The Mitsubishi MR-J4 servo drive just threw an error and your machine is stopped. You need the diagnosis now — not a manual chapter, not a theory explanation. This guide covers every common MR-J4 error code, tells you what caused it, and walks you through the exact fix.
The MR-J4 series (MR-J4-A, MR-J4-B, MR-J4-DU) is installed on thousands of CNC machines, packaging lines, and assembly systems worldwide. It’s dependable, but when something goes wrong in the servo loop, the drive protects itself by shutting down and displaying an error code. Most errors are fixable on-site if you know what to check.
How to Read MR-J4 Error Codes
Mitsubishi MR-J4 error codes follow the format AL.xx or AL.xx.x, displayed on the drive’s 5-digit LED. The first two digits identify the error category; additional digits provide sub-detail.
To check error history:
- Press the MODE button until the display shows [AL_h]
- Press SET to view the most recent error
- Press UP/DOWN to scroll through the last 8 errors
- To clear error history: hold MODE + SET for 3 seconds in [AL_h] screen
Always check error history. The current displayed error may be a secondary fault — the root cause could be an earlier error that triggered a chain reaction.
Critical Error Codes — Address These Before Anything Else
AL.12 — Memory Error
What it means: The drive’s internal parameter memory has been corrupted. The drive cannot operate with invalid parameters.
Possible causes:
- Power interruption during parameter write
- EEPROM cell degradation from excessive write cycles
- Electrical surge or noise corrupting memory
- Drive firmware glitch
Troubleshooting steps:
- Power cycle the drive. If AL.12 returns on every power-up, the EEPROM is damaged and the drive needs replacement.
- If the drive boots normally after a power cycle, immediately back up all parameters using MR Configurator2 software.
- Perform a parameter initialization: set PC18 to “1” (initialize all parameters to defaults), then re-enter your application parameters.
- After restoring parameters, monitor for AL.12 recurrence over the next 48 hours. If it returns, replace the drive.
Parts to check: Drive unit (EEPROM), power supply stability
AL.16 — Encoder Communication Error
What it means: The drive cannot establish communication with the motor encoder. This is the most common MR-J4 error in the field.
Possible causes:
- Encoder cable disconnected or damaged
- Encoder cable connector pins bent or corroded
- Cable length exceeds specification (max 30m for standard cable)
- EMI from nearby power cables or servo drives
- Motor encoder failure
- Wrong motor set in parameter PA01
Troubleshooting steps:
- Power off. Disconnect the encoder cable at the motor side and drive side. Inspect both connectors for bent pins, corrosion, or pushed-back contacts.
- Measure cable continuity wire by wire. Any open circuit = replace the cable immediately.
- Check cable routing. The encoder cable must not run parallel to motor power cables. Maintain at least 20cm separation, or use a grounded steel divider in the cable tray.
- Verify parameter PA01 (motor code) matches your motor nameplate exactly. A mismatch selects the wrong encoder protocol.
- Connect a known-good motor with its encoder cable. If AL.16 clears, the original motor’s encoder has failed. If AL.16 persists, the drive’s encoder receiver circuit may be damaged.
Parts to check: Encoder cable (most common failure point), motor encoder, drive CN2 connector
AL.17 — Board Error
What it means: The drive’s internal CPU or circuit board has detected a fault. This is a hardware-level error.
Possible causes:
- CPU malfunction
- Internal power supply circuit failure
- PCB damage from surge, moisture, or contamination
- Drive overheating causing component failure
Troubleshooting steps:
- Power cycle the drive. If AL.17 returns immediately, the drive has a hardware fault.
- Check the drive environment: ambient temperature, ventilation, dust contamination.
- Inspect for physical damage: burnt smell, discolored PCB, bulging electrolytic capacitors.
- This error is not field-repairable. The drive must be replaced.
Parts to check: Drive unit (replacement required)
Overload and Current Error Codes
AL.24 — Motor Overload
What it means: The motor’s thermal protection model inside the drive has detected sustained operation above rated torque. The drive shuts down to prevent motor insulation damage.
