FANUC SP9079 Alarm 79: Abnormal Initial Test Operation and Troubleshooting

FANUC SP9079 means that the spindle amplifier detected abnormal operation during its initial test or control-program initialization. The CNC displays SP9079, while the spindle amplifier displays alarm 79.

For the FANUC αi Series covered by Maintenance Manual B-65285EN/04, alarm 79 is associated with Dual Check Safety and the official action is to replace the spindle amplifier or its control printed-circuit board. The Series 30i-A Dual Check Safety manual B-64004EN/02 describes the condition as a failure to execute the spindle safety functions at power-up and specifies replacement of the SPM control printed-circuit board. For the αi-B Series covered by B-65515EN/01, FANUC describes alarm 79 as an error detected during control-program initialization and instructs technicians to replace the spindle amplifier.

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

What Does FANUC SP9079 Mean?

SP9079 is an internal spindle-amplifier initialization alarm. It indicates that the drive did not complete its required initial test or control-program initialization normally. On applicable Dual Check Safety systems, FANUC further explains that the spindle safety functions expected at power-up were not executed correctly.

The wording varies slightly by amplifier generation:

• αi Series: Abnormal operation was detected in the initial test.

• Series 30i-A Dual Check Safety: The spindle safety functions at power-up were not executed.

• αi-B Series: An error was detected during initialization of the spindle-amplifier control program.

These descriptions identify the same diagnostic area: the spindle amplifier's internal startup processing. Alarm 79 is not described by FANUC as a spindle-speed command error, motor overload, feedback-cable fault, power-line phase problem, guard-door request error, or ordinary machine-program problem.

The official procedure is therefore hardware-oriented. Confirm the alarm and installed equipment, then replace the component specified by the maintenance manual for that amplifier series.

SP9079 and Dual Check Safety

FANUC B-65285EN/04 states that alarm 79 occurs when Dual Check Safety is in use. B-64004EN/02 places SP9079 among the spindle safety-function self-diagnostics and explains that the required power-up safety functions were not executed.

This does not mean that a technician should disable Dual Check Safety, modify a safety-speed value, or bypass a guard circuit. SP9079 reports a failure of the spindle amplifier's internal initial test or initialization process. It is different from an alarm caused by a commanded speed exceeding a configured safety limit.

The αi-B maintenance manual uses the broader description control-program initialization error and directs replacement of the spindle amplifier. Always apply the manual edition for the exact spindle amplifier, CNC, software series, and safety configuration rather than assuming that every FANUC generation uses the same serviceable assembly.

1. Confirm SP9079 and Amplifier Alarm 79

Before replacing hardware, confirm both ends of the alarm report:

• The CNC alarm screen shows SP9079.

• The spindle amplifier status display shows 79.

• The alarm occurs during power-up or spindle-amplifier initialization.

• The displayed number is not 74, 75, 76, 77, or 78, which identify different self-test or comparison failures.

Record the CNC screen and amplifier display before switching off the machine. Also record:

• The complete CNC series and model

• The full spindle amplifier or SPM model number, including every suffix

• The spindle software series and edition, when available

• Whether Dual Check Safety is installed and enabled

• Whether the alarm appears at every power-up or only intermittently

• Whether the spindle amplifier, control board, CNC hardware, or wiring was serviced immediately before the alarm began

The timing is important. An alarm that appears as the drive initializes supports the SP9079 diagnosis. If a different alarm appears only after the spindle starts rotating, diagnose the number actually displayed rather than treating it as alarm 79.

2. Distinguish SP9079 From SP9076

SP9076 and SP9079 can both be described as safety-function execution problems, but FANUC assigns them to different conditions.

The practical distinction is the stage at which the drive detects the failure. Alarm 76 concerns safety-function execution monitoring, whereas alarm 79 concerns the initial startup test. Always use the actual amplifier display and the applicable alarm table.

3. Follow the Replacement Instruction for the Installed Series

FANUC's replacement wording is not identical across all amplifier generations.

This series distinction matters. A board-first repair sequence may be appropriate where FANUC identifies the control printed-circuit board as a replaceable unit, but it is not a universal instruction for every SPM. Some assemblies are serviced as complete spindle amplifiers.

Do not select a repair path solely from the two-digit alarm display. Confirm the complete hardware identity and use the maintenance manual that matches the installed unit.

4. Do Not Add Unrelated Troubleshooting Steps

For confirmed alarm 79, the cited FANUC procedures do not instruct technicians to:

• Change spindle motor initialization parameters

• Adjust safety-speed parameters

• Edit PMC ladder logic

• Reverse motor power phases

• Replace the spindle feedback cable or encoder

• Test motor insulation as the primary alarm-79 remedy

• Replace the CNC CPU card

• Reload part programs or restore CNC SRAM data

Those actions belong to other alarms and should be performed only when a separate diagnostic result supports them. Unnecessary parameter or ladder changes can create additional faults and can compromise a validated safety configuration.

