When a Mitsubishi MELSERVO servo amplifier flashes an AL.xx fault code, production stops — and every minute counts. This guide covers the complete alarm list for the MR-J family, including MR-J2S fault codes, MR-J3 alarm codes, and MR-J4 alarm codes, explaining what each code means, what causes it, and how to diagnose the root problem fast. Whether you are a controls engineer troubleshooting a live fault or a maintenance technician trying to decide between repair and replacement, this reference is built for you.
The Mitsubishi MELSERVO MR-J Family at a Glance
Mitsubishi Electric's MELSERVO MR-J line encompasses several generations of AC servo amplifiers, each backward-compatible in general concept but meaningfully different in architecture and network connectivity.
MR-J2 / MR-J2S is the legacy generation still running in thousands of North American facilities. It uses an analog/pulse command interface and a serial encoder connection, with no high-speed fiber-optic network. These units are no longer in production, but field populations remain large, making repair and quality spare parts critical to keeping older machines alive.
MR-J3 introduced SSCNET III, Mitsubishi's high-speed fiber-optic servo network that dramatically reduces wiring and improves synchronization. MR-J3 remains common in packaging and machine-tool applications commissioned through roughly 2016.
MR-J4 is the current generation. It supports SSCNET III/H (the faster, second revision of the fiber network) as well as several application-specific variants: the -A suffix denotes a general-purpose analog/pulse-command amplifier, and the -TM suffix with EtherNet/IP adds Ethernet-based network connectivity for integration with non-Mitsubishi controllers. MR-J4 also includes improved safety functions and expanded tuning capability.
All three generations display faults in the AL.xx format on a small seven-segment LED on the amplifier face. While many alarm numbers are shared, their exact behavior, sub-codes, and resettability differ by generation. The industries most reliant on MR-J amplifiers include packaging machinery, CNC machine tools, semiconductor handling, robotics, and general OEM automation throughout the United States.
How MELSERVO Alarms and Warnings Work
The MR-J amplifier uses two distinct fault classifications: alarms and warnings.
An alarm (AL.xx, displayed without a decimal prefix on some units) is a protective shutdown condition. When an alarm triggers, the amplifier immediately disables the output, the servo motor coasts or dynamic brakes to a stop depending on the fault type, and the drive latches in a fault state. The alarm code remains on the display until it is cleared. Most alarms require the operator to remove the cause and then perform a reset — either by cycling control power or by asserting the RES (reset) input signal. Some severe alarms, particularly those involving internal hardware damage, will not clear with a software reset alone; the underlying hardware condition must be corrected first.
A warning is a pre-fault or advisory condition. The amplifier continues to run, but the display alternates between the current status and the warning code, and a warning output bit is asserted to the controller. Warnings are your early-warning system — they indicate a condition approaching a fault threshold. Common examples include AL.E6 (emergency stop / forced stop warning on MR-J4) and battery-low warnings on absolute encoder systems.
The alarm history — typically the last several faults — is stored in non-volatile memory and can be read through the amplifier's parameter unit, MR Configurator2 software, or the controller network. This history is invaluable for diagnosing intermittent faults.
Always confirm alarm codes against the specific instruction manual for your amplifier generation. Alarm numbering and behavior vary between MR-J2S, MR-J3, and MR-J4, and using the wrong manual can lead to misdiagnosis.
Common Mitsubishi MELSERVO MR-J Alarm Codes
The table below covers the most widely encountered standard alarms across the MR-J family, organized by fault category. Use this as a rapid field reference.
