FANUC SP9043 Alarm 43: Differential-Speed Position Coder Disconnection

FANUC SP9043 means that the position-coder signal from the master spindle used for differential spindle speed control has been disconnected or detected as abnormal. The CNC may display SP9043 or SSPA:43 DISCONNECT POSITION CODER DEF. SPEED, while the spindle amplifier displays 43.

On applicable systems using Submodule SW, FANUC identifies the monitored differential-speed signal at connector JYA3S. The first repair direction is to inspect the position-coder feedback path and replace a defective cable. The spindle amplifier or its control PCB is considered only after the external signal path, connector condition, parameters, shielding, and cable routing have been verified.

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

What Does FANUC SP9043 SSPA:43 Mean?

SP9043 is not a general statement that every spindle encoder signal has failed. It is associated with the position-coder signal used by differential spindle speed control. FANUC's αi-series maintenance documentation describes alarm 43 as disconnection of the master-spindle position-coder signal used in this control mode. The applicable 0i-D maintenance manual identifies the corresponding signal at JYA3S in Submodule SW.

The position coder supplies rotational position and speed information for the master spindle. If the signal disappears, becomes intermittent, or cannot be interpreted reliably, the control can no longer maintain the commanded speed relationship between the master and slave spindles. It stops the operation and reports SP9043.

The alarm wording can vary with the CNC display language, software series, and screen width. Relevant versions include:

• SP9043

• SSPA:43 DISCONNECT POSITION CODER DEF. SPEED

• Alarm 43: Position coder signal for differential speed mode disconnected

Always confirm the number shown on both the CNC and the spindle amplifier. A different spindle alarm involving a position coder must be diagnosed by its own number.

How Differential Spindle Speed Control Works

Differential spindle speed control commands the rotational speed of one spindle, called the slave spindle, relative to another spindle, called the master spindle. FANUC documents this function for applications such as rigid tapping at the center of a rotating workpiece.

In that application, the master spindle holds and rotates the workpiece at a constant speed. The slave spindle drives the tapping tool at a speed calculated relative to the master-spindle motion. This arrangement can avoid the time otherwise required to stop the workpiece spindle before tapping.

The master-spindle position coder must be mechanically connected at a 1:1 ratio, either directly or through a 1:1 gear or timing-belt arrangement, for the documented configuration. Its feedback is routed to the differential-control system so the slave side can account for the master spindle's actual rotation.

Example FANUC differential spindle speed control configuration. The master-side position-coder signal is routed through Submodule SW for use by the slave-side spindle control.

During differential rigid tapping, the master spindle must remain at a constant speed. Its acceleration or deceleration during tapping can prevent the intended speed relationship from being maintained. The combined master-spindle speed and slave-side rigid-tapping command must also remain within the permitted slave-spindle speed. These operating restrictions help explain why reliable master-spindle feedback is essential, but SP9043 itself still identifies a feedback-signal problem rather than a cutting-program or speed-limit alarm.

Why JYA3S Matters—and Why the “S” Can Be Misleading

Submodule SW is used with supported spindle-switching and differential-control arrangements. Its connector families distinguish main-side and sub-side interfaces, including JYA3M and JYA3S for position-coder or external one-rotation signals.

In the differential spindle speed configuration documented by FANUC, the master-spindle position-coder signal is connected through JYA3S. Therefore, do not assume that the “S” suffix means the alarm must come from a position coder physically mounted on the slave spindle. Trace the actual machine wiring and the FANUC configuration diagram.

Reference connection arrangement for FANUC Submodule SW. Actual connector use and wiring depend on the installed spindle amplifier and machine configuration.

The second figure is a general Submodule SW connection reference. It shows the connector families and main/sub-side switching arrangement, but it is not a substitute for the machine builder's electrical drawing. The exact amplifier type, optional module, motor sensors, switching contactors, and feedback routing can differ by machine.

SP9043 Uses the Alarm 27 Troubleshooting Method

FANUC maintenance manuals direct technicians to troubleshoot alarm 43 using the procedure for alarm 27, the standard αi position-coder disconnection alarm. This does not make SP9043 and SP9027 identical. It means that the same categories of failure—parameters, connectors, cable continuity, contamination, shielding, routing, and amplifier input circuitry—must be checked on the differential-speed signal path.

For the related standard position-coder alarm, see the FANUC SP9027 Alarm 27 Position Coder Disconnection guide.

The most useful first observation is when alarm 43 occurs:

Recording the timing before disturbing the wiring can prevent an intermittent fault from being hidden during inspection.

