type, software series, parameters, and wiring. Photograph the connectors and cable route before disturbing them. The exact configuration—not the alarm text alone—determines the correct test point.

2. If the Alarm Appears Before the Spindle Moves

When the alarm is present with the drive inactive or before the spindle rotates, follow the first branch of the alarm-73 procedure but apply it to the spindle-sensor channel.

Verify the Spindle-Sensor Parameters

Compare the installed sensor with the detector settings in the applicable FANUC AC Spindle Motor Parameter Manual B-65280EN and the machine-builder parameter list. Verify the settings that define:

• Sensor or detector type

• Feedback selection

• Number of teeth, pulses, or detector resolution where applicable

• Rotation direction

• Spindle-to-motor relationship

• Use of a second feedback channel

• Machine-specific spindle-position options

An incorrect detector selection can make a connected, functioning sensor appear abnormal. This is especially relevant after parameter loss, backup restoration, amplifier replacement, or conversion work.

Do not initialize parameters or copy settings from a similar machine. Save the existing values and correct only differences confirmed by the applicable documentation.

Inspect the Spindle-Sensor Cable and Connector

With machine power isolated and the specified discharge time observed, inspect the complete spindle-sensor feedback path:

• Connector engagement and locking hardware

• Bent, recessed, corroded, or contaminated contacts

• Cable damage near the spindle, cabinet entry, clamps, and cable carrier

• Oil or coolant inside the connector or cable jacket

• Poor strain relief or an excessive bend radius

• Nonstandard splices, extensions, or substituted cables

• Conductor continuity using the approved disconnected-cable procedure

If the feedback cable is disconnected or electrically open, FANUC's referenced procedure directs replacement of the cable. Confirm the complete cable part number and connector arrangement before ordering a replacement.

Check and Adjust the Spindle Sensor

An incorrectly adjusted sensor may not produce a usable feedback signal. Follow the detector-specific adjustment method and use the correct measuring equipment, check board, test points, and waveform criteria.

If the signal cannot be adjusted correctly or cannot be observed, FANUC's alarm-73 procedure directs replacement of the connection cable and sensor. Apply that direction to the actual spindle-sensor hardware used by the machine.

Do not use “adjust the encoder” as a universal procedure. The installation may use an αiBZ sensor, analog αiCZ sensor, position coder, or another documented spindle detector, and each has its own mounting and signal requirements.

3. If Movement of the Cable or Another Axis Triggers SP9084

If the spindle is stopped but the alarm appears when another axis, spindle head, or cable carrier moves, an intermittent feedback cable is a strong suspect. FANUC specifically identifies a broken conductor when the alarm follows cable movement.

Inspect sections that flex or move repeatedly. A cable can pass a stationary continuity check yet open momentarily at a particular bend. Look for:

• Internal conductor fatigue

• Jacket stretching or flattening

• Damage at cable-carrier links

• Loose crimp terminals

• Connector pins moving inside the housing

• A cable pulled tight at the end of travel

• Coolant entry that changes contact resistance during movement

Replace a confirmed intermittent cable; a movement-sensitive fault can return unpredictably during production.

FANUC responses to contamination vary by series: B-65285EN/04 specifies cleaning, while the βiSVSP manual specifies connector replacement. Follow the applicable manual and replace damaged seals, contacts, or housings.

4. If SP9084 Appears While the Spindle Rotates

An alarm that occurs only during rotation points toward vibration-sensitive connections, marginal sensor adjustment, grounding, shielding, routing, or electrical noise.

Check Frame Grounding and Shield Termination

The αi-B maintenance manual expands the alarm-73 procedure to include low-impedance frame grounding and detailed feedback-shield requirements. Verify that:

• The spindle, detector housing, and amplifier cabinet use the documented grounding points.

• Ground conductors contact clean, unpainted surfaces where specified.

• The feedback-cable shield is continuous through intermediate connections.

• Cable clamps provide the specified low-impedance shield termination.

• Unshielded conductor lengths at each end are kept as short as required.

• No improvised shield connection or ground loop has been introduced.

Restore the original design rather than adding random ground wires. Incorrect grounding can increase noise or create an unsafe current path.

Separate Sensor Feedback From Power Wiring

FANUC instructs technicians to separate the sensor feedback cable from motor power leads if they are bundled together or routed in parallel over a long distance. Check the entire route, including cabinet entrances and cable carriers.

High-current switching can induce noise into low-level feedback signals. Maintain the separation, crossing method, shielding, and clamping specified by the applicable amplifier descriptions and machine wiring documentation.

Verify the Correct Cable Construction

The αi-B manual specifies twisted-pair conductors for sensor and thermistor circuits and a shielded feedback cable. A cable that fits physically can still have the wrong impedance, pairing, shield coverage, or conductor arrangement.

Use the exact FANUC cable or a replacement that fully meets the documented electrical and mechanical specification. Pay particular attention to any relay section, repaired connector, or locally manufactured cable.

