05.01.2026 by Viktor Siebert
Repair of a Mitsubishi AC Servo Motor HG-KR43J with Encoder Communication Error
Initial situation and fault description.
The Mitsubishi AC Servo Motor HG-KR43J was sent to us with a recurring and reproducible fault condition. During operation in combination with a MR-J4-40B Mitsubishi Servo Drive Unit, the following alarm occurred repeatedly:
AL. 20.3 – Encoder communication error: Receive data error 3
According to the Mitsubishi diagnostic description, this alarm explicitly refers to the checking method of AL.20.1 and indicates a serious communication fault between the servo amplifier and the motor encoder.
In this case, an absolute encoder of type OBA24-R was installed.
The customer additionally reported that the fault initially occurred sporadically but increased in frequency over time. Safe machine operation was no longer possible, as the servo system repeatedly entered a fault state and axis movements were interrupted.
Market situation and specifics of the HG-KR series
The HG-KR series, and especially the HG-KR43J, has been discontinued for some time. Genuine new motors are practically no longer available on the open market.
What is currently offered almost exclusively consists of:
- used motors
- visually refurbished units
- so-called refurbished motors from China without a verifiable history
Our experience clearly shows that these motors often look cosmetically refurbished and freshly painted on the outside, but are heavily worn internally. Typical findings include:
- contaminated or already damaged encoders
- worn or corroded motor bearings
- improperly installed sealing elements
- missing or incorrect encoder adjustment
Encoder-related issues in particular very quickly lead to communication alarms such as AL.20.x or AL.21 when used with MR-J4 servo systems.
Incoming inspection and diagnosis in our workshop
After receiving the motor, a mechanical and electrical incoming inspection was carried out:
- visual inspection of the motor housing
- inspection of connectors and sealing surfaces
- inspection of the encoder connection
- measurement of insulation resistance
- mechanical check of shaft and bearings
Even during manual rotation, irregular running characteristics were noticeable. After opening the encoder area, significant contamination inside the encoder housing was found. Fine particles, abrasion and residues had accumulated in the area of the encoder disk and electronics.
In parallel, the motor bearings already showed audible damage. This combination explains the fault condition very well:
Mechanical irregularities combined with a contaminated encoder system lead to incorrect data telegrams and ultimately to communication interruptions with the MR-J4 servo amplifier.
Repair process step by step
After consultation with the customer, the scope of repair was clearly defined. The goal was a technically sustainable repair, not merely a short-term fault elimination.
The following work was carried out:
- complete disassembly of the motor
- professional cleaning of all assemblies
- removal and inspection of the OBA24-R absolute encoder
- assessment of the encoder condition
In this case, the internal contamination of the encoder was already so severe that reliable operation could no longer be guaranteed.
The encoder was therefore replaced. Depending on the extent of damage, we also offer encoder repair services as an alternative.
In addition, the following measures were taken:
- replacement of all motor bearings
- renewal of sealing elements
- inspection and servicing of all connectors
- reinstallation and precise adjustment of the encoder
Encoder adjustment is particularly critical for the HG-KR series, as even minimal deviations can immediately result in communication or position errors.
Testing, parameterization and adjustment with MR-J4-40B
After mechanical refurbishment, the motor was tested on our MR-J4 test bench.
The HG-KR43J was operated with a MR-J4-40B Servo Drive Unit, exactly matching the combination used later in the customer’s machine.
The following tests were performed:
- initialization of the absolute position system
- encoder communication test
- reference run and position monitoring
- continuous operation under varying load conditions
- thermal monitoring
- verification of feedback signals
Only after stable and error-free communication over an extended period was the motor approved for shipment.
To mentioned Mitsubishi Motor: Mitsubishi HG-KR43J AC Servo Motor
Link zum erwähnten: Mitsubishi MR-J4-40B AC Servo Drive Unit
More details about our Mitsubishi repair services can be found here:
Mitsubishi motor Repair by Industrypart
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Our expert team is happy to help!
Technical specifications Mitsubishi HG-KR43J
| Parameter | Value |
|---|
| Manufacturer | Mitsubishi Electric |
| Model | HG-KR43J |
| Motor type | AC Servo Motor |
| Rated power | approx. 400 W |
| Rated voltage | 3AC 108 V |
| Rated current | approx. 2.6 A |
| Rated speed | 3000 rpm |
| Maximum speed | approx. 3000 rpm |
| Encoder | Absolute encoder OBA24-R |
| Protection class | IP65 |
| Cooling | Natural cooling |
| Weight | approx. 1.5 kg |
| Compatible drive | MR-J4-40B |
Typical alarm messages and causes for the HG-KR43J
Encoder and communication alarms
| Alarm code | Description | Typical cause |
|---|
| AL.16 | Encoder error 1 | Damaged encoder cable or loose connector |
| AL.20 | Encoder error 2 | Data error, EMC interference, encoder defect |
| AL.21 | Encoder error 3 | Hardware defect inside the encoder |
| AL.25 | Absolute position lost | Battery empty or disconnected |
Electrical and power-related alarms
| Alarm code | Description | Typical cause |
|---|
| AL.30 | Regenerative error | Excessive braking energy |
| AL.32 | Overcurrent | Short circuit or insulation fault |
| AL.33 | Overvoltage | Excessive regenerative energy |
Mechanical and thermal alarms
| Alarm code | Description | Typical cause |
|---|
| AL.46 | Servo motor overheat | Overload or insufficient cooling |
| AL.50 | Overload 1 | Continuous mechanical resistance |
| AL.51 | Overload 2 | Short-term extreme overload |
Sustainability and economic efficiency of repair
Especially for discontinued motors such as the HG-KR43J, professional repair is often the only economically viable solution.
Instead of installing unreliable used motors with an unknown history, the customer receives a technically tested and precisely adjusted original motor.
Advantages of repair:
- significant cost savings compared to replacement procurement
- avoidance of downtime risks caused by counterfeit motors
- sustainable use of existing resources
- verified functionality with the original MR-J4 servo amplifier
Conclusion
The Mitsubishi HG-KR43J is a clear example of how critical encoder condition and mechanical integrity are in modern servo systems.
By replacing and precisely adjusting the encoder, renewing the bearings, and performing comprehensive testing with the MR-J4-40B, the motor was restored to a reliable and long-term stable operating condition.
In this case, a professional repair is not only technically sound, but also the most ecological and economical decision.