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Mitsubishi MR-J4 Servo on Press: AL.16 and AL.20 Encoder Alarm Fix

admin

Mitsubishi MR-J4 servo amplifiers are widely used on servo presses in Asia. AL.16 and AL.20 are the most common encoder-related alarms.

## AL.16: Encoder Error 1

This alarm indicates a communication error between the servo amplifier and the encoder. The MR-J4 uses a high-speed serial encoder (MELSERVO protocol) that is sensitive to cable quality and noise.

Causes specific to press applications:
- Cable damage at the slide flex point (most common)
- Electrical noise from the main drive inverter coupling into the encoder cable
- Loose connector at the encoder (vibration from press operation loosens connectors over time)
- Encoder cable too long without proper shielding

Diagnosis:
1. Check the encoder cable at the point where it enters the slide housing - flex the cable manually and watch for alarm triggering
2. Check connector MR-J4CN2: look for bent pins, corrosion, or loose locking mechanism
3. Measure cable shield continuity: should be under 1 ohm from amplifier end to encoder end
4. Check cable routing: minimum 100mm separation from power cables, cross at 90 degrees if they must intersect

Fix:
- Replace cable with Mitsubishi MR-EKCBL flexible cable (rated for 10 million flex cycles)
- Add ferrite core on encoder cable at amplifier end (Mitsubishi part: MR-BFV0.5)
- Re-route cable away from power cables

## AL.20: Encoder Error 2

AL.20 is a more serious encoder fault indicating the encoder itself has detected an internal error.

Causes:
- Encoder overheating (motor temperature above 80C)
- Encoder battery failure (for absolute encoders)
- Physical damage to encoder from press vibration or crash
- Encoder supply voltage out of range

Diagnosis:
1. Check motor temperature: MR-J4 reports motor temperature via parameter display. Above 80C triggers AL.20
2. Check encoder battery voltage: should be above 3.0V. Replace if below 3.2V
3. Check encoder supply voltage at CN2 connector: pin 1 (+5V) should be 4.75-5.25V

## After Encoder Alarm: Absolute Position Recovery

After clearing AL.16 or AL.20 on an absolute encoder system:
1. Perform home position return (parameter PA14 = 1)
2. Verify the home position matches the mechanical reference
3. Run 5 slow-speed strokes and verify position repeatability
4. Do NOT run production until home position is confirmed

anna_controls

AL.16 is usually cable or connector. AL.20 is usually the encoder itself. Quick test: swap the encoder cable with a known good one. If AL.16 clears, replace the cable. If it persists, the encoder needs replacement. For AL.20, there's no cable fix —the encoder's internal communication has failed.

raj_maintenance

We stock spare MR-J4 encoder cables for every press. They cost $180 each and they fail every 18-24 months on our high-SPM lines due to flex fatigue. We learned to replace them on schedule rather than waiting for failure. An encoder cable failure during production means an emergency stop, a homing procedure, and at least 30 minutes of lost production. The scheduled replacement takes 15 minutes during planned downtime.