1. Confirm the Exact Alarm and Installed Configuration

Before changing parameters or replacing parts, record:

• The complete CNC series and model

• The CNC message and the 43 shown on the spindle amplifier

• The complete spindle amplifier model and every suffix

• Whether Submodule SW is installed

• Which physical spindle is defined as the master and which is the slave

• The position-coder type and cable part number when available

• Whether the alarm occurs at power-up, while stopped, when an axis or cable moves, or only during spindle rotation

• Any recent work on the spindle, amplifier, Submodule SW, position coder, cable, connector, or machine parameters

Do not diagnose this alarm from the text “position coder” alone. SP9041, SP9042, SP9043, SP9047, and other spindle alarms identify different signal conditions. The amplifier number is the reliable starting point.

2. Trace the Master-Spindle Position-Coder Signal Path

Use the machine electrical drawing and the applicable FANUC connection manual to identify the complete signal path. On the documented differential-speed arrangement, this includes the master-spindle position coder, its feedback cable and intermediate connectors, Submodule SW JYA3S, and the relevant spindle-amplifier connection.

Verify that:

• Each cable is connected to the specified interface rather than a physically similar connector.

• JYA3S and all intermediate plugs are fully seated and locked.

• Connector shells, keys, pins, and sockets are not bent, pushed back, corroded, or loose.

• No unauthorized adapter or repaired cable changes the pin assignment or shielding.

• The actual routing matches the machine builder's drawing.

Never interchange JYA3M and JYA3S merely as a test. A connector that fits physically may carry a different selected signal, and arbitrary swapping can create additional faults.

3. Check the Applicable Sensor Parameters

FANUC's alarm-27 procedure includes incorrect sensor parameters as a possible cause when the motor is deactivated. Verify the motor- and sensor-specific spindle parameters against:

• A known-good machine backup

• The exact spindle motor and position-coder specifications

• The applicable edition of the FANUC Spindle Motor Parameter Manual B-65280EN

• Machine-builder commissioning data

Look especially for a mismatch introduced after amplifier replacement, memory restoration, option changes, or parameter editing. Do not copy values from a similar machine solely because it uses a similar motor.

Parameter initialization is not the first universal remedy. Initialization can overwrite machine-specific settings and may introduce new spindle alarms or unsafe operation. Perform it only when the applicable procedure and a verified parameter source show that the data is missing or incorrect, and preserve a complete backup first.

4. Inspect and Test the Position-Coder Feedback Cable

With machine power isolated according to FANUC and machine-builder safety procedures, inspect the feedback cable over its full accessible length. Check for:

• Cuts, crushing, abrasion, sharp bends, heat damage, or stretched sections

• Broken connector locks or strain relief

• Conductors that fail intermittently when the cable flexes

• Loose, recessed, oxidized, or contaminated contacts

• Repairs that interrupt the cable shield or alter the conductor arrangement

• Incorrect cable type or connector assignment

If the alarm occurs when the machine or cable carrier moves, test the harness while carefully flexing the suspected sections during a proper continuity or signal test. The machine may be stationary and the spindle motor deactivated even though machine movement flexes the feedback cable. This symptom often points to a conductor or contact that opens only at a particular bend.

Replace a damaged or intermittent cable with the correct FANUC or machine-builder specification. Do not install an unshielded generic cable or bypass the designed connector path.

5. Check for Coolant or Oil Inside the Connector

Coolant, oil, conductive residue, and corrosion can interrupt or distort the low-level feedback signal. Disconnect power, open only the interfaces permitted by the service procedure, and inspect JYA3S and the position-coder cable connectors.

If contamination is found, correct the source of ingress as well as the immediate connector problem. Applicable αi documentation instructs technicians to clean a connector containing coolant; applicable βiSVSP documentation may require connector replacement. Follow the instruction for the exact hardware rather than applying one procedure universally.

After cleaning or replacement, allow the connector to dry fully, inspect the contacts, reconnect it securely, and confirm that the cable seal and strain relief prevent recurrence.

6. Check Shielding, Grounding, and Cable Routing

If SP9043 appears only while the spindle rotates, electrical noise becomes a major suspect. FANUC's corresponding alarm-27 procedure specifically directs technicians to check cable shielding and to separate the signal cable from the servo or spindle motor power lead.

Confirm that:

• The specified shielded feedback cable is installed.

• The shield termination and frame-ground clamp match the FANUC connection manual.

• The shield has not been cut during a cable repair.

• Feedback and motor-power cables are not bundled together.

• Adequate separation is maintained along the full cable route, including inside the electrical cabinet and cable carrier.

• Excess cable is not coiled beside the spindle amplifier, contactor, reactor, or motor-power wiring.

Separating the cables only near the connector is not enough if they run together elsewhere. Test after restoring the complete shield and routing arrangement.

