Introduction

Servo drives in modern Okuma CNC machines are highly reliable components, but after years of continuous industrial operation they can develop complex faults that combine electrical, thermal, and mechanical symptoms.

In this repair case, we received an Okuma MIV04-1-B3 servo inverter unit, part number 1006-2214, with a customer complaint of unstable axis behavior and repeated servo alarms.

The machine owner reported that the drive occasionally worked for a short period, but the faults became more frequent over time. This type of complaint is particularly important because it often indicates progressive aging of the power electronics, rather than a simple wiring or parameter issue.

At Schirmer GmbH – CNC Elektronik, we repair Okuma drives and control electronics using real test procedures that replicate actual CNC operating conditions as closely as possible. This ensures that intermittent faults are identified and verified under load — not only on a bench.

Initial condition and fault symptoms in the CNC machine

The Okuma MIV04-1-B3 was installed into our CNC test environment to reproduce the exact customer issue under real machine conditions.

During the first test run, clear differences in axis behavior were observed:

On the Z-axis, the drive triggered Alarm A.L. 06 sporadically, even while the axis was not moving.
When attempting to move the Z-axis, the drive repeatedly and reproducibly triggered Alarm A.L. 28.
On the Y-axis, movement was initially possible, but it was accompanied by strong mechanical noise and noticeable vibration. After a short time, the Y-axis operation also ended with Alarm A.L. 06.
Additionally, the integrated cooling fan started, but produced abnormal audible running noise, suggesting mechanical wear and reduced cooling efficiency.

This combination of:

electrical alarms,
vibration and mechanical symptoms,
and thermal-related irregularities

was a strong early indicator that the problem was not caused by a simple parameter mismatch or external cable issue.

Technical explanation of the relevant alarms

The following alarms are among the most common and most important fault indicators in Okuma MIV servo drives. Understanding their meaning helps identify whether the root cause is mechanical load, thermal stress, or internal electronic degradation.

Alarm A.L. 06 – Inverter Overload

In the Okuma MIV series, A.L. 06 is defined as Inverter Overload.

This alarm is triggered when the inverter’s overload protection detects that the drive has been operating above its permissible load range for too long. Depending on the situation, the root cause may include:

Increased mechanical load (axis friction, worn ball screw, damaged bearings)
Incorrect motor parameters or tuning
Thermal problems (insufficient cooling, blocked air channels, fan failure)
Incorrect current measurement signals
Aging-related degradation of IGBT modules, drivers, or power board components

The critical detail is that this alarm can appear not only during motion, but also during standby conditions if the drive internally detects abnormal current regulation behavior.

Alarm A.L. 28 – DIFF Over (Position Deviation Too Large)

A.L. 28 indicates a DIFF Over error, which occurs when the deviation between the commanded position (setpoint) and the actual feedback position becomes too large.

This can be caused by:

unstable control loops
distorted or unstable current signals
encoder communication problems
power stage issues (drive no longer controlling motor torque correctly)
mechanical load spikes or slipping couplings

In combination with strong vibration and abnormal noise, A.L. 28 is considered highly critical, because it may indicate either:

unstable electrical drive output, or
mechanical instability on the axis.

In this case, the symptoms strongly suggested an electrical/thermal root cause inside the drive.

Detailed repair story from our workshop

After removing the drive from the test machine, the unit was brought to our workshop for systematic inspection and diagnostics.

Visual inspection and cooling system condition

The first step was a detailed visual inspection.
We immediately found:

noticeable dust and contamination inside the cooling channels
deposits on the heatsink area
contamination around the fan and airflow openings

The cooling fan itself showed rough mechanical running behavior. This matched the noise observed during machine testing and indicated that the drive’s cooling capacity was likely reduced.

Since Okuma MIV drives operate in control cabinets and industrial environments, it is common that dust and cabinet contamination slowly reduce airflow over the years. This creates long-term thermal stress on the power electronics.

Electrical input and signal stability tests

Next, we performed electrical testing of the drive’s internal supply rails and feedback signals.

The main control voltages were within tolerance.
However, during load operation, we observed abnormal behavior:

current feedback signals showed instability
the deviations were not constant, but increased as the unit warmed up
the longer the drive ran, the more noticeable the signal distortion became

This is exactly the kind of pattern that explains why the alarm A.L. 06 sometimes occurred even at standstill, and why it became more reproducible after motion commands.

Power board and power stage inspection

The power board and power section were removed and tested separately.

The diagnostics showed that several areas of the power stage were thermally stressed beyond the normal range. This was consistent with aging-related degradation.

Over time, thermal cycling can cause:

weakening of solder joints
degradation of semiconductor switching performance
drift of measurement components
increased resistance in high-current paths

As a result, the drive can no longer regulate current precisely. This leads to unstable servo behavior, which in the CNC machine appears as:

vibration
noise
unstable axis motion
and ultimately DIFF Over alarms such as A.L. 28.

Control board (ICB1) verification

In parallel, the control board (ICB1) was checked.

This step is extremely important, because Okuma drives rely on stable communication and reference signals from the OSP control system.

We ensured that:

no corrupted reference signals were present
encoder link communication remained stable
no abnormal interference or connector corrosion was affecting the system

After cleaning, inspection, and verification, the control board was assessed as stable.

Preventive fan replacement

A key part of the repair was the preventive replacement of the cooling fan.

Even if the fan still spins, a worn fan bearing can lead to:

reduced airflow
higher internal temperature
increased thermal stress
repeated overload alarms over time

In industrial drives, a worn fan is one of the most common reasons why a repaired unit fails again after a few months.

After replacing the fan, we fully cleaned the cooling surfaces and reassembled the unit.

Final testing under real CNC conditions

After completing the repair, the drive was installed again into our CNC test machine.

Both the Z-axis and Y-axis now showed:

stable, smooth motion
no abnormal vibration
no unusual noise
stable current behavior under load
consistent performance during acceleration and braking cycles

Most importantly:

✅ Alarm A.L. 06 did not reoccur
✅ Alarm A.L. 28 did not reoccur

Thermal long-term test

A special focus was placed on thermal stability.

The drive was operated for an extended period under repeated load cycles to ensure that the repaired unit remained stable even as the temperature increased.

The power stage temperatures remained within normal operating limits, confirming that:

the cooling system was restored
the electronics no longer entered critical thermal zones
and the current regulation remained stable.

Preventive recommendations for machine owners

Based on this repair case, we recommend the following preventive measures for Okuma MIV drives:

Regular cleaning of drive units and cabinet cooling channels
Early replacement of fans when unusual noise is detected
Verification of cabinet ventilation and ambient temperature
Avoiding long-term operation close to maximum load limits
Functional testing when early signs of vibration or abnormal noise appear

In many cases, early maintenance can prevent major downtime and costly production interruptions.

Conclusion

This Okuma MIV04-1-B3 repair case clearly demonstrates how closely electrical, thermal, and mechanical effects are linked in modern servo drives.

Alarms such as A.L. 06 and A.L. 28 should not be treated as isolated issues. In many cases, they represent a progressive aging process inside the power electronics — especially when combined with thermal and vibration symptoms.

Through systematic diagnostics, professional repair work, and real CNC testing under load, the full machine availability can be restored reliably and secured long-term.

If your Okuma servo drive shows recurring alarms, unstable axis behavior, or unusual noise, our team at Schirmer GmbH – CNC Elektronik can diagnose and repair the unit professionally — including testing under realistic CNC conditions.