FANUC Series 15-M SV026 and SV027 After Battery Replacement
A customer recently contacted us about an older machining center equipped with a FANUC Series 15-M control.
The machine had been working before the CNC battery was replaced. Unfortunately, the battery was changed while the control was powered off. When the machine was switched back on, two servo alarms appeared immediately:
SV026 – ILLEGAL AXIS ARRANGE SV027 – ILL DGTL SERVO PARAMETER
The axes would not become servo ready, and the machine could no longer operate normally.
At first glance, these alarms can make it look like there is a problem with the servo amplifier or one of the motors. In this case, however, the timing of the failure made us look at the CNC parameters first.
Why the Battery Replacement Matters
On older FANUC controls, the CNC battery is not simply there for the clock or alarm history. It is used to retain important memory contents while the machine is powered off.
The FANUC Series 15 maintenance documentation specifically notes that CNC memory contains information such as programs, offsets and parameters, and that the battery maintains this data when external power is removed. FANUC also instructs maintenance personnel to replace the memory backup battery with the CNC powered on.
This is why replacing a weak or disconnected memory battery with the control powered off can be risky.
If the battery was already discharged, or if backup power was interrupted during replacement, some CNC data may no longer be valid when the control starts again.
That does not automatically mean that every parameter has been erased. Sometimes only part of the machine configuration is affected. This is exactly why we do not recommend performing a memory clear as the first troubleshooting step.
What SV026 Actually Tells Us
According to the FANUC Series 15 alarm documentation, SV026 means that the servo axis arrangement is not set correctly.
The main parameter associated with this alarm is:
Parameter 1023 – Servo axis arrangement
The FANUC Series 15 parameter documentation describes Parameter 1023 as the setting used to determine the servo axis arrangement. More complicated machines may use different arrangements for synchronized, tandem or additional controlled axes.
On a simple three-axis machining center, the CNC normally has to associate the machine's X, Y and Z axes with the correct servo axis numbers.
If Parameter 1023 has become zero, duplicated, or otherwise incorrect, the CNC may no longer know how the logical machine axes correspond to the servo system.
That is when SV026 appears.
The important point is that there is no universal value that should simply be entered into Parameter 1023.
A three-axis VMC, a four-axis machine, a five-axis machine and a machine using tandem control may all require different settings.
Copying Parameter 1023 from another FANUC 15-M machine just because the control looks the same can create additional problems.
SV027 Usually Points to the Digital Servo Configuration
The second alarm in this case was:
SV027 – ILL DGTL SERVO PARAMETER
FANUC's Series 15 alarm table is quite specific about this alarm.
It lists several conditions that can generate SV027, including:
- An incorrect motor type in Parameter 1874
- An invalid number of pulses per motor revolution in Parameter 1876
- An invalid motor rotation direction in Parameter 1879
For Parameter 1879, the Series 15 documentation specifies valid rotation-direction values of 111 or -111.
Additional digital-servo parameters can also be involved depending on the control version and machine configuration.
For example, FANUC troubleshooting references for related Series 15 controls also identify parameters such as the position-feedback pulse setting, servo-axis number and flexible feed-gear settings as areas that may need to be checked when troubleshooting an illegal digital servo parameter alarm.
This is why replacing the servo amplifier is not our first move when SV026 and SV027 suddenly appear immediately after a battery or memory problem.
The amplifier may be perfectly good.
The CNC may simply no longer have the correct information describing the motor connected to it.
What We Check First
On a machine with this history, we normally start by leaving the machine in Emergency Stop and checking the parameters before attempting any axis movement.
The first parameters we would inspect are:
1023 – Servo axis arrangement 1874 – Servo motor type 1876 – Motor/feedback pulse-related setting 1879 – Motor rotation direction
We would also look through the surrounding servo parameter area and compare the values between axes.
One obvious warning sign is finding parameter pages containing large groups of zeros where machine-specific values would normally be expected.
If X, Y and Z suddenly all have zero or obviously identical servo settings after a battery problem, that is much more suspicious than a single amplifier failure.
Do Not Perform a Full Memory Clear Yet
This is one of the most important points in this repair.
Do not immediately perform:
MEMORY CLEAR MASTER CLEAR PARAMETER CLEAR or any similar complete initialization procedure.
There may still be valid machine-builder information in the CNC.
Clearing everything can turn a partial parameter-loss problem into a complete machine recovery project.
Before changing or clearing anything, photograph or record every parameter page that can still be accessed.
We also recommend recording the current diagnostics, alarm history and PMC-related information where possible.
No Parameter Backup? Start With the Machine, Not Google
This is where older CNC machines can become difficult.
A FANUC parameter manual explains what a parameter does, but it usually does not tell you the exact value that the machine builder originally used for a particular machine.
Those values depend on the mechanics, motors, amplifiers, gear ratios, axis configuration and options installed by the machine manufacturer.
A recent discussion in the CNC community involved a machine that had lost parameters after a battery failure. One of the useful suggestions was to check the electrical cabinet for old parameter paperwork and, if possible, obtain the original machine-specific backup from the builder rather than trying to reconstruct everything from a generic FANUC manual.
That matches what we normally recommend in the field.
Before attempting to rebuild the servo configuration, search the machine carefully.