7. Check Control-Board Seating Where the Manual Specifies It

For applicable βiSVSP hardware, FANUC's alarm-27 procedure includes checking that the control PCB is inserted fully when the alarm occurs as the feedback cable is moved. If this model-specific condition applies, isolate power, observe the specified discharge time, and have qualified personnel inspect and reseat the board according to the maintenance manual.

Do not generalize this step to every αi spindle amplifier or remove boards without the correct procedure. Record connector locations and equipment data before disturbing the amplifier.

8. Evaluate the Position Coder Only After the Cable Path

A defective position coder can cause loss of feedback, but SP9043 does not by itself prove that the encoder is damaged. The field procedure supplied for this article includes position-coder replacement as a possible later step; FANUC's brief SP9043 remedy first points to the cable, and the detailed manual directs technicians to the alarm-27 procedure. First verify the parameters, connections, cable continuity, contamination, shielding, and routing.

If supported test equipment or substitution with a known-compatible device isolates the fault to the coder, replace it according to the motor and machine procedure. Preserve the documented mechanical coupling and 1:1 relationship required by the differential-control configuration. Complete any orientation or commissioning checks required by the machine builder.

Do not replace the coder merely because the alarm text contains “position coder.” An open cable or failed amplifier input can produce the same detected signal loss.

9. Replace the Spindle Amplifier or Control PCB Last

FANUC includes the spindle amplifier or control PCB as a possible cause in the alarm-27 troubleshooting path used for alarm 43. Replacement becomes reasonable when all of the following have been established:

• The correct sensor parameters are loaded.

• The correct connectors and signal path have been confirmed.

• The feedback cable passes continuity, flex, shielding, and routing checks or has been replaced with a known-good compatible cable.

• Connectors are clean, dry, undamaged, and fully seated.

• The position coder has been proven serviceable or the fault remains with a known-good feedback source.

• The alarm still occurs on the same amplifier input.

Before replacement, record the full amplifier model, suffixes, control-board number, spindle software information, and machine options. Do not select a replacement by output rating or physical appearance alone. If a replacement is required, browse FANUC servo, spindle, and power drives or send the complete model information to REACO CNC for a compatibility check.

Difference Between SP9027 and SP9043

FANUC tells technicians to use the alarm-27 troubleshooting method for alarm 43 because the electrical failure modes are similar. Still, always trace the connector path belonging to the displayed alarm and installed configuration.

Difference Between Alarm 41, Alarm 42, Alarm 43, and Alarm 47

Exact alarm text varies by spindle software series. Use the maintenance manual for the installed amplifier and diagnose the number actually displayed.

FANUC SP9043 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm SP9043/SSPA:43 on the CNC and 43 on the spindle amplifier.

2. Record the CNC, amplifier, Submodule SW, position coder, and exact alarm timing.

3. Identify the master spindle and trace its differential-speed feedback path through JYA3S using the machine drawing.

4. Verify motor- and sensor-specific parameters against a known-good source; do not initialize them blindly.

5. Isolate power and inspect connector seating, pins, cable damage, strain relief, and contamination.

6. Test for intermittent continuity when the harness or machine assembly moves.

7. If the alarm occurs during rotation, verify the full cable shield, grounding, and separation from power wiring.

8. On applicable βiSVSP hardware, inspect control-board seating according to its maintenance procedure.

9. Test the position coder only after the cable path is proven normal.

10. Replace the correct spindle amplifier or control PCB only after the external feedback circuit has been eliminated as the cause.

11. Retest differential spindle speed control and the complete rigid-tapping sequence 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 27 and 43

• FANUC AC Spindle Motor βi Series, Servo Amplifier βi Series Maintenance Manual, B-65325EN/02, SP9043 and Alarm 27 troubleshooting

• FANUC Series 0i-MODEL D Maintenance Manual, B-64305EN/03, SP9043 / JYA3S alarm description

• FANUC AC Spindle Motor αi Series Parameter Manual, B-65280EN/05, Differential Spindle Speed Control

• FANUC Servo Amplifier αi Series Descriptions, B-65282EN/06, Submodule SW connectors and total connection diagram

Always use the manual edition that applies to the exact CNC, spindle amplifier, spindle software, Submodule SW, position coder, and machine configuration.

Need Help With FANUC SP9043 Alarm 43?

REACO CNC provides independent troubleshooting, testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers and feedback systems. Send us the CNC alarm screen, amplifier display, complete CNC model, exact spindle amplifier model and suffix, Submodule SW details, position-coder information, cable part number, and the operating state in which alarm 43 occurs.

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 wiring, parameter, replacement, and verification procedure depends on the exact CNC, spindle amplifier, spindle software, Submodule SW, position-coder type, and machine.