| Alarm | Name | Likely Cause | First Check |
|---|---|---|---|
| AL.10 | Undervoltage | Low or missing main circuit supply; power supply sag during acceleration | Measure L1/L2/L3 voltage at amplifier terminals; check fuses, contactors, and supply capacity |
| AL.30 | Regenerative Error / Regenerative Overload | Excessive regenerative energy; external regen resistor open, undersized, or disconnected | Verify regen resistor wiring and resistance; review deceleration profile and duty cycle |
| AL.32 | Overcurrent | Output stage (IGBT) failure; motor winding short; damaged motor cable; phase-to-phase short | Disconnect motor and check insulation; inspect cable and connector for shorts; if fault persists with motor disconnected, amplifier output stage is suspect |
| AL.33 | Overvoltage | Bus voltage exceeded during deceleration; regen resistor unable to dissipate energy; faulty bus capacitors | Check regen resistor and wiring; lengthen decel ramp; measure bus capacitance if fault is not load-related |
| AL.16 | Encoder Error (Malfunction) | Encoder cable damaged or shielding compromised; connector seated improperly; encoder electronics failed | Inspect encoder connector and cable end-to-end; swap with known-good cable; if fault remains, encoder or amplifier feedback circuit may be failed |
| AL.20 | Encoder Normal Communication Error | Serial communication loss between amplifier and encoder; poor connector contact; EMI on feedback cable | Reseat encoder connector; verify cable routing away from power conductors; check shield grounding |
| AL.24 | Main Circuit Error | Internal power supply fault inside the amplifier; pre-charge circuit failure; bus fault | Cycle control power; if fault persists immediately at power-up, the amplifier's internal power board requires inspection |
| AL.25 | Absolute Position Erased | Encoder backup battery low or dead; battery cable disconnected; first power-up after battery replacement | Replace encoder battery; perform absolute position reference (home) procedure per machine OEM documentation |
| AL.37 | Parameter Error | Parameter data corrupted (power loss during write, EEPROM wear); incorrect parameter restored from backup | Reload parameters from backup or re-enter from machine documentation; check for EEPROM fault sub-code |
| AL.45 | Main Circuit Overheat | Amplifier internal temperature exceeded limit; blocked ventilation; ambient temperature too high; cooling fan failed | Check enclosure temperature and airflow; inspect and replace cooling fan if not spinning; verify amplifier is not undersized for duty cycle |
| AL.46 | Servo Motor Overheat | Motor thermistor tripped; motor overloaded or ventilation blocked; thermistor wiring open | Allow motor to cool; verify thermistor wiring continuity; review load duty cycle and motor sizing |
| AL.50 | Overload 1 | Continuous overload exceeding thermal model; mechanical binding; incorrect motor/amplifier pairing | Check mechanical load for binding or increased friction; verify motor and amplifier capacity match application; review tuning gains |
| AL.51 | Overload 2 | Instantaneous peak overload beyond rated; high-inertia load with aggressive acceleration; tuning instability | Review acceleration ramps; check inertia ratio; inspect for mechanical shock or jam condition |
| AL.52 | Error Excessive (Following Error) | Commanded position not reached within tolerance; motor stall; encoder feedback lost; gain too low | Check for mechanical binding; verify encoder signal quality; review following-error tolerance parameter and servo gains |
| AL.8A | Serial Communication Timeout | Communication loss between amplifier and host controller (SSCNET III/III/H fiber); broken or dirty fiber connector; node failure upstream | Inspect fiber optic connectors for contamination or damage; verify all network nodes are powered; check for upstream axis fault breaking the chain |
| AL.E6 | Emergency Stop / Forced Stop Warning | EMG (emergency stop) input deasserted; safety circuit open; controller-commanded forced stop | Verify EMG signal wiring and safety circuit continuity; confirm controller is not commanding a forced stop state; note this is a warning on MR-J4, not a hard alarm |
Generation note: AL.8A (serial communication timeout) and AL.E6 (forced stop warning) are specific to MR-J3 and MR-J4 SSCNET network amplifiers and do not exist on MR-J2S. Conversely, some MR-J2S alarm sub-codes for AL.16 and AL.20 differ from MR-J3/J4 behavior. AL.30 behavior also varies — on MR-J4, the regenerative overload threshold and detection logic are more granular. Always cross-reference the generation-specific instruction manual before drawing a diagnostic conclusion.
