FANUC SP9041 Alarm 41: Position Coder One-Rotation Signal Error and Troubleshooting

FANUC SP9041 means that the spindle amplifier detected the one-rotation signal from the position coder at an incorrect position. The CNC typically displays SP9041 / SSPA:41, while the spindle amplifier displays alarm 41.

FANUC Maintenance Manual B-65285EN/04 describes alarm 41 as an incorrect generation position of the αi position coder one-rotation signal. The official procedure directs technicians to check the position-coder parameters, observe the coder Z signal, inspect cable shielding and routing, and finally test the spindle amplifier or its control board.

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

Safety Warning Before Troubleshooting SP9041

Testing the one-rotation signal may require controlled spindle rotation and, in some procedures, measurement of a live feedback signal. This work must be performed by qualified personnel using the applicable FANUC test equipment, electrical drawings and machine-builder procedure.

Do not probe an amplifier connector with an improvised lead, run the spindle at high speed to reproduce the alarm, or change detector and gear-ratio parameters by trial and error. Isolate machine power and observe the specified DC-link discharge time before inspecting connectors, cables, grounding or mechanical couplings. Keep personnel clear of the spindle, belt and tool-change mechanism during any controlled rotation test.

Back up all CNC, PMC and spindle parameters before changing any setting. A wrong detector type, tooth count, mounting direction or gear ratio can cause incorrect spindle-position feedback and affect orientation, rigid tapping, threading, spindle synchronization or Cs contour control.

What Does FANUC SP9041 Mean?

A spindle position coder normally supplies incremental A/B feedback together with a reference signal that occurs once per revolution. FANUC calls this reference the one-rotation signal; for the αi position coder it is commonly examined as the Z signal.

Alarm 41 does not simply mean that the feedback cable is disconnected. It means that the amplifier judged the position at which the one-rotation signal occurred to be incorrect relative to the other position feedback and the configured detector relationship.

This distinction is important:

• SP9041 / alarm 41: the one-rotation signal occurs, but its detected position is incorrect or inconsistent.

• SP9042 / alarm 42: the one-rotation signal is not generated.

• SP9047 / alarm 47: the counted A/B position-coder pulses are outside the expected range for one revolution.

• SP9027 / alarm 27: the αi position-coder signal is disconnected or cannot be detected correctly.

The first task is therefore to determine whether a one-rotation signal exists. If no signal can be observed, the diagnosis moves toward the coder, feedback cable and connection. If the signal is present, the diagnosis focuses on its occurrence once per expected revolution, the position-coder settings, the mechanical relationship to the spindle and the applicable gear-ratio data.

What the SFLG1/FGRD Test Shows

On supported spindle software, FANUC SERVO GUIDE can observe SFLG1 (Spindle Flag 1). The FANUC SERVO GUIDE Operator's Manual identifies SFLG1 bit 0, FGRD, as the one-revolution-signal detection flag. When FGRD is 1, the internal flag indicates that a one-rotation signal has been generated.

SFLG1 is not available on every FANUC spindle-software edition. B-65285EN/04 lists it for supported software such as applicable 9D50/11 and later editions, with separate minimum editions for other software series. Confirm software compatibility and the required SERVO GUIDE version before relying on this data.

Use the result only as a diagnostic branch:

Do not treat the whole SFLG1 word as a single on/off status. Select and interpret the FGRD bit according to the applicable SERVO GUIDE manual. Some FANUC configurations also provide a CNC/PMC one-rotation detection-state signal such as PC1DTA, but its availability and use depend on the control function and machine sequence.

1. Confirm SP9041 and Record the Exact Configuration

Before changing a parameter or replacing a position coder, confirm:

• The CNC displays SP9041, SSPA:41 or the series-specific equivalent.

• The spindle amplifier display shows 41.

• The alarm belongs to the correct spindle on a multi-spindle machine.

• No accompanying alarm identifies position-coder disconnection, missing one-rotation feedback or abnormal A/B pulse count.

