Mitsubishi CNC Alarms: M70, M80, M700 Servo and Spindle Fault Diagnosis

Mitsubishi CNC controls are everywhere in Japanese machine tool brands — Mazak, Okuma, Mori Seiki, Brother, and dozens of others run Mitsubishi Meldas hardware under the hood even when the interface looks OEM. Knowing how to read and diagnose Mitsubishi alarms is a skill that transfers across a huge portion of the machines you will see in precision shops.

This guide covers the servo and spindle alarms on the M70, M80, and M700 series — the most common Mitsubishi control generations in production today.

How Mitsubishi Alarm Numbers Work

Mitsubishi uses a four-digit alarm numbering system with a letter prefix on some series. The number range tells you the subsystem:

• 0001–0099 — Emergency stop and safety interlock alarms
• 0100–0199 — X-axis servo faults
• 0110–0119 — X-axis overtravel
• 0200–0299 — Spindle system faults
• 0300–0399 — Y-axis servo faults
• 0400–0499 — Z-axis servo faults
• 0500–0599 — Additional axis faults (4th axis, B-axis, etc.)
• 1000–1999 — Program and parameter alarms

On M70/M80 series the axis assignment follows the same pattern but you may see them prefixed with the channel number on multi-axis machines.

The Most Common Mitsubishi Servo Alarms

0100: X-Axis Servo Fault

This is the most common servo alarm on Mitsubishi-controlled machines. It fires when the servo drive detects a position error, overcurrent, or feedback fault on the X-axis.

Diagnostic sequence:

1. Check the alarm detail code shown in the alarm list (DIAGN screen). The sub-code after the main alarm number tells you the specific fault type.
2. Sub-code 1 = position loop error (following error exceeded). Check for mechanical binding, way lube, or drive current limit.
3. Sub-code 4 = drive overcurrent. Check the motor for insulation breakdown and the drive power module.
4. Sub-code 8 = encoder communication fault. Check the encoder cable and connector at the motor.
5. Power cycle the machine after addressing the root cause. Mitsubishi servo alarms require a full power cycle to clear.

Field tip: On M70/M80, press DIAGN > SERVO to see real-time current and position error for each axis. This shows you whether the drive is struggling mechanically (high current, position error growing) before the alarm trips.

0010: Servo Power Off

The servo system has de-energized. This sounds obvious but it is often caused by something upstream, not the servo itself.

Common causes:

• Emergency stop circuit open — check all E-stop buttons and door interlocks
• Main servo power supply undervoltage — measure the DC bus voltage at the drive
• Thermal overload on the servo transformer
• Control relay K1 (servo on relay) failed or not energizing

Diagnostic steps:

1. Check the DC bus voltage at the drive module — should be approximately 280-320V DC for a 200V system.
2. Verify the E-stop chain is fully released and all safety interlocks are satisfied.
3. Check the servo on relay in the electrical cabinet. On most machines this is labeled K1 or labeled SERVO ON in the electrical drawings.
4. Look at the power supply LEDs. Mitsubishi MDS-series power supplies have indicator LEDs for overcurrent, undervoltage, and regeneration alarms.

0110: X-Axis Positive Overtravel

The X-axis has exceeded its positive software or hardware travel limit.

Most common causes:

• Absolute encoder datum lost after battery failure
• Axis homed incorrectly after a crash
• Workpiece offset error placing the tool outside the work envelope
• Hardware limit switch trigger (actuator bent or switch failed)

Diagnostic steps:

1. If the machine was recently powered on after a long shutdown, check the absolute encoder battery voltage. Mitsubishi uses lithium backup batteries — if the battery dies, the absolute position is lost and the machine must be re-referenced.
2. Jog the axis manually in the opposite direction (negative) to clear the overtravel zone.
3. Re-home the axis using the machine reference cycle.
4. If the limit switch is physically triggered, inspect the actuator and limit switch for damage.

Spindle Alarms: 0200-Series

0200: Spindle System Fault

The spindle drive has faulted. The sub-code tells you the type.

Sub-code breakdown:

• Sub-code 1 = speed deviation alarm — spindle not reaching commanded speed within the timeout period
• Sub-code 2 = spindle drive overcurrent
• Sub-code 4 = spindle encoder fault
• Sub-code 8 = overvoltage on spindle drive DC bus (usually regeneration issue)
• Sub-code 16 = communication fault between control and spindle drive (SSCNET or analog)

For sub-code 1 (speed deviation):

1. Run the spindle at a low speed command (S200 RPM). Watch the actual speed feedback on the DIAGN screen.
2. If actual speed is significantly lower than commanded, check V-belt tension, spindle brake release, and gear engagement.
3. If actual speed tracks but trips on high speed ranges, check the high-speed spindle motor winding resistance.

For sub-code 4 (encoder fault):

1. Check the spindle encoder cable for damage, particularly at the cable entry point on the spindle motor.
2. On Mitsubishi spindle drives (MDS-B or MDS-C series), check the encoder connector at the drive for corrosion or bent pins.
3. Verify the encoder power supply voltage at the drive output (usually 5V).

Using the Mitsubishi Diagnostic Screen

Every Mitsubishi M70/M80/M700 has a built-in diagnostic interface that most technicians underuse.

Key diagnostic screens:

• DIAGN > SERVO: Real-time servo current, position error, and drive status for each axis
• DIAGN > SPINDLE: Spindle speed command vs. actual, current, and drive status
• DIAGN > I/O: PLC input/output monitor — essential for tracing relay and switch faults
• PARAM > SERVO: Servo gain parameters — check these if following error is high without mechanical cause
• ALARM > HISTORY: Logged alarm history with timestamps — look for patterns in intermittent faults

The DIAGN screens can resolve many servo and spindle faults without touching anything in the cabinet.

Absolute Encoder Battery Replacement

This is the most frequently missed maintenance item on Mitsubishi-controlled machines. The absolute encoder battery is a 3.6V lithium cell (Mitsubishi part ER6V or equivalent). When it fails, the machine loses its axis reference positions and will not run until re-referenced.

Warning signs:

• Alarm 1037 (battery low) or 1038 (battery error) appearing at startup
• Machine requiring re-homing on every power cycle
• Position values drifting between power cycles

Replace the battery with the machine powered ON. Replacing it with the machine off will cause the encoder to lose its position data immediately. After replacement, verify all axis positions against a known reference before running parts.

Mitsubishi Alarm Codes on AxisMD

AxisMD carries the full Mitsubishi CNC alarm database for M70, M80, and M700 series controllers. Every alarm code in the servo, spindle, and program categories has step-by-step diagnostic procedures.

Look up any Mitsubishi alarm code in the Mitsubishi alarm database for field-tested root cause analysis and repair procedures.

Machine down time is expensive. Knowing the alarm before you open the cabinet changes everything.