FANUC SP9042 Alarm 42: No Position Coder One-Rotation Signal and Troubleshooting

FANUC SP9042 means that the spindle amplifier did not detect the position coder's one-rotation signal. The CNC typically displays SP9042 / SSPA:42, while the spindle amplifier displays alarm 42.

FANUC Maintenance Manual B-65285EN describes alarm 42 as a condition in which the one-rotation signal of the position coder is not generated. The official troubleshooting direction is to verify the detector parameters, check the one-rotation signal at the approved test point, inspect or replace the connection cable and position coder, and test the spindle amplifier only after the feedback path has been confirmed.

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

Safety Warning Before Troubleshooting SP9042

Checking a one-rotation signal can require controlled spindle rotation and measurement on energized diagnostic equipment. This work must be performed by qualified personnel using the applicable FANUC manual, machine electrical drawings and machine-builder procedure.

Do not probe JYA3, JYA4 or another feedback connector with an improvised lead. Do not repeatedly run the spindle at high speed to search for a missing pulse. On applicable older hardware, FANUC specifies the PSD check pin on the spindle check board, not direct probing of the position-coder connector. On supported newer systems, approved diagnostic software may be used instead.

Isolate machine power and observe the specified DC-link discharge time before disconnecting the coder cable, opening connectors or replacing the spindle amplifier. Keep personnel clear of the spindle, belt, gearbox, chuck and tool-changing mechanism during any controlled rotation test.

Back up all CNC, PMC and spindle parameters before changing detector settings. Selecting the wrong detector type or tooth count can produce incorrect position feedback and unexpected spindle behavior.

What Does FANUC SP9042 Mean?

A spindle position coder normally provides incremental position feedback together with a reference pulse that occurs once per revolution. FANUC calls this reference the one-rotation signal. With an αi position coder, technicians commonly examine the corresponding Z signal.

SP9042 is issued when the amplifier cannot obtain this one-rotation reference under the required detection condition. This can happen because:

• The spindle-sensor type or related detector setting does not match the installed hardware.

• The one-rotation pulse is not being produced by the position coder.

• The pulse is produced at the coder but does not reach the spindle amplifier because of an open, damaged, incorrectly connected or intermittent cable.

• The installed detector is connected to the wrong interface for the selected configuration.

• The applicable one-rotation detection-speed condition is not satisfied or is configured incorrectly.

• The spindle amplifier input or control board does not detect a verified signal.

Alarm 42 does not by itself prove that the complete position coder is electrically disconnected. A full or intermittent loss of the position-coder signal is covered by related alarms such as SP9027. SP9042 specifically focuses on the missing one-rotation reference.

Difference Between SP9041 and SP9042

SP9041 and SP9042 both concern the one-rotation signal, but FANUC defines different detection conditions:

This distinction changes the troubleshooting order. For SP9041, parameters describing the sensor relationship and applicable mechanical ratios can be important because the signal is detected at the wrong position. For SP9042, first establish whether a one-rotation signal exists at all.

Do not treat alarm 41 and alarm 42 as interchangeable translations of “abnormal one-rotation signal.” Record the number on the spindle amplifier and follow the procedure for that exact number. For the adjacent fault, see the FANUC SP9041 One-Rotation Signal Position Error guide.

1. Confirm SP9042 and Record the Exact Configuration

Before changing parameters or replacing the encoder, confirm:

• The CNC displays SP9042, SSPA:42 or the series-specific equivalent.

• The spindle amplifier display shows 42.

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

• No accompanying alarm identifies a complete coder disconnection, abnormal A/B pulse count, spindle-sensor fault or motor-sensor fault.

• The machine condition that triggers the alarm is known, such as the first spindle rotation, orientation, rigid tapping, threading or entry into another position-control mode.

Record the complete CNC model, spindle amplifier model and suffix, spindle software series and edition, position coder or spindle-sensor model, connector label, cable number and current spindle parameters. Also note whether the alarm appeared after parameter restoration, amplifier replacement, coder replacement, cable work, spindle repair or gearbox work.

If the machine operated correctly before maintenance, compare the current detector settings and physical connections with the approved machine backup and electrical drawing. A replacement part that looks identical may use a different detector interface or parameter selection.

2. Identify the Actual Feedback Device and Connector

The technician's description identifies the position coder at JYA3, which is correct for a common αi position-coder configuration. However, JYA3 must not be treated as the universal position-coder connector for every FANUC spindle system.