Diagnosing the Most Common MELSERVO Failures
Most field calls trace back to five alarm families. Here is how to work through each one systematically, separating application issues from hardware failures.
AL.50 / AL.51 — Overload: Start by distinguishing configuration causes from mechanical or electrical ones. If the alarm appears only after a recent program change or increased production rate, the motor or amplifier may be undersized for the new duty cycle — a commissioning problem, not a hardware fault. If the alarm appears with no process change, look for increased mechanical resistance: worn bearings, contaminated ball screw, misalignment, or a foreign-object jam. On MR-J2S units, encoder feedback degradation can cause the amplifier to fight a phantom position error, mimicking overload. When the overload appears at rest or at minimal load, the amplifier's internal current sensing or thermal model circuitry may be faulty — that is a hardware repair scenario. For more context, see our guide on servo drive repair symptoms and fault codes.
AL.33 / AL.30 — Overvoltage and Regenerative Overload: These almost always share a root cause. During deceleration, the motor acts as a generator. If the regenerative energy cannot be absorbed — because the external regen resistor is open, undersized, or its wiring is damaged — the DC bus voltage rises until the amplifier trips. First verify the regen resistor resistance and wiring. If regen hardware is intact, lengthen the decel ramp or reduce cycle rate. If AL.33 fires at light load with a normal decel profile, suspect degraded DC bus capacitors — a well-known wear-out mechanism in MR-J2S amplifiers — which is a textbook component-level repair.
AL.32 — Overcurrent: Disconnect the motor and power the amplifier. If AL.32 clears, the fault is external: check motor winding insulation resistance and inspect the motor cable for chafed insulation or connector pin damage. If AL.32 persists with the motor disconnected, the amplifier's IGBT output stage has failed — this requires component-level repair of the power board and is not field-fixable.
AL.16 / AL.20 — Encoder / Feedback Errors: Begin with the cable. Encoder cables routed near servo power cables without adequate shielding or grounding are the most common cause on all MR-J generations. Reseat both ends of the encoder connector, verify shield termination, and swap with a known-good cable if available. If the fault persists through a new cable, the encoder itself (on the motor) or the encoder interface circuitry inside the amplifier has failed — both require a repair or replacement decision.
Repair vs. Replace Your MELSERVO Amplifier
When an MR-J amplifier faults, you have two legitimate paths: send it to Flexa for component-level repair, or swap it with an in-stock replacement. Here is how to make the right call.
Repair makes the most sense when:
- The fault is a repeatable hardware failure (failed IGBT, degraded bus caps, failed encoder interface) and the unit is otherwise in good condition.
- The unit is an MR-J2S — these are out of production, and matching the exact model and parameter set is easier with a repaired unit than sourcing a used replacement of unknown history.
- You want to preserve a known-good parameter configuration without re-commissioning.
- You have time for a 24–48 hour diagnostic turnaround and a written quote before committing.
Our Mitsubishi servo drive repair service covers component-level work on all MR-J generations with a 2-year repair warranty on completed repairs.
An in-stock replacement is the faster path when:
- Your line is down and you need same-day ship.
- You have no spare and cannot afford repair turnaround time.
- The damaged unit has sustained severe physical damage.
Flexa stocks Mitsubishi MR-J amplifiers, including the Mitsubishi MR-J2S-10B servo amplifier for legacy MR-J2S applications, the Mitsubishi MR-J4-200TM-EIP servo amplifier for EtherNet/IP network applications, and the Mitsubishi MR-J4-350A4-RJ servo amplifier for higher-capacity MR-J4 installations. Browse the full in-stock servo drives collection for current availability.
One important note: a replacement amplifier must match the original unit's model number, capacity, and network interface type exactly. Do not substitute an MR-J4-A (analog/pulse) for an MR-J4 SSCNET unit without engineering review — the network interface is fundamentally different. Request a free MELSERVO repair quote or a replacement recommendation from our team before ordering if you are uncertain.