We have seen old machines where the only surviving parameter record was:
- A printed parameter sheet inside the electrical cabinet
- A floppy disk stored with the electrical drawings
- Handwritten parameter changes inside the maintenance manual
- A backup kept by the previous maintenance technician
- A factory parameter sheet supplied with the machine
Check the cabinet door pockets, electrical drawing folders and documentation boxes.
On a machine that is 20 or 30 years old, one sheet of paper can save many hours of troubleshooting.
Record the Servo Motor and Amplifier Numbers
If no backup can be found, the next step is to document the hardware.
Take clear photographs of the FANUC nameplates on every servo motor.
Typical FANUC motor part numbers begin with something similar to:
A06B-xxxx-Bxxx
We would normally record:
X-axis servo motor Y-axis servo motor Z-axis servo motor Fourth/fifth-axis motor if fitted Servo amplifier model numbers Spindle motor model Spindle amplifier model
This information is extremely useful when determining whether the motor-type parameters remaining in the CNC make sense.
It also gives the machine builder, FANUC service engineer or repair company something concrete to work with.
An Identical Machine Can Help — But Be Careful
If another identical machine is available, its parameter backup can be extremely useful.
But “same FANUC controller” is not enough.
Ideally, the donor machine should have the same:
- Machine manufacturer
- Machine model
- Machine generation
- Number of controlled axes
- Servo motor models
- Servo amplifier configuration
- Gearbox and ballscrew arrangement
- Optional rotary axes
- CNC software/options
Two machines can both use a FANUC 15-M and still have very different machine parameters.
For this reason, we prefer using another machine's parameters as a comparison reference first, rather than immediately loading the complete file.
This Is a Common Problem on Older CNC Equipment
Parameter loss following battery problems is not unique to this particular machine.
There are many reports from technicians and machine owners dealing with older FANUC controls after battery failure. In one Reddit CNC discussion, a machine owner reported complete parameter loss after a battery failure and was advised to obtain the machine-specific parameter backup rather than relying only on a generic controller parameter manual.
Another machinist described recovering an older FANUC-controlled machine after motherboard work caused parameter loss; in that case, having made a parameter backup beforehand was what made recovery possible.
These community examples are anecdotal rather than official FANUC service instructions, but they illustrate an important maintenance lesson: the parameter backup is often more valuable than the replacement battery itself.
Clearing SV026 and SV027 Is Only the Beginning
A machine is not necessarily repaired just because these two alarms disappear.
If battery-backed CNC data was actually lost, other machine-specific information may also need to be checked.
Depending on the machine, this can include:
- Reference positions
- Stored stroke limits
- Electronic gear settings
- Servo gain parameters
- Backlash compensation
- Pitch-error compensation
- Spindle parameters
- Tool changer settings
- PMC keep relays
- PMC timers and counters
- Macro variables
- Work offsets
This is why we strongly recommend resisting the temptation to keep changing random parameters until the alarms disappear.
A wrong parameter can sometimes clear an alarm while creating a much more dangerous problem when the axis is finally enabled.
Battery Replacement on Older FANUC Controls
The safest approach is to create a backup before touching the memory backup battery.
FANUC documentation for older controls warns that the CNC battery preserves programs, offsets and parameters while external power is removed, and battery replacement procedures commonly require the CNC to remain powered during replacement.
However, working inside an energized electrical cabinet presents a serious electrical hazard.
Battery replacement with the control energized should therefore only be carried out by trained personnel following the machine manufacturer's and FANUC's safety procedures.
Before replacement, make a copy of everything you can:
CNC parameters Servo parameters Programs Offsets PMC data Macro variables Machine-builder settings
And keep that backup somewhere other than inside the machine.
Our Conclusion on This FANUC 15-M Case
With the sequence reported by this customer — normal machine operation, battery replacement with the control powered off, followed immediately by SV026 and SV027 — the first thing we would investigate is the CNC and servo parameter configuration.
We would not condemn the servo amplifier based on these two alarms alone.
The immediate priorities are:
- Preserve whatever data is still in the CNC.
- Check Parameter 1023 and the digital-servo parameter area.
- Record all motor and amplifier model numbers.
- Find the original machine parameter backup if possible.
- Contact the original machine builder with the machine model and serial number.
- Only initialize or rebuild the servo parameters after the actual machine configuration has been confirmed.
If you have a FANUC Series 15-M machine showing SV026 ILLEGAL AXIS ARRANGE and SV027 ILL DGTL SERVO PARAMETER, especially after battery replacement or a long period with the machine powered off, send us:
- A photo of the alarm screen
- Machine manufacturer and model
- Machine serial number
- CNC model
- Photos of the servo motor nameplates
- Photos of the servo amplifier nameplates
- Photos of Parameters 1023, 1874, 1876 and 1879
- Any old parameter sheets or backup files you can find
From those details, it is usually possible to determine whether the problem is primarily parameter loss, incorrect servo initialization, or an actual hardware fault.
Technical References
FANUC Series 15 Programming / Alarm Documentation, GFZ-62564E/02 – SV026 and SV027 alarm descriptions.
FANUC Series 15 Parameter Manual, B-62560E/02 – servo axis arrangement and Parameter 1023 information.
FANUC Series 15/150 Model B Maintenance Manual – CNC memory backup battery precautions and battery replacement information.
Reddit r/CNC – field discussion concerning machine-specific parameter recovery following battery failure.
Reddit r/Machinists – field example involving parameter recovery on an older FANUC control.