FANUC SP9027 Alarm 27: Position Coder Signal Disconnection Troubleshooting

FANUC SP9027 means that the spindle amplifier detected a disconnected or abnormal αi position coder signal. On a FANUC Series 0i-D/0i Mate-D, the CNC may show SP9027 / SSPA:27 DISCONNECT POSITION CODER, while the spindle amplifier displays 27.

FANUC separates the diagnosis according to when the alarm occurs: while the motor is deactivated, when the feedback cable is moved, or while the spindle is rotating. This timing helps distinguish an incorrect sensor setting or permanent open circuit from an intermittent conductor fault or an electrical-noise problem.

For the surrounding spindle alarm numbers, see the FANUC Serial Spindle Alarm Codes SP9001–SP9033.

What Does FANUC SP9027 Mean?

FANUC B-65285EN/04 defines amplifier alarm 27 as a disconnected αi position coder signal. The Series 0i-D/0i Mate-D maintenance manual describes the related CNC alarm as an abnormal spindle position coder signal and identifies connector JYA3 for that control and connection arrangement.

The position coder provides spindle-position feedback used by functions such as spindle orientation and threading. Alarm 27 concerns the availability of that feedback signal between the position coder and spindle amplifier. It is not the same as CNC-to-amplifier serial communication alarm 24.

The term position coder should also not be replaced automatically with motor encoder. FANUC distinguishes the αi position coder from the motor sensor and separate spindle sensor. Their signal-disconnection alarms have different amplifier numbers.

Is JYA3 Always the Correct Connector?

No. JYA3 is correct for the αi position coder in applicable configurations, including the Series 0i-D alarm-table example. FANUC amplifier connection documentation also identifies JYA4 for an αi position coder S in certain configurations. Main/sub-spindle arrangements may use labels such as JYA3M and JYA3S.

Before disconnecting anything, verify:

• The exact CNC series and spindle amplifier model

• Whether the detector is an αi position coder, αi position coder S, or another sensor type

• The connector designation in the machine builder's electrical drawing

• The position-coder cable specification and pin arrangement for the installed hardware

Do not force a cable into JYA3 merely because SP9027 appears. A wrong connector or pin arrangement can create another fault or damage the equipment.

Main Causes of FANUC Alarm 27

FANUC documentation identifies the following diagnostic directions:

• Incorrect sensor-setting parameters. The selected detector type or related sensor configuration does not match the installed position coder.

• Incorrect, loose or disconnected feedback-cable connection. A connector may be installed at the wrong interface, incompletely seated or wired incorrectly.

• A broken feedback-cable conductor. The fault may become intermittent when the spindle or cable moves.

• Coolant contamination inside a connector. Contamination can cause poor contact and an intermittent signal.

• Incorrect cable shielding. Alarm 27 that appears during spindle rotation can be caused by inadequate shielding between the sensor and spindle amplifier.

• Feedback cable routed with motor power wiring. Electrical noise can interfere with the position-coder signal.

• A faulty spindle amplifier or control printed-circuit board. FANUC lists amplifier-side replacement after parameter and cable checks for an alarm present while the motor is deactivated.

The alarm 27 procedure does not identify cutting load, spindle acceleration time, motor U/V/W phase sequence, belt slip or CNC serial-communication cable as general causes.

1. Confirm the Alarm and Record When It Occurs

Before changing parameters or replacing hardware, confirm:

• The CNC shows SP9027 or the series-specific equivalent.

• The spindle amplifier display shows 27.

• The alarm concerns the spindle position coder, not alarm 24, 41, 42, 47, 73 or 84.

• The exact operating condition in which the alarm appears.

Record whether alarm 27 occurs:

1. As soon as the system is powered or the motor remains deactivated

2. Only when the spindle, cable chain or feedback cable moves

3. Only while the spindle motor rotates

Also record the complete CNC model, spindle amplifier model and suffix, position-coder type, connector label, cable number, recent parameter work and any recent replacement or cabinet rewiring. Different FANUC control series can map amplifier alarm 27 to different CNC alarm numbers, so the amplifier display and applicable manuals must be checked together.

2. If Alarm 27 Occurs While the Motor Is Deactivated

When alarm 27 is present with the motor deactivated, FANUC first directs the technician to check the position-sensor parameters and feedback cable.

Check the sensor-setting parameters

Compare the current spindle parameters with:

• The exact installed position-coder type

• The complete spindle motor and amplifier configuration

• The original machine parameter backup

• The applicable FANUC AC Spindle Motor αi/βi Series Parameter Manual, B-65280EN

• The machine builder's commissioning data

Do not initialize the spindle parameters or copy values from a similar machine without a verified backup. Detector-type settings and parameter numbers vary by CNC series and spindle configuration.

Some applicable systems use a feedback-disconnection detection bit identified as PCLS. FANUC states that disconnection detection is normally enabled and that any temporary disabling used for sensor waveform adjustment must be reversed after adjustment. Do not suppress SP9027 by permanently disabling the detection function.

Check the feedback-cable connection

After safely isolating power according to the FANUC and machine builder procedures, inspect the complete coder-to-amplifier path:

• Correct connector for the installed coder type

• Correct pin arrangement and cable specification

• Fully seated and locked connectors

• Bent, pushed-back, contaminated or corroded contacts

• Cuts, crushing, sharp bends or pulling damage

• Damage near clamps, cabinet doors, joints and moving cable sections

If the connections are correct but the cable cannot reliably carry the signal, FANUC directs replacement of the feedback cable.

