The Mitsubishi MR-J family (MR-J2S, MR-J3, MR-J4, and the newer MR-J5) covers most of the servo amplifiers you'll find on CNC machining centers, semiconductor handlers, packaging lines, and robotic cells. When the absolute