FANUC B-65280EN defines a spindle sensor as a detector connected to JYA3 or JYA4, depending on the detector and amplifier arrangement. Supported devices can include an αi position coder, α position coder S, separate αiBZ sensor, separate αiCZ sensor or another specified detector. JYA3 can also be used for an external one-rotation proximity-switch input in applicable configurations.

Before checking the cable, determine:

• The exact detector model installed on the machine

• Whether it is used as the spindle sensor or as part of another feedback arrangement

• The correct amplifier connector for that detector

• Whether the one-rotation signal comes from the coder itself or an external switch

• Whether the detector is directly coupled to the spindle or driven through a gear, coupling or timing belt

Use the exact spindle-amplifier manual and machine electrical drawing to identify the pins and test points. Do not move a cable from JYA3 to JYA4 merely because one connector appears unused.

3. Verify the Position-Coder and Detector Parameters

FANUC's first official alarm 42 check is the sensor parameter setting. The exact parameter numbers and bit definitions vary by CNC generation, spindle software and detector type. In the αi/βi parameter groups described by B-65280EN, the following are relevant examples:

For an αi position coder, B-65280EN gives a specific detector-type selection and states that the spindle-sensor tooth-count bits are set according to that detector. Do not copy individual bit values from another machine, because the correct choice changes with the installed feedback device and software edition.

Parameter 4004 is especially easy to misuse. It selects the edge type of an external one-rotation proximity switch connected in a supported arrangement. It is not a general Z-signal adjustment for every αi position coder.

Parameter 4394#2 also requires care. In supported software, it changes whether the one-rotation signal is detected only at 10 min⁻¹ or above, or regardless of spindle speed. The manual lists detector and software restrictions. Do not change this bit simply to suppress a low-speed alarm without verifying the installed sensor and applicable edition.

If SP9042 appeared after a parameter restore, compare the entire detector-related group with the approved machine backup. Do not change only one visible bit while leaving an incompatible sensor type, tooth count or feedback arrangement elsewhere.

4. Check Whether the One-Rotation Signal Is Generated

FANUC B-65285EN directs technicians to check the PSD check pin on the spindle check board for alarm 42. If the signal is not generated once per revolution, the official procedure specifies replacement of the connection cable and position coder.

PSD is not a generic pin on every modern FANUC spindle amplifier. It belongs to the applicable check-board procedure. Use it only when the exact maintenance manual identifies the spindle check board and its test connection for the installed SPM.

On supported spindle software, FANUC SERVO GUIDE can provide a non-contact diagnostic alternative. In SFLG1, bit 0 FGRD is the one-revolution-signal detection flag. When FGRD becomes 1, the internal status indicates that a one-rotation signal has been generated. Availability depends on the CNC, spindle software and SERVO GUIDE version.

Do not interpret the entire SFLG1 word as a single one-rotation status. FGRD is the relevant bit. Do not use SERVO GUIDE to change parameters or run an unsafe test program merely to reproduce the alarm.

5. Inspect the Feedback Cable and Connectors

With power safely isolated, inspect the complete feedback path from the position coder to the assigned SPM interface:

• Confirm that the cable is installed at the connector specified for the actual detector.

• Check that both ends are fully seated and locked.

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

• Check the cable for crushing, abrasion, excessive bending, pulling damage and broken strain relief.

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

• Verify continuity and insulation using the approved procedure and cable specification.

• Compare the cable part number and pin assignment with the machine drawing.

If coolant or oil has entered a connector, clean and inspect it using the approved method and correct the source of contamination. Connector contamination is a useful practical check, but B-65285EN does not list it as a separate universal alarm 42 step. A cleaned connector must still pass the signal test.

When the signal appears or disappears as the cable moves, suspect an intermittent conductor or contact. Replace a cable when testing confirms damage or an open path. Do not splice a precision feedback cable or substitute an unverified cable solely because the connectors fit.

Although shielding and power-cable separation are explicit official checks for related alarms 41 and 47, the concise alarm 42 procedure focuses on the missing signal, connection cable, coder and SPM. Verify shielding and routing whenever cable specification or electrical noise is in doubt, but do not let a general grounding adjustment replace the required one-rotation signal test.

6. Test the Position Coder

If the detector settings are correct and the cable path has been verified, determine whether the position coder itself produces the required reference pulse.