• The operating condition that triggers the alarm is known, such as first orientation after power-on, rigid tapping, threading, spindle synchronization or another position-control operation.

Record the complete CNC model, spindle amplifier model and suffix, spindle software series and edition, position-coder or spindle-sensor type, connector designation, cable number and current spindle parameters. Also record any recent work involving the spindle amplifier, coder, timing belt, gearbox, spindle motor, parameter restoration or machine wiring.

If the machine previously operated correctly and SP9041 appeared immediately after parameter loading or mechanical work, compare the current state with the approved machine-specific backup and the pre-maintenance mechanical arrangement before replacing hardware.

2. Identify the Actual Position-Feedback Device and Connector

The phrase position coder does not make one connector universal across every FANUC spindle system. B-65280EN shows an αi position coder connected to JYA3 in a typical αi configuration. The same manual also shows that separate spindle detectors can be connected to JYA3 or JYA4, and B-65285EN identifies JYA4 for some type-B sensor connections.

Before inspecting the cable, verify:

• Whether the installed feedback device is an αi position coder, α position coder S, separate αiBZ/αiCZ sensor or another supported detector

• Whether the coder is mounted directly on the spindle, connected by a 1:1 gear or timing belt, or fitted to another mechanically related shaft

• Whether the machine uses an external one-rotation proximity switch instead of the coder's own reference signal

• The correct amplifier connector and pin assignment in the exact amplifier manual and machine electrical drawing

JYA3 is common for the αi position coder in applicable configurations, but it must not be stated as the fixed position-coder connector for every FANUC system. On some systems JYA3 is also used for the external one-rotation proximity-switch input, while another spindle-sensor connection is assigned elsewhere.

3. Check Whether the Z Signal Occurs Once Per Revolution

FANUC B-65285EN/04 directs technicians to observe the position-coder Z signal. If the signal is not generated once per rotation, the manual specifies replacement of the position coder.

The signal must be checked with the approved FANUC method for the installed hardware. Depending on the amplifier generation, this may involve SERVO GUIDE, a spindle check board, a servo check pin board or direct observation at a specified test point. Use the applicable manual rather than probing the feedback connector directly.

During the approved low-speed test, determine:

1. Whether a Z/one-rotation pulse is present.

2. Whether it occurs once for each expected coder revolution.

3. Whether the occurrence position is repeatable rather than drifting or appearing intermittently.

4. Whether the result changes when the cable, connector, spindle belt or mechanical coupling moves.

If no Z signal is present, inspect the connection and cable before condemning the coder. A missing signal at the amplifier does not prove that the coder failed if the feedback path is open, contaminated or incorrectly connected. If a correct signal reaches the amplifier once per revolution but alarm 41 remains, continue with the parameter, mechanical-relationship and amplifier-side checks.

4. Verify the Position-Coder and Spindle-Sensor Parameters

FANUC's official alarm 41 procedure places the sensor settings first. The exact parameters depend on the CNC, spindle software and installed detector. For the αi/βi parameter groups described in B-65280EN/10, the following are relevant examples:

In the cited manual, an αi position coder is selected by the applicable 4002 setting, and its standard sensor-tooth setting is defined by the matching detector table. Do not copy these bits from another machine without verifying the detector and manual edition.

Also confirm that the selected parameter data belongs to the correct spindle. A correct value stored under the wrong spindle number will not correct a multi-spindle machine. After detector-related parameter changes, follow the FANUC-specified power-off/on procedure because the detector circuitry can be initialized according to those settings.

Do not disable position-feedback alarm detection merely to clear SP9041. Suppressing the alarm removes the warning without correcting an incorrect one-rotation relationship.

5. Check the Applicable Gear-Ratio Parameters

The phrase gear-ratio parameter can refer to different relationships. Identify the installed detector arrangement before selecting a parameter group.

