Mitsubishi MR-J4 AL.16 vs AL.20: Encoder Communication Faults Explained
Table of Contents
The distinction between AL.16 and AL.20 is the communication stage. AL.16 concerns initial communication. AL.20 concerns normal communication after initialization. “Normal communication” does not mean the motor must be turning—an AL.20 fault can occur while the axis is stationary. See Mitsubishi’s MR-J4 Troubleshooting Manual.
This guide focuses on conventional rotary-servo applications using MR-J4-A and MR-J4-B amplifiers. Linear encoders, multi-axis amplifiers and special operating modes have additional checks.
Read the Detailed Code Before Touching the Cable
A maintenance report that says “encoder alarm” leaves too much unanswered. Record the detailed code, affected axis, amplifier model and motor model before disconnecting anything.
The following examples are a partial list, not a complete alarm table.
| Detailed code | Meaning, paraphrased | Why it matters |
|---|---|---|
| 16.1 | Receive-data fault during initialization | Directs you to the initial communication checks |
| 16.3 | Another initial receive-data fault | Its checks also include configuration and control-power stability |
| 20.1 | Receive-data fault during normal communication | Includes cable and shield-termination checks |
The detailed-code remedies overlap. They can involve connections, configuration, environmental interference, the encoder or the amplifier. None of these three codes proves that a particular component has failed. Use the applicable model column in the official troubleshooting tables.
Before starting work, capture:
- The full alarm code and available alarm history.
- Whether the fault appeared at startup, at standstill or during a particular movement.
- Any recent motor, cable, amplifier or parameter changes.
- Whether other axes were affected at the same time.
Keep a parameter backup before making changes. A factory reset can remove useful evidence and introduce a second problem.
- Related Product
Need a Replacement Altivar Drive?
Browse new original Altivar drives for maintenance, troubleshooting, and replacement projects.
- New original parts
- Fast response
- Global shipping
Use the Fault Timing to Choose the First Check
The circumstances surrounding the alarm often make the investigation more manageable. Treat them as clues, not proof.
| Observed pattern | Useful next investigation |
|---|---|
| First appeared after maintenance | Compare the installed parts, connections and settings with the documented configuration |
| Returns near the same axis position | Inspect the corresponding moving-cable section under safe isolation |
| Appears around contactor or other equipment switching | Compare event timing and review cable routing and shielding |
| Several axes fail together | Look for a shared event before assuming several motors failed independently |
For example, an alarm that began immediately after a cable replacement deserves a close look at that replacement. Confirm its complete part number and intended application. A connector that fits mechanically is not enough to establish compatibility.
Conversely, a fault that returns at the same point in travel calls for a different investigation. Identify which cable section bends there, then inspect it with the machine safely isolated. Do not try to reproduce the alarm by pulling or bending a live cable.
Inspect the Connections With Power Safely Isolated
Electrical inspection belongs to qualified personnel following the machine’s isolation procedure. Secure any load that could move, isolate the relevant supplies, observe the specified discharge period and verify the equipment is safe before touching connections. Switching off the operator panel is not electrical isolation. Mitsubishi’s MR-J4 safety instructions explain the residual-voltage hazard.
Photograph both ends of the encoder connection before disturbing them. Then examine the connector seating, locking mechanism, contacts, cable entry and strain relief. Look for contamination, damaged contacts, crushing or a cable pulled tight against its connector.
Avoid changing several things at once. If a connector is reseated, a cable replaced and parameters altered in the same intervention, a successful restart will not tell you which change mattered.
A clean-looking cable is not a conclusive test result. If the visual inspection finds nothing, the next step is a suitable electrical or substitution test performed against the correct documentation—not an automatic motor order.
- PLC Solutions
Find the Right PLC for Your System
Explore new original PLC CPUs, I/O modules, power supplies, and communication units for maintenance and automation projects.
- New original parts
- Fast response
- Global shipping
Check Shielding Before Blaming the Encoder
Encoder communication depends on the cable installation as well as the connected equipment.
Mitsubishi’s EMC Installation Guidelines describe shielded encoder cabling and shield grounding at both ends. The MR-J4 safety instructions also call for separation between power and signal cables.
During an inspection, compare the actual installation with those requirements:
- Has the encoder cable been rerouted alongside power wiring?
- Was a shield termination disturbed during connector replacement?
- Has an extension or intermediate connection been added?
- Did the fault start after another drive or switching device was installed?
An alarm occurring when a contactor switches is worth investigating, but timing alone does not establish electrical noise as the cause. Record the pattern and check the installation before adding filters, ferrites or ground connections.
After a Replacement, Verify the Configuration
If the fault started after an amplifier or motor change, compare the full model numbers with the approved machine configuration. Include suffixes and the cable assembly reference.
Check the relevant communication settings in the manual for that amplifier family. Do not copy a parameter number or value from a different MR-J4 variant. Preserve the original settings and document each change.
Where hardware isolation is necessary, a qualified technician may use a confirmed-compatible, known-good component under a controlled test procedure. Compatibility must be established first. Borrowing a cable or motor from a neighboring axis simply because it looks similar can complicate the diagnosis.
A useful test record states what changed, what stayed unchanged and whether the original fault returned under the same conditions.
Resetting AL.16 and AL.20
For the AL.16 and AL.20 entries, Mitsubishi’s reset table permits power cycling, while ordinary alarm reset and CPU reset are marked unavailable. Correct the cause before restarting; repeated power cycling is not a repair.
Keep the run command disabled and follow the machine’s restart procedure. Once the axis is ready for a controlled test, verify more than an alarm-free display.
Check startup, the operating condition associated with the original fault and the relevant machine cycle. If work affected position references or mechanical connections, complete the required reference and positioning checks before releasing the machine.
Record the corrective action and test conditions. “Restarted successfully” is a weaker maintenance record than “repaired the damaged connection, then verified startup and the previously failing movement.”
- HMI Replacement
Upgrade or Replace Your HMI
Find compatible HMI panels for discontinued models, machine upgrades, and operator interface replacement.
- New original parts
- Fast response
- Global shipping
Power your projects with brand-new, original Omron, Mitsubishi, Schneider Servo – in stock, ready now!
Before Ordering Replacement Parts
If testing identifies a component that needs replacement, prepare its complete model number, the amplifier and motor nameplate details, cable reference, detailed alarm code and relevant photographs.
Those details make a parts inquiry useful. You can contact Kwoco with that information when checking a replacement requirement.
For alarms outside encoder communication, refer to the broader Mitsubishi MR-J4 Servo Alarm Codes guide.
Contact Us
Just fill out your name, email address, and a brief description of your inquiry in this form. We will contact you within 24 hours.
Product Category
Hot Sale Products
Omron CPU Unit CJ2M-CPU32
Omron PLC CJ1W-PA202 Supplier in China
Mitsubishi MR-JE Series 400w Servo Drive MR-JE-40A
Need a Fast Match?
Share your model or spec and our team will help you find the right fit-fast.
- Expert support
- Quick response
- Competitive price
- +86-755-81481609
- [email protected]
You May Also Find These Topics Interesting
10 Things You Need to Know About Omron CPU Type
10 Things You Need to Know About Omron CPU Type As an engineer with years of experience in the industrial automation field,
Capacitor Reforming: Bringing Your VFD Back to Life After Storage
This article explores the critical process of capacitor reforming for Variable Frequency Drives (VFDs) that have been in storage. If you’re in
PLC Guide: Concepts, Applications & Wiring Essentials
This document provides a comprehensive analysis of the Programmable Logic Controller (PLC), covering its core concepts, powerful capabilities, and fundamental wiring methodologies.