Possible causes:
- Mechanical overload — jam, binding, excessive friction
- Repeated rapid acceleration cycles without sufficient cooling time
- Motor brake not releasing
- Motor rated current parameter wrong
- Motor fan failure (on force-cooled motors)
Troubleshooting steps:
- Disconnect the motor from the mechanical load. Run the motor in speed mode at low speed. If AL.24 does not occur, the problem is mechanical — check the load path.
- Check the brake: measure the brake release voltage at the motor terminals. If the brake is not receiving power, check the brake power supply and wiring.
- Verify parameter PA01 matches the motor nameplate. Wrong motor code = wrong thermal model.
- Monitor the torque meter in MR Configurator2 during operation. If torque consistently exceeds 80% of rated, the motor may be undersized for the application.
- For force-cooled motors, verify the cooling fan is running. A failed fan causes the motor to overheat even at moderate load.
Parts to check: Motor brake, mechanical load path, cooling fan
AL.25 — Drive Overload
What it means: The drive’s output current exceeded 150% of rated for longer than the allowable time limit.
Possible causes:
- Drive undersized for the motor or application
- Acceleration/deceleration times too aggressive
- Mechanical binding causing sustained high torque demand
- Regenerative energy not being properly dissipated
Troubleshooting steps:
- Check the drive’s rated current vs. the motor’s rated current. The drive rating must be equal to or greater than the motor.
- Increase acceleration time (PB06) and deceleration time (PB07) by 50% and retest.
- Run the motor unloaded. If AL.25 still occurs, the drive may have an internal current sensing fault.
- Monitor load ratio during normal operation. If it peaks above 100% frequently, consider upgrading to the next drive size.
Parts to check: Drive sizing, mechanical load, acceleration parameters
AL.30 — Regenerative Overvoltage
What it means: The DC bus voltage exceeded the safe limit during motor deceleration. The drive cannot absorb the regenerative energy fast enough.
Possible causes:
- No regenerative resistor installed
- Regenerative resistor failed (open circuit)
- Regenerative resistor value too high (insufficient braking capacity)
- Deceleration time too short for the load inertia
- Multiple drives sharing a power supply without coordinated braking
Troubleshooting steps:
- Check if a regenerative resistor is installed. For MR-J4 drives, check the resistor connection at P and C terminals.
- Measure the resistor value. Compare to the specification in the MR-J4 manual for your drive size. An open circuit = failed resistor.
- Increase deceleration time (PB07) by 50% and test.
- If the application has high-inertia loads (large rotating drums, heavy flywheels), you may need a larger resistor or a regenerative converter unit.
- Check parameter PC10 (regenerative resistor selection). It must match the installed resistor type.
Parts to check: Regenerative resistor, deceleration parameters, resistor parameter setting
AL.31 — Undervoltage
What it means: The DC bus voltage dropped below the minimum operating level during motor acceleration or operation.
Possible causes:
- Input power supply voltage too low or unstable
- Power supply capacity insufficient for the drive’s peak current demand
- Input cable too thin (voltage drop under load)
- Magnetic contactor chattering or worn contacts
- Input reactor too large (excessive voltage drop)
Troubleshooting steps:
- Measure the input voltage at the drive terminals (L1, L2, L3 for 3-phase; L1, L2 for single-phase) while the motor is accelerating. If voltage drops below 160V (200V class), the supply is inadequate.
- Check the input cable cross-section. For MR-J4-200W or larger, use at least 2mm² cable. Undersized cable causes voltage drop.
- Inspect the magnetic contactor on the input side. Pitted or worn contacts cause intermittent voltage loss.
- Verify the power supply can deliver the drive’s peak current demand (typically 300% of rated for 0.5 seconds during acceleration).
Parts to check: Input power cable gauge, magnetic contactor, power supply capacity
Position and Speed Error Codes
AL.32 — Overspeed
What it means: The motor speed exceeded 120% of the rated speed.