Spindle-to-motor gear ratio — parameters 4056 to 4059

These parameters define motor rotations per spindle rotation for the active gear range. The value is selected by the CTH1A/CTH2A gear or clutch state. Confirm both the numerical data and the actual input selection. Correct numbers in the wrong selected range can still create an incorrect calculated relationship.

Motor-sensor-to-spindle arbitrary ratio — parameters 4171 to 4174

These parameters apply to supported configurations that use motor-sensor feedback to produce spindle-position feedback, including the external one-rotation-signal method. They are not the normal universal adjustment for a directly connected αi position coder.

Spindle-sensor-to-spindle arbitrary ratio — parameters 4500 to 4503

Later B-65280EN editions define these parameters for supported configurations in which spindle-sensor feedback must be converted by a mechanical ratio. They are selected by the applicable gear state and require the correct numerator/denominator relationship. Software-edition restrictions apply.

For a normal αi position coder mounted directly or through a 1:1 gear or timing belt, do not invent a ratio to compensate for a mechanical error. Verify the actual installation against the machine drawing and original commissioning data. If the coder is on another shaft or the mechanical ratio has changed, obtain the exact tooth counts and use the parameter group specified for that detector configuration.

6. Inspect the Position-Coder Cable and Connectors

If the one-rotation signal cannot be observed reliably, inspect the full coder-to-amplifier feedback path with power safely isolated:

• Confirm the cable is connected to the correct interface for the installed detector.

• Check that both connectors are fully seated and locked.

• Inspect pins and sockets for bending, push-back, corrosion or poor contact.

• Check the cable for crushing, abrasion, sharp bends, pulling damage and broken strain relief.

• Pay particular attention to sections that move with the spindle head or cable carrier.

• If coolant or oil has entered a connector, clean it using the approved procedure, inspect the contacts and correct the source of contamination.

Coolant inspection is a sensible practical check, especially when the signal changes with movement, but the αi alarm 41 procedure in B-65285EN/04 does not list coolant intrusion as a separate universal cause. It is explicitly relevant to related intermittent feedback-cable procedures. Do not let connector cleaning replace the required Z-signal and parameter checks.

Replace the feedback cable when testing confirms an intermittent or open conductor, incorrect cable specification or damaged shielding. Do not replace the position coder solely because the signal is missing at the amplifier connector.

7. Check Shielding, Routing and Grounding

FANUC B-65285EN/04 specifically lists faulty shielding and a signal cable bundled with a servo-motor power lead as alarm 41 diagnostic directions. Its connection illustration shows a shielded sensor cable connected to frame ground through the specified cable clamp.

Check that:

• The correct shielded feedback cable is installed.

• Shield termination and frame-ground clamping follow the applicable B-65282EN connection procedure.

• The position-coder cable is routed separately from spindle-motor, servo-motor and other high-current power leads.

• The cable is not coiled beside contactors, reactors, braking components or amplifier power wiring.

• The machine frame and designated grounding points are secure and free of corrosion.

• Recent cabinet rewiring has not changed the approved separation or shield termination.

Do not add arbitrary ground connections at both cable ends. The correct shield arrangement depends on the FANUC cable specification and machine design. Incorrect grounding can create rather than eliminate noise.

8. Inspect the Mechanical Coder-to-Spindle Relationship

B-65280EN shows the αi position coder directly connected to the spindle or connected through a gear or timing belt at a 1:1 ratio in its typical configuration. If the coder is mechanically coupled, verify that the physical relationship remains stable:

• The timing belt has the specified tension and is not slipping.

• Pulleys, gears, keys, couplings and mounting fasteners are secure.

• The coder shaft and coupling are not loose or damaged.

• The installed gear or pulley tooth counts match the machine design.

• Gear change completes correctly and the CTH1A/CTH2A state matches the actual mechanism.

A loose coupling or slipping timing belt can change the angular relationship between the spindle and coder even if an electrical pulse is still present. This check is especially relevant when SP9041 began after spindle, belt, pulley, gearbox or encoder work.