Our Mitsubishi MELSERVO Repair Process
Flexa's repair process is designed to give plant engineers and purchasing managers full visibility and zero surprises.
- Free diagnostic within 24–48 hours. Ship your unit to our Lewisville, TX facility. We perform a full incoming inspection and fault analysis at no charge.
- Written quote before any work begins. You approve the repair scope before we proceed — no hidden charges.
- Component-level repair. We work at the board level: IGBT power modules, DC bus capacitors, gate driver circuits, encoder interface hardware, control board components, and internal power supply sections. We do not simply swap sub-assemblies.
- Full load testing. Every repaired MELSERVO amplifier is tested under load on a servo motor before it ships back to you. We verify proper operation across the operating speed range and confirm alarm-free operation.
- 2-year warranty on all completed repairs — one of the strongest coverage terms in the industry for servo amplifier repair.
- Nationwide mail-in service. We serve customers across the United States. Rush turnaround is available — ask when you contact us.
If you work with other servo platforms, we also provide Allen-Bradley Kinetix 300 servo drive repair and a broad range of additional servo and motion control repair services.
Frequently Asked Questions
What does an AL. alarm mean on a Mitsubishi MELSERVO amplifier?
An AL. code is a fault alarm displayed on the seven-segment LED of an MR-J series servo amplifier. When triggered, the amplifier disables its output, stops the motor, and latches the fault code until the condition is corrected and the drive is reset. The code identifies the fault category — voltage, current, thermal, feedback, communication, or parameter — so technicians can direct their diagnosis efficiently.
How do I reset an MR-J alarm, and why does it come back?
Most MR-J alarms reset by cycling control power or asserting the RES input signal after the fault condition is resolved. If an alarm returns immediately or after a short interval, the underlying hardware or application condition has not been corrected. Resetting without fixing the root cause — mechanical binding, failed component, wiring fault — will not result in a lasting fix and may cause further damage to the amplifier or motor.
Do the same alarm codes apply to MR-J2S, MR-J3, and MR-J4?
Many alarm numbers are shared across MR-J generations — AL.32 (overcurrent), AL.50 (overload 1), and AL.16 (encoder error) exist on all three — but their sub-codes, detection thresholds, and reset behavior can differ. Additionally, some alarms are generation-specific: AL.8A (SSCNET communication timeout) and AL.E6 (forced stop warning) apply to MR-J3 and MR-J4 only. Always confirm codes against the instruction manual for your specific amplifier generation.
Can you still repair an obsolete MR-J2S amplifier?
Yes. Flexa repairs MR-J2S amplifiers regularly. Although the MR-J2S is out of production, replacement components for its power stage, bus capacitors, and encoder interface are still sourced through our supply chain. Repairing a known-good MR-J2S unit is often preferable to sourcing a used replacement of uncertain history, particularly when the original parameter configuration needs to be preserved without a full recommission.
What warranty comes with a MELSERVO repair?
Flexa provides a 2-year warranty on all completed MELSERVO amplifier repairs. This covers the repaired components and workmanship against defects under normal operating conditions. The warranty applies to MR-J2S, MR-J3, and MR-J4 repairs alike. Specific warranty terms and conditions are detailed on our warranty page, which you can review before approving any repair quote.
Decode Your MELSERVO Alarm — Get Help Today
A flashing AL.xx code does not have to mean extended downtime. Flexa Systems is a US-based industrial automation repair and parts company in Lewisville, TX, specializing in Mitsubishi MELSERVO amplifiers across all MR-J generations. We offer a free diagnostic on every unit received, with a written quote before any work begins. Our repairs carry an industry-leading warranty, and we maintain in-stock MR-J amplifiers for facilities that need a fast replacement rather than a repair cycle.
Call us at (254) 254-0005, request a free MELSERVO repair quote online, or visit our Mitsubishi servo drive repair service page to learn more about what we cover. Whether your amplifier needs a component-level fix or you need a verified in-stock spare shipped today, Flexa has both paths ready.