Possible causes:
- Speed command too high from the PLC or controller
- Parameter PB03 (speed limit) set incorrectly
- Motor runaway due to encoder feedback loss
- External force driving the motor beyond rated speed
Troubleshooting steps:
- Check the speed command signal from the PLC. Is it within the expected range?
- Verify parameter PB03 (internal speed limit) is set correctly for your motor.
- If the motor runs away when enabled, the encoder feedback is likely lost or reversed. Check encoder cable and PA01 setting.
- For vertical axes with gravity loads, check that the mechanical brake holds when power is off. A failed brake allows the load to drive the motor overspeed.
Parts to check: Speed command, encoder feedback, mechanical brake
AL.35 — Command Frequency Excessive
What it means: The pulse command frequency from the controller exceeds the drive’s maximum input frequency.
Possible causes:
- PLC pulse output frequency too high for the drive’s configured input mode
- Electronic gear ratio (PB08/PB09) set incorrectly
- Wiring error on the pulse input terminals
Troubleshooting steps:
- Calculate the required pulse frequency: motor speed (rpm) × encoder resolution × gear ratio. Verify this is within the MR-J4’s maximum input frequency (4 Mpps for differential input, 200 kpps for open collector).
- Check parameters PB08 and PB09 (electronic gear ratio). Adjust if the resolution is too high for the command frequency.
- Verify pulse input wiring matches the selected input mode (differential: P5R/P5L/SD; open collector: P5R/SD).
Parts to check: Pulse input wiring, gear ratio parameters, PLC pulse output
AL.37 — Excessive Deviation
What it means: The position error between the commanded position and the actual motor position exceeds the allowable deviation set in parameter PA10.
Possible causes:
- Load inertia too large for the servo loop tuning
- Servo gains too low (PB06, PB07, PB08, PB09)
- Mechanical jam or excessive friction
- Position command too aggressive (high speed + high acceleration)
- Encoder feedback degraded
Troubleshooting steps:
- Increase the allowable deviation in parameter PA10. This is a temporary workaround — find the root cause.
- Run auto-tuning (MR Configurator2 → Tuning → Auto-tuning). The MR-J4’s 2-degree-of-freedom auto-tuning handles most applications.
- If auto-tuning doesn’t resolve it, increase the position loop gain (PB07) in small increments (10% at a time) and test.
- Check the mechanical system for binding, contamination, or worn guides.
- Verify the load inertia ratio (PB06) is set correctly. If the actual inertia is much higher than the setting, the servo loop cannot compensate.
Parts to check: Servo gain parameters, mechanical system, inertia ratio setting
Encoder-Specific Error Codes
AL.41 — Overspeed at Power On
What it means: The motor was rotating when the drive was powered on. The drive detected shaft rotation during initialization.
Possible causes:
- External force spinning the motor (gravity on vertical axis, airflow on fan)
- Motor shaft was already in motion when drive was enabled
- Motor brake failed to hold
Troubleshooting steps:
- Ensure the motor is stationary before powering on or enabling the drive.
- On vertical axes, verify the brake engages when power is removed.
- Add a delay in the PLC program: wait 500ms after brake engagement before enabling the servo.
Parts to check: Mechanical brake, PLC enable sequence
AL.42 — Encoder Error
What it means: The encoder detected an internal fault — typically a signal integrity issue within the encoder itself.
Possible causes:
- Motor encoder hardware failure
- Encoder cable noise or signal degradation
- Encoder connector contamination (oil, coolant ingress)
- Motor bearing failure causing shaft vibration that damages the encoder
Troubleshooting steps:
- Replace the encoder cable with a known-good Mitsubishi genuine cable. Non-genuine cables often have incorrect impedance and shielding.
- Inspect the encoder connector at the motor for oil or coolant contamination. Clean with contact cleaner if needed.