5. Treat Sensor Wear and Spindle Runout as Supplemental Checks

Technician experience sometimes finds physical wear on a spindle-mounted sensor or target. If inspection shows rubbing, impact marks, metal debris, or repeated sensor damage, check the mechanical cause before fitting another sensor.

Possible supplemental checks include:

• Sensor-to-target clearance

• Sensor bracket security

• Spindle rear-end runout

• Bearing condition

• Axial or radial movement

• Damage caused by a previous crash or spindle repair

These are reasonable machine-specific checks, but they are not a substitute for the official parameter, cable, sensor-signal, shielding, and amplifier sequence. Runout limits and adjustment values must come from the spindle or machine-builder documentation.

6. Replace the Spindle Amplifier Only After External Checks

If the correct spindle-sensor parameters, cable, connector, sensor adjustment, grounding, shielding, routing, and cable specification have been verified, the spindle-amplifier input circuit may be faulty.

For αi Series hardware covered by B-65285EN/04, the referenced procedure allows replacement of the spindle amplifier or spindle-amplifier control printed-circuit board. For βiSVSP hardware, FANUC specifies the control printed-circuit board or complete βiSVSP. The αi-B manual directs complete spindle-amplifier replacement after the preceding checks. Follow the service level for the exact installed series.

Before choosing replacement hardware, verify the complete FANUC part number and suffixes, hardware revision, sensor support, software compatibility, connector layout, parameter backup, and commissioning 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.

Why CNC CPU Replacement Is Not the First SP9084 Step

The cited FANUC maintenance manuals do not list CNC CPU-card replacement as the standard action for alarm 84. SP9084 is handled through the spindle-sensor version of the alarm-73 procedure: parameters, cable, sensor adjustment, contamination, shielding, routing, and spindle-amplifier hardware.

CNC CPU replacement appears in procedures for other alarms, including certain Dual Check Safety result mismatches, but it should not be imported into SP9084 without separate diagnostic evidence. Replacing the CNC main board before checking the spindle-sensor circuit risks unnecessary cost and may introduce parameter, software, or compatibility problems.

Difference Between SP9084 and Related Sensor Alarms

Confirm the exact alarm number before choosing a detector, connector, cable, or parameter procedure.

FANUC SP9084 Diagnostic Table

What Not to Do With Alarm 84

• Do not assume that every SP9084 system uses JYA4 without checking the amplifier and machine drawings.

• Do not confuse the spindle sensor with the motor sensor.

• Do not replace the CNC main board as an initial alarm-84 action.

• Do not copy sensor parameters from another machine without verifying the complete configuration.

• Do not keep using an intermittent cable held in a temporary position.

• Do not bundle the feedback cable with motor power wiring.

• Do not add improvised grounds or shield connections.

• Do not replace a worn sensor repeatedly without checking the mechanical cause.

• Do not replace the spindle amplifier before completing the documented external checks.

Recommended Troubleshooting Order

1. Confirm SP9084 on the CNC and alarm 84 on the spindle amplifier.

2. Record the machine state, alarm timing, complete amplifier model, spindle-sensor type, parameters, and wiring arrangement.

3. Confirm whether the machine uses a separate spindle sensor and identify its exact connector; verify JYA4 only where documented.

4. If the alarm is present before motion, check the detector parameters, cable connection, continuity, and sensor adjustment.

5. If cable or machine movement triggers the alarm, inspect for a broken conductor, loose contact, excessive tension, or coolant entry.

6. Replace the feedback cable or service the connector according to the applicable FANUC manual.

7. If the alarm occurs during rotation, verify sensor mounting, signal quality, frame grounding, shielding, and cable routing.

8. Separate the feedback cable from spindle and servo motor power leads.

9. Inspect for sensor wear and verify spindle runout only when physical evidence supports that branch.

10. Replace the correct spindle sensor and cable if the signal cannot be adjusted or observed.

11. Replace the supported spindle-amplifier control board or complete amplifier only after the external causes are eliminated.

12. Retest spindle rotation, orientation, positioning, and all 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 73 and 84 and spindle check-terminal table

• FANUC AC Servo Motor αi-B Series, AC Spindle Motor αi-B Series, Servo Amplifier αi-B Series Maintenance Manual, B-65515EN/01, Alarm Codes 73 and 84 (SP9073 and SP9084)

• FANUC AC Servo Motor βi Series, AC Spindle Motor βi Series, Servo Amplifier βiSVSP Maintenance Manual, B-65325EN/02, Alarm Codes 73 and 84 and JYA4 check-terminal assignments

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

• Applicable FANUC Servo Amplifier Descriptions manual and machine-builder wiring documentation for the installed feedback channel, grounding, shielding, and routing requirements

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

Need Help With FANUC SP9084 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 model, spindle-amplifier model and suffix, spindle-sensor type, parameter backup, connector photos, wiring information, 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, sensor channel, parameters, cable installation, and machine-builder design.