Check the amplifier side only after parameters and cable

If the exact sensor settings and a confirmed good cable do not clear the alarm, FANUC B-65285EN/04 lists replacement of the spindle amplifier or its control printed-circuit board. The βi SVPM maintenance manual gives the equivalent direction for the applicable SVPM.

Verify the complete part number, suffix, board number, software support and machine configuration before replacing an amplifier or control board. Do not select a replacement only by physical appearance or base model.

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

3. If Alarm 27 Appears When the Cable Moves

An alarm that reacts to cable or spindle movement strongly indicates an intermittent connection, poor connector contact or broken conductor.

With machine power isolated and the DC link safely discharged, inspect:

• The cable section that bends as the spindle head or cable carrier moves

• Strain relief at both connectors

• Areas where the cable rubs against a panel, bracket or cable-chain edge

• Loose contacts that move inside the connector housing

• Evidence of coolant, oil or corrosion in the connector

FANUC directs replacement of the cable when an internal conductor may be broken. If coolant has entered the connector, clean the connector using the approved procedure and eliminate the source of contamination before retesting.

Do not rely only on a stationary continuity test for an intermittent cable. A conductor can make contact while still and open when flexed. Use the machine builder's approved cable-test procedure and protect the connected amplifier and sensor electronics during testing.

4. If Alarm 27 Occurs While the Spindle Rotates

When the alarm is generated during spindle rotation, FANUC directs attention to shielding and cable routing.

Check that:

• The required shielded position-coder cable is installed.

• The cable shield and frame-ground connection follow the applicable FANUC connection manual.

• The shield is secured with the specified cable clamp or grounding method.

• The sensor cable is separated from spindle-motor, servo-motor and other high-current power leads.

• Excess signal cable is not coiled beside drives, reactors, magnetic contactors or power wiring.

• Recent cabinet or machine rewiring has not changed the approved cable route.

If the signal cable is bundled with a motor power lead, route the cables separately according to the machine electrical design. Do not add unverified grounding connections at both ends, because an incorrect shield termination can create additional noise problems.

After correcting shielding and routing, retest the spindle through the operating condition that previously produced the alarm. If alarm 27 remains, continue with the applicable parameter, cable and amplifier-side checks rather than replacing the position coder without test evidence.

Should the Position Coder Be Replaced?

The FANUC alarm 27 procedure in B-65285EN/04 does not list immediate position-coder replacement as the standard action. It emphasizes sensor parameters, feedback cable, cable movement, coolant, shielding, routing and amplifier-side hardware.

Position-coder replacement can become appropriate when separate signal testing confirms that the coder itself is faulty. However, FANUC explicitly places coder signal and one-rotation-signal tests in procedures for related alarms such as 41 and 42. Do not describe every SP9027 case as a failed encoder.

Likewise, reseating the spindle amplifier control board is not stated as a universal first step for alarm 27. Check board installation only when the exact amplifier manual, recent maintenance history or inspection provides a reason to do so.

Difference Between SP9027 and Related Sensor Alarms

Always diagnose the number actually displayed on the spindle amplifier. Similar words such as sensor, coder, encoder and disconnection do not make these alarms interchangeable.

FANUC SP9027 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm SP9027 on the CNC and alarm 27 on the spindle amplifier.

2. Record whether the alarm occurs with the motor deactivated, when the cable moves or during spindle rotation.

3. Identify the exact CNC, spindle amplifier, position coder, connector and cable specification.

4. Back up spindle parameters and compare the sensor settings with the actual hardware and original machine data.

5. Isolate machine power and observe the required DC-link discharge procedure.

6. Inspect the correct feedback connector and cable; replace the cable if the connection is correct but the signal remains disconnected.

7. For a movement-related alarm, check for an intermittent conductor, poor connector contact and coolant contamination.

8. For a rotation-related alarm, correct cable shielding, grounding and separation from motor power leads.

9. If correct parameters and a confirmed good cable do not clear the alarm, test or replace the exact spindle amplifier/control board specified for the machine.

10. Replace the position coder only when separate testing identifies it as faulty.

11. Retest safely and diagnose any new amplifier alarm separately.

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 27

• FANUC Series 0i-D/0i Mate-D Maintenance Manual, B-64305EN/03, SP9027 alarm list

• FANUC AC Servo Motor βi Series, AC Spindle Motor βi Series, Servo Amplifier βi Series Maintenance Manual, B-65325EN/01, Alarm Code 27

• FANUC AC Spindle Motor αi/βi Series Parameter Manual, B-65280EN, detector settings and feedback-disconnection detection

• FANUC Servo Amplifier αi Series Descriptions, B-65282EN, position-coder connectors, shielding and cable installation

• Beijing FANUC SP9027 technical guidance, position coder disconnection troubleshooting

Always use the manual edition and electrical drawing that apply to the exact CNC, spindle amplifier, position coder, software and machine configuration.

Need Help With FANUC SP9027 Alarm?

REACO CNC provides independent FANUC position-coder cable diagnosis, spindle amplifier testing, repair, compatibility checking and replacement support. Send us the CNC alarm screen, amplifier display, complete CNC model, exact spindle amplifier model and suffix, position-coder type, connector/cable labels, alarm timing and details of any recent parameter or wiring work.

Visit the FANUC Technical Support Center for more FANUC alarm guides, or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The applicable parameter, cable, sensor and amplifier procedure depends on the exact CNC, spindle amplifier, position coder and machine configuration.