A controlled power cycle may be used as part of the machine builder's normal confirmation procedure, especially because SP9079 is detected during initialization. Repeated power cycling is not the repair specified by FANUC. If the CNC consistently reports SP9079 and the amplifier reports 79, proceed with the applicable hardware-replacement procedure.

5. Select the Correct Replacement Hardware

Before ordering a control board or complete spindle amplifier, verify:

• The full FANUC part number and every suffix

• The amplifier family and hardware revision

• Whether the control printed-circuit board is a separately supported service part

• Compatibility with the CNC and spindle software series

• Compatibility with the spindle motor and feedback configuration

• Compatibility with the installed Dual Check Safety configuration

• Connector arrangement and machine-builder requirements

Do not choose a replacement only by cabinet appearance, current rating, or the base part number. Similar-looking amplifiers may use different control hardware, software, connector assignments, or safety support.

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

6. Replace the Specified Assembly Safely

Spindle-amplifier replacement involves hazardous DC-bus voltage and safety-related hardware. The work should be performed by qualified personnel using the applicable FANUC manual and the machine builder's lockout, discharge, and commissioning procedures.

Before removal:

1. Record the alarm screens, model numbers, connector locations, and installed configuration.

2. Stop the machine according to the builder's procedure.

3. Isolate incoming power and apply the required lockout/tagout controls.

4. Wait the specified discharge time and verify the safe condition before touching the amplifier.

5. Protect connectors and fiber-optic cables from contamination, bending, and incorrect reconnection.

When the manual permits control-board replacement, preserve the machine-specific configuration and follow the documented board-transfer or setup procedure. When the applicable manual specifies the complete spindle amplifier, install an exact compatible unit rather than improvising a component-level repair in the machine.

7. Validate the Repair and Safety Functions

After installing the specified hardware, power up the machine according to the applicable procedure and confirm:

• SP9079 and amplifier alarm 79 no longer appear

• The spindle amplifier completes initialization normally

• No new hardware, software, or compatibility alarm is displayed

• The spindle operates correctly under the machine builder's test procedure

• The applicable Dual Check Safety parameters are verified

• Required safety-function checks and acceptance tests are completed by authorized personnel

FANUC's Dual Check Safety documentation requires the safety parameters and functions to be checked after replacement of a safety-related part. Clearing the alarm alone is not sufficient evidence that the safety system has been fully validated.

If a different alarm appears after replacement, record and diagnose that new number separately. It may identify a compatibility, configuration, communication, or connection problem rather than a recurrence of SP9079.

Related Internal Self-Test Alarms

Several FANUC spindle alarms concern internal self-tests, but they do not identify the same failure.

For the related RAM-test condition, see the FANUC SP9016 Alarm 16: Dual Check Safety RAM Error.

Do not substitute the procedure for one alarm number for another. The exact two-digit amplifier display determines which internal test failed and which assembly FANUC directs technicians to replace.

FANUC SP9079 Diagnostic Table

What Not to Do With Alarm 79

• Do not bypass or disable Dual Check Safety to make the alarm disappear.

• Do not alter certified safety parameters without the required authorization and validation procedure.

• Do not assume that a CNC CPU card is the cause; the cited alarm-79 procedures point to spindle-amplifier hardware.

• Do not replace motor, encoder, or power wiring without evidence from another alarm or test.

• Do not install a control board or amplifier whose complete specification has not been verified.

• Do not return the machine to production before the required post-replacement safety checks are complete.

Recommended Troubleshooting Order

1. Confirm SP9079 on the CNC and alarm 79 on the spindle amplifier.

2. Record whether the alarm occurs during every power-up or spindle-amplifier initialization.

3. Record the complete CNC model, spindle amplifier model and suffix, software information, and Dual Check Safety configuration.

4. Identify the exact FANUC maintenance manual and replacement level for the installed amplifier series.

5. Do not change parameters, ladder logic, motor wiring, or feedback components without evidence of a separate fault.

6. Obtain the exact compatible control printed-circuit board or complete spindle amplifier specified by the applicable procedure.

7. Isolate power and replace the specified assembly using the FANUC and machine-builder safety instructions.

8. Power up the machine and confirm that alarm 79 no longer appears.

9. Verify the applicable safety parameters and complete the required Dual Check Safety functional checks.

10. If another alarm appears, diagnose that number 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 79

• FANUC Series 30i-A Dual Check Safety Operator's Manual, B-64004EN/02, Spindle Alarm SP9079 and post-replacement safety checks

• FANUC AC Servo Motor αi-B Series, AC Spindle Motor αi-B Series, Servo Amplifier αi-B Series Maintenance Manual, B-65515EN/01, Alarm Code 79 (SP9079)

Always use the manual edition that applies to the exact CNC, spindle amplifier, spindle software, Dual Check Safety configuration, and machine.

Need Help With FANUC SP9079 Alarm?

REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers and control boards. Send us the CNC alarm screen, amplifier display, complete CNC model, exact spindle amplifier model and suffix, software 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 applicable replacement and verification procedure depends on the exact CNC, spindle amplifier, software, safety configuration, and machine.