Use the approved test method for the installed coder. The test should establish:

1. Whether the incremental feedback is present.

2. Whether the one-rotation/Z reference is present.

3. Whether the reference occurs once for each expected coder revolution.

4. Whether the signal remains stable during controlled rotation and cable movement.

The presence of A/B incremental feedback does not prove that the one-rotation channel is healthy. A coder can provide motion feedback while its Z/reference output is missing. Conversely, an absent signal at the SPM does not prove coder failure until the cable and connector path have been checked.

If no one-rotation signal is generated and the approved cable test is satisfactory, replace the position coder with the exact compatible model. Verify the complete FANUC number, connector arrangement, mechanical mounting and machine-specific alignment procedure.

Do not replace the coder only because the alarm text mentions an encoder. Confirm the missing signal first whenever the hardware permits.

7. Check Mechanical Drive of the Coder When Applicable

Some αi position-coder arrangements are directly connected to the spindle; others use a specified 1:1 gear or timing belt. If the coder is mechanically driven, confirm that it actually rotates with the spindle:

• The timing belt is installed and has the specified tension.

• The belt, pulley, gear, key or coupling is not broken or slipping.

• Coder mounting screws and shaft couplings are secure.

• The installed mechanical ratio matches the machine design.

• Recent spindle repair has not left the coder uncoupled.

A stationary coder cannot generate a once-per-revolution pulse even if its electrical cable is intact. This mechanical check is configuration-dependent and is not a reason to change gear-ratio parameters by trial and error.

Parameters 4056–4059 define the spindle-to-motor gear ratio for the active gear range, but FANUC's alarm 42 procedure does not identify them as the universal first correction. If a pulse is present but its position or calculated relationship is wrong, the condition is closer to the SP9041 diagnostic branch.

8. Test the Spindle Amplifier Last

The spindle amplifier or its control printed-circuit board becomes the correct final diagnostic direction only after the following have been confirmed:

• The detector parameters match the installed position coder and software.

• The approved one-rotation test has been performed.

• The connection cable, connectors and pin assignment are correct.

• The position coder generates a valid one-rotation signal.

• Any applicable mechanical coder drive is intact.

If a valid signal reaches the SPM but alarm 42 remains, FANUC B-65285EN specifies replacement of the SPM or SPM control printed-circuit board. Test with an exact compatible unit when possible.

Verify the complete FANUC part number and suffix, hardware series, control-board number, spindle software compatibility and detector interface before installation. A spindle amplifier selected only by output rating or physical appearance may not support the original feedback configuration.

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, SP9047, SP9082 and SP9086

The wording can vary by FANUC generation and detector family. Always identify the actual feedback device and use the exact number displayed by the spindle amplifier.

FANUC SP9042 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm SP9042 on the CNC and alarm 42 on the spindle amplifier.

2. Record the complete CNC, spindle amplifier, spindle software, detector, connector and cable information.

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

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

5. Verify the exact detector type, tooth-count and applicable one-rotation detection settings against B-65280EN and the approved machine backup.

6. Use the FANUC-specified diagnostic method: PSD on the applicable spindle check board or SFLG1#0 FGRD where supported.

7. Confirm whether the one-rotation/Z signal occurs once per expected coder revolution.

8. Inspect the correct cable, connectors, pin assignment and moving cable sections.

9. Test the cable and coder separately when the approved procedure allows.

10. On mechanically driven coders, check the belt, pulley, gear, coupling and mounting.

11. Replace the connection cable or exact position coder when testing confirms the missing signal source.

12. If a valid signal reaches the SPM and alarm 42 remains, test or replace the exact compatible spindle amplifier/control board.

13. Retest at controlled low speed, then verify orientation, threading, rigid tapping and other required 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, Alarm Code 42

• FANUC AC Spindle Motor αi/βi Series, Built-in Spindle Motor BiI Series Parameter Manual, B-65280EN, detector settings, connector definitions and one-rotation detection parameters

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

• FANUC Servo Amplifier αi Series Descriptions, B-65282EN, detector connections, test hardware and feedback-cable requirements

• Beijing FANUC SP9042 technical guidance, position-coder one-rotation signal troubleshooting

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

Need Help With FANUC SP9042 Alarm?

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

Visit the FANUC Technical Support Center for more FANUC alarm guides, browse FANUC spindle amplifiers and drives, or contact REACO CNC for assistance.

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