- If a new cable doesn’t resolve it, the encoder itself has failed. The motor needs replacement or factory repair.
- Check the motor bearing by turning the shaft by hand. Rough or noisy rotation indicates bearing failure, which will damage the encoder if not addressed.
Parts to check: Encoder cable, motor encoder, motor bearing
AL.52 — Encoder Battery Error
What it means: The battery that maintains absolute encoder position data has failed or is depleted.
Possible causes:
- Battery voltage below 2.8V
- Battery not installed (on absolute encoder motors)
- Battery holder contact corrosion
Troubleshooting steps:
- Replace the battery (Mitsubishi MR-J3BAT or MR-J4BAT depending on your system). Replace it while the drive is powered on to preserve position data.
- If the battery was dead for an extended period, the absolute position is lost. You must re-home the axis.
- Check the battery holder contacts for corrosion. Clean if needed.
- If using an incremental encoder motor, you don’t need a battery. Check if PA01 is set correctly for your motor type.
Parts to check: Encoder battery, battery holder contacts
Complete MR-J4 Error Code Quick Reference
Error Code Description Severity Typical Fix Time AL.12 Memory error High 15 min–drive replacement AL.13 Clock error High Drive replacement AL.15 Storage error High Parameter reset or drive replacement AL.16 Encoder communication error High 15–60 min AL.17 Board error Critical Drive replacement AL.19 Serial communication error Medium 15–30 min AL.1A Motor combination error High 5 min (correct PA01) AL.1B ENCB malfunction High Drive replacement AL.1C Motor overheating Medium 15–60 min AL.20 Encoder Z-phase error High 15–30 min AL.24 Motor overload Medium 15–60 min AL.25 Drive overload Medium 15–60 min AL.30 Regenerative overvoltage Medium 15–30 min AL.31 Undervoltage Medium 15–30 min AL.32 Overspeed High 15–30 min AL.33 Overvoltage Medium 15–30 min AL.35 Command frequency excessive Low 15 min (parameter) AL.37 Excessive deviation Medium 15–60 min AL.41 Overspeed at power on Low 5 min (sequence fix) AL.42 Encoder error High 15–60 min AL.43 Encoder phase error High 15–30 min AL.45 Main circuit device overheat Medium 30–60 min AL.46 Motor overheating (estimated) Medium 15–60 min AL.47 Power supply phase failure Medium 15–30 min AL.50 Overload 1 (software) Medium 15–60 min AL.51 Overload 2 (hardware) Critical Drive replacement AL.52 Encoder battery error Low 5 min (replace battery) AL.71 SSCNET communication error High 15–30 min AL.72 SSCNET transmission error High 15–30 min AL.74 SSCNET transmission cycle error Medium 15 min (parameter) AL.7A SSCNET communication error 2 High 15–30 min AL.8A USB communication error Low Check cable AL.8E Watchdog error Critical Drive replacement AL.92 Battery voltage low warning Low Replace battery AL.96 Home position shift Low Re-home axis AL.99 Drive internal fault Critical Drive replacement MR-J4 vs MR-J3 Compatibility Notes
Many users ask if they can swap an MR-J4 drive for a failed MR-J3 unit. Here’s the direct answer:
Feature MR-J3 MR-J4 Encoder resolution 18-bit 22-bit Encoder protocol Different serial format Updated protocol Auto-tuning 1-DOF 2-DOF (improved) Parameter structure Different numbering Different numbering Motor compatibility J3 motors only J4 motors only SSCNET version SSCNET III SSCNET III/H Max input frequency 1 Mpps 4 Mpps MR-J3 and MR-J4 are not interchangeable. The encoder protocols differ, the parameter structures are different, and the connectors are not pin-compatible. If you have a failed MR-J3 drive, replace it with an MR-J3. If you’re upgrading an entire axis, replace both the motor and drive together with J4 components.
One exception: the MR-J4-DU model is specifically designed as a J3 replacement. It accepts J3 motor encoders and maps J3 parameters automatically. If you need to upgrade from J3 without replacing the motor, the MR-J4-DU is your only option.