Do not apply the same conclusion to every belt-driven spindle. The detector arrangement and ratio must be identified first. FANUC treats external one-rotation proximity-switch systems and motor-sensor feedback configurations separately from the standard position-coder arrangement.

9. Test the Spindle Amplifier Last

If the following have all been confirmed, amplifier-side hardware becomes a valid diagnostic direction:

• The exact sensor and gear-ratio parameters match the installed machine.

• The Z/one-rotation signal is present once per expected revolution and reaches the amplifier correctly.

• The feedback cable, connectors, shield, routing and grounding are correct.

• The coder-to-spindle mechanical relationship is secure and repeatable.

FANUC B-65285EN/04 then specifies replacement of the spindle amplifier or spindle-amplifier control printed-circuit board. Test with an exact compatible unit when possible. Verify the full FANUC part number, suffix, hardware series, control-board number and spindle-software compatibility before installation.

Do not select a replacement only from output rating or physical appearance. A mismatched amplifier or control board can introduce software, detector or configuration alarms that are separate from SP9041.

If replacement support is required, browse FANUC servo, spindle and power drives or send REACO CNC the complete amplifier model for compatibility checking.

Difference Between SP9027, SP9041, SP9042 and SP9047

Alarm 46 is also related: FANUC states that its one-rotation signal cannot be detected normally during thread cutting and directs technicians to troubleshoot it as alarm 41. Always confirm the number on the spindle amplifier rather than treating all spindle encoder alarms as the same fault.

FANUC SP9041 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm SP9041 on the CNC and alarm 41 on the spindle amplifier.

2. Record the exact CNC, spindle amplifier, spindle software, coder/sensor, connector and machine configuration.

3. Back up all CNC, PMC and spindle parameters.

4. Identify whether the installed device is an αi position coder, another spindle sensor or an external one-rotation switch.

5. Where supported, monitor SFLG1#0 FGRD in SERVO GUIDE to determine whether a one-rotation signal is detected.

6. Observe the Z/one-rotation signal using the approved FANUC test method and confirm that it occurs once per expected revolution.

7. Verify the applicable sensor type, mounting direction, tooth-count and one-rotation settings against B-65280EN and the machine backup.

8. Check the correct gear-ratio group and confirm that the active CTH1A/CTH2A selection matches the actual gearbox or clutch state.

9. Inspect the correct feedback connector and cable; address contamination, damaged contacts or intermittent conductors.

10. Correct cable shielding, frame-ground clamping and separation from motor power leads.

11. Inspect the timing belt, gears, pulley, coupling and coder mounting when the detector is mechanically driven.

12. If the signal, parameters, cable installation and mechanical relationship are confirmed, test or replace the exact spindle amplifier/control board.

13. Retest first at low speed and without cutting load, then verify orientation and other position-control 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 Code 41 and SERVO GUIDE observable spindle data

• FANUC AC Spindle Motor αi/βi Series, Built-in Spindle Motor Bi Series Parameter Manual, B-65280EN/10, detector settings, typical detector configurations and gear-ratio parameters

• FANUC SERVO GUIDE Operator's Manual, B-65404EN/01, SFLG1 and FGRD one-revolution-signal detection flag

• FANUC Servo Amplifier αi Series Descriptions, B-65282EN, detector connections, shield termination and cable routing

• Beijing FANUC SP9041 technical guidance, position-coder one-rotation signal troubleshooting

Always use the manual edition that applies to the exact CNC, spindle amplifier, spindle software, position coder, detector connection and machine. Parameter numbers, connector assignments and available SERVO GUIDE data differ between FANUC generations.

Need Help With FANUC SP9041 Alarm?

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

Visit the FANUC Technical Support Center for more FANUC alarm guides, read the FANUC SP9027 Position Coder Disconnection guide, or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The correct signal test, parameter, cable, mechanical and amplifier procedure depends on the exact CNC, spindle amplifier, detector, software and machine-builder configuration.