30.08.2026 by Viktor Siebert

With older CNC machines, a failed servo motor is often not simply an electrical defect. Especially in motors that have operated for many years in close proximity to the machining area, coolant, oil, contamination, and aging seals can gradually cause damage that is initially barely visible from the outside.

This was exactly the type of damage we found on a Mitsubishi HC202BS AC Servo Motor that arrived at our workshop for repair.

The motor is equipped with a Mitsubishi UA-5-00 brake and an OSE104S2 encoder. During the initial inspection, there were already strong indications that the actual problem was on the feedback side. The encoder had failed.

After opening the motor, however, it became clear that this was not an isolated encoder defect. Coolant had penetrated areas of the motor over an extended period. Particularly critical was the fact that the fluid had reached the inside of the encoder and eventually destroyed it.

At the same time, the motor itself showed poor insulation. Contamination and coolant residue were found inside the motor.

This resulted in a typical chain of damage in an older CNC servo motor:

Aging seals → coolant ingress → contamination and moisture inside the motor → deterioration of insulation → coolant ingress into the encoder → failure of position feedback.

For this reason, the repair was not limited to replacing the encoder. The motor was mechanically overhauled completely and subsequently tested on an original Mitsubishi MDS-B-SVJ2-20 Servo Drive.

Initial Diagnosis

The motor nameplate provided clear identification:

Characteristic	Specification
Manufacturer	Mitsubishi Electric
Device class	AC Servo Motor
Model	HC202BS
Input	3 AC
Voltage	138 V
Current	11 A
Output power	2 kW
Rated speed	2000 r/min
Standard	IEC 34-1
Nameplate year	1994
Brake	Mitsubishi UA-5-00
Encoder	Mitsubishi OSE104S2
Test drive after repair	Mitsubishi MDS-B-SVJ2-20

During the subsequent inspection, three areas were the primary focus:

• Condition of the OSE104S2 encoder
• Insulation condition of the motor winding
• Mechanical and structural condition of the motor, including the seals

The encoder proved to be defective. After opening it, the cause became clear: coolant had entered the encoder housing.

The encoder failure therefore could not be regarded as a random electronic defect, but rather as the consequence of a sealing problem that had existed for an extended period.

In addition, the motor insulation test showed a poor condition. Contamination and coolant residue inside the motor confirmed that the fluid had not only reached the encoder area.

This is a crucial point in a professional repair. If only the encoder had been replaced, the actual cause of the damage would have remained inside the motor.

Actual Cause of Failure

The failure analysis revealed a clear chain of damage:

• The motor seals had aged and become porous after many years of operation.
• This allowed coolant to penetrate areas that are intended to remain dry.
• Coolant and contamination entered the motor interior.
• The insulation condition of the motor deteriorated.
• Fluid reached the OSE104S2 encoder.
• The encoder was damaged by the penetrating coolant and eventually failed completely.
• Replacing only the encoder would not have eliminated the underlying cause.

The final point is particularly important.

Installing a new encoder on a motor that remained contaminated and inadequately sealed would not have constituted a sustainable repair. The entire motor therefore had to be considered as a system.

Mitsubishi servo documentation also demonstrates how critical feedback is to the servo system. Mitsubishi CNC Drives separately monitor motor-side detector faults, communication faults, and incorrect feedback data. The available Mitsubishi MDS-D/DH manual lists, among other faults, initialization errors, Detector Data Error, and Main Side Detector Communication Error. The manual also explicitly mentions the OSE104 as an incremental detector type. However, these alarm specifications originate from the MDS-D/DH generation and are therefore not presented here as direct alarm codes for the older MDS-B-SVJ2-20.

Why Coolant Is So Problematic for a Servo Motor

Coolant is intended to cool and lubricate the tool and workpiece during machining. Inside a servo motor or encoder, however, it has no place.

Gradual ingress is particularly problematic.

An aged seal does not necessarily fail completely from one day to the next. Initially, only very small quantities may penetrate. Over time, moisture, oil components, and contamination accumulate inside the motor.

This can progressively deteriorate the electrical insulation condition.

The situation is even more critical with the encoder. The feedback system provides the servo system with information about the actual motor movement. If this information is incorrect or completely absent, the control system can no longer operate correctly.

For this reason, Mitsubishi troubleshooting procedures systematically distinguish between the encoder, detector cable, and drive unit when diagnosing detector problems. In the event of a Detector Communication Error, for example, the connector connections, detector cable, shielding, and finally the encoder or drive are checked.

In our repair case, this distinction was clear: the OSE104S2 itself had been damaged by coolant ingress.

Repair Measures

Due to the extent of the damage, the repair was not limited to the encoder.

The Mitsubishi HC202BS was completely refurbished.

The main work steps included:

• Complete disassembly of the servo motor
• Inspection of the motor interior for coolant and contamination
• Intensive cleaning of the affected areas
• Inspection of the electrical insulation condition
• Complete mechanical overhaul of the motor
• Reworking or replacement of wear components relevant to sealing
• Replacement of the defective Mitsubishi OSE104S2 encoder
• Inspection of the Mitsubishi UA-5-00 brake
• Reassembly of the motor
• Final electrical and mechanical inspection
• Functional test using an original Mitsubishi Servo Drive

Cleaning was particularly important.

Coolant residue and the contamination bound to it must not simply remain inside the motor after an overhaul. The electrical condition must be reassessed after cleaning and refurbishment.

Only then does it make sense to install the replacement encoder and proceed with the final functional test.

Final Test on Mitsubishi MDS-B-SVJ2-20

A decisive part of a servo motor repair is testing the motor together with suitable servo electronics.

For this reason, the HC202BS was tested after the overhaul on an original Mitsubishi MDS-B-SVJ2-20 Servo Drive.

The objective is not merely to determine whether the shaft rotates.

The feedback system must operate reliably in the interaction between the motor, encoder, and Servo Drive. The drive must correctly detect and regulate the motor movement.

This system test is particularly important after an encoder failure.

A motor may initially appear electrically and mechanically normal and still have problems with position or speed feedback. Only testing on the Servo Drive demonstrates whether the motor and feedback system are once again functioning correctly together.

After replacement of the OSE104S2, the complete mechanical overhaul, and restoration of the motor to a proper condition, the HC202BS was successfully tested on the Mitsubishi MDS-B-SVJ2-20.

Customer Feedback

No documented verbatim customer feedback is available for this repair case. We therefore deliberately refrain from using a fabricated customer quotation.

To mentioned Mitsubishi Motor:

Mitsubishi HC202BS AC Servo Motor
Mitsubishi MDS-B-SVJ2-20 AC Servo Drive Unit
Encoder Mitsubishi OSE104S2

More details about our Mitsubishi repair services can be found here:
Mitsubishi motor Repair by Industrypart

📞 Feel free to contact us with any questions about your Mitsubishi drive technology.
Our expert team is happy to help!

Technical Data: Mitsubishi HC202BS

The following table contains only information that can be clearly derived from the available nameplate and documented repair data.

Technical characteristic	Value
Manufacturer	Mitsubishi Electric
Model	HC202BS
Alternative spelling	HC 202 BS / HC-202BS
Device type	AC Servo Motor
Input	3 AC
Motor voltage	138 V
Motor current	11 A
Output power	2 kW
Rated speed	2000 r/min
Brake	Mitsubishi UA-5-00
Encoder	Mitsubishi OSE104S2
Nameplate standard	IEC 34-1
Nameplate year	1994
Serial number	M62556002
Test drive	Mitsubishi MDS-B-SVJ2-20
Frequency	Cannot be clearly determined from the available documentation
Weight	Not documented
Protection class	Not documented
Software version	Not specified for the motor
Axes	Not specified as a motor characteristic

This repair case clearly demonstrates why theoretical protection conditions and the actual operating conditions inside a machine tool must be considered separately.

Even if a motor operates without problems for many years, elastomeric seals age. A seal that originally provided sufficient protection may no longer offer the same level of protection after decades of operation.

Interaction with Other Devices

The following system configuration is documented for the motor repaired in this case:

• Mitsubishi HC202BS AC Servo Motor
• Mitsubishi OSE104S2 Encoder
• Mitsubishi UA-5-00 Brake
• Mitsubishi MDS-B-SVJ2-20 Servo Drive used for testing

Other motor, drive, or CNC combinations are deliberately not described as compatible because no definitive approval for such combinations is available in the provided documentation.

The OSE104 belongs to a Mitsubishi encoder family that is also mentioned in later Mitsubishi CNC documentation. The available MDS-D/DH manual, for example, explicitly identifies the OSE104 and OSE105 as incremental detectors.

Functional Description

Together with the Servo Drive and encoder, the Mitsubishi HC202BS forms a closed-loop servo control system.

The Servo Drive generates the electrical motor control signals based on commands received from the CNC control. The encoder detects the actual movement of the motor and feeds this information back to the system.

In simplified form, the control loop consists of:

CNC Control → Servo Drive → HC202BS Servo Motor → Mechanical Axis → OSE104S2 Feedback → Servo Drive

The encoder is therefore not merely an auxiliary component.

Without reliable feedback, the Servo Drive cannot reliably determine whether the requested movement has actually been executed.

For detector-related problems, Mitsubishi troubleshooting documentation therefore includes inspections of connectors, detector cables, shielding, and the detector itself. In cases involving incorrect feedback data, mechanical mounting and excessive vibration are also considered.

The Mitsubishi UA-5-00 brake adds a holding function to the motor. It is not part of the actual servo control process itself, but rather provides an additional mechanical function.

Alarm and Error Codes in the Mitsubishi CNC Servo System

Important note: No exact MDS-B alarm table for the HC202BS or the MDS-B-SVJ2-20 used in this case was found in the provided files. The following table therefore contains only genuine Mitsubishi alarm codes from the provided MDS-D/DH Instruction Manual. They illustrate typical fault groups involving motors, encoders, and Servo Drives, but must not be interpreted as a 1:1 alarm table for the MDS-B-SVJ2-20.

Alarm	Error name	Meaning	Reset	Action according to manual
18	Main side detector: Initial communication error	Initial communication with motor-side detector failed	PR	Check parameters, connectors, cable, detector, and drive
22	Detector data error	Incorrect feedback data from position detector	AR	Check detector mounting, vibration, cable, and detector
24	Grounding	Motor power cable is in contact with FG	PR	Check insulation of motor and power cable
2B	Main side detector: Error 1	Motor-side detector reports an error	Depends on detector	Perform detector-specific diagnostics
2C	Main side detector: Error 2	Motor-side detector reports an error	Depends on detector	Perform detector-specific diagnostics
2D	Main side detector: Error 3	Motor-side detector reports an error	Depends on detector	Perform detector-specific diagnostics
2E	Main side detector: Error 4	Motor-side detector reports an error	Depends on detector	Perform detector-specific diagnostics
2F	Main side detector: Communication error	Communication with motor-side detector interrupted	PR	Check connectors, cable, shielding, detector, and drive
31	Overspeed	Motor speed exceeds allowable value	PR	Check parameters, feedback, and drive
32	Power module error, overcurrent	Overcurrent detected in power module	PR	Check motor cable, motor insulation, detector cable, and drive
3A	Overcurrent	Excessive motor current detected	PR	Check motor, cable, insulation, and operating conditions
46	Motor overheat / Thermal error	Thermal protection of motor or detector activated	NR	Check load, wiring, motor, and detector
50	Overload 1	Motor or drive overload level ≥ 100%	NR	Check load and mechanical conditions
51	Overload 2	Very high current command present for a defined period	NR	Check motor cable, feedback, and load

The Mitsubishi documentation explicitly includes insulation testing of the motor and motor power cable as part of troubleshooting for grounding and overcurrent faults. Contact with oil is also specifically mentioned as an inspection point in the event of a grounding fault.

This is precisely what makes the HC202BS repair case technically relevant: the poor insulation condition and the coolant ingress were not two unrelated observations. They were consequences of the same aging and sealing failure process.

Technical Block

Assembly	Designation	Function	Finding / Repair
Servo motor	Mitsubishi HC202BS	Generates mechanical axis movement	Completely mechanically overhauled
Motor winding	Component of HC202BS	Electromagnetic torque generation	Poor insulation condition due to contamination and coolant residue
Encoder	Mitsubishi OSE104S2	Position and speed feedback	Defective due to coolant ingress, replaced
Brake	Mitsubishi UA-5-00	Mechanical holding function	Inspected during overhaul
Seals	Motor sealing components	Protect the motor interior against contamination and fluids	Aged and porous, identified as the cause of coolant ingress
Motor interior	HC202BS	Protected internal area containing electrical and mechanical components	Contaminated with dirt and coolant residue, thoroughly cleaned
Feedback system	OSE104S2 / Servo Drive	Provides actual motor movement data to the servo control system	Restored by encoder replacement and system testing
Test Servo Drive	Mitsubishi MDS-B-SVJ2-20	Control and functional testing of the repaired motor	Used for final functional test
Final condition	Mitsubishi HC202BS	Complete servo motor assembly	Mechanically overhauled, encoder replaced and successfully tested on original Mitsubishi Servo Drive