For genuine Mitsubishi MR-J4 servo drives, motors, and cables, visit Wavetop Auto. We stock MR-J4, MR-J4-DU, and MR-J3 series components and can verify compatibility before you order.
Preventive Maintenance Schedule
Interval Action Prevents Monthly Check encoder cable connectors for looseness AL.16, AL.42 Monthly Verify cabinet temperature below 45°C AL.17, AL.45 Monthly Check motor brake operation on vertical axes AL.41, AL.32 Quarterly Tighten all terminal block screws AL.31, intermittent faults Quarterly Measure motor insulation resistance (megger test) Motor winding failure Quarterly Inspect encoder connector for oil/coolant ingress AL.42 Semi-annually Clean drive cooling fan and heat sink AL.45, overheating Semi-annually Check regenerative resistor value AL.30 Annually Replace encoder battery (absolute encoder systems) AL.52, AL.92 Annually Back up all parameters via MR Configurator2 AL.12 data loss Annually Full system ground integrity check Intermittent alarms When to Replace vs. Repair
Symptom Repair Replace AL.16 (encoder comm error) Replace encoder cable Motor encoder is dead AL.17 (board error) Not field-repairable Always replace the drive AL.30 (overvoltage) Replace regenerative resistor Drive braking circuit failed AL.42 (encoder error) with new cable — Motor encoder failed AL.45 (overheat) with clean fan Improve cabinet ventilation Drive thermal sensor degraded Intermittent AL.16 at high speed only Replace encoder cable with shielded type Drive encoder circuit degraded Multiple different alarms randomly Check grounding and power quality Drive reaching end of life AL.51 (hardware overload) — Drive output stage damaged, replace How to Order the Correct MR-J4 Replacement
Getting the right replacement the first time saves days of downtime. Here’s what you need before you order:
- Drive model number — Found on the drive nameplate. Example: MR-J4-200A. The suffix matters: A = analog/pulse, B = SSCNET, DU = J3 replacement.
- Motor model number — From the motor nameplate. Example: HG-SR202. This determines the encoder type and power rating.
- Voltage class — 200V or 400V. Check your input power supply.
- Control mode — Position, speed, or torque. This affects parameter setup on the replacement drive.
- Parameter backup — If possible, extract parameters from the failed drive before removal. If the drive won’t power on, refer to your machine builder’s documentation.
At Wavetop Auto, we verify every drive-motor combination before shipping. Send us the model numbers and we’ll confirm compatibility — no guessing, no returns.
FAQ
Q1: Can I clear an MR-J4 error without fixing the root cause?
No. The MR-J4 will not allow the alarm to be cleared while the fault condition persists. You can reset by pressing SET or cycling power, but the alarm will return immediately if the condition still exists.
Q2: My drive shows AL.16 only during high-speed operation. What should I check?
This is almost always an encoder cable issue. At high speed, the encoder signal frequency increases and marginal cables fail to transmit clean signals. Replace the cable with a genuine Mitsubishi shielded encoder cable. Route it at least 20cm from power cables.
Q3: How do I back up MR-J4 parameters before replacing a drive?
Connect a PC running MR Configurator2 via USB or RS-422. Go to Parameter → Read All, then save to file. After installing the replacement drive, use Parameter → Write All to restore. This takes 2 minutes and eliminates manual parameter entry errors.
Q4: Can I use a larger MR-J4 drive on a smaller motor?
Yes, within reason. A 1kW drive can run a 400W motor. You must set parameter PA01 to the correct motor code — the drive then automatically adjusts current limits to match the motor. Do not use a drive smaller than the motor rating.
Q5: The drive displays AL.99. What does this mean?
AL.99 indicates an internal drive fault that is not user-diagnosable. Power cycle the drive. If AL.99 returns, the drive must be replaced. Do not attempt to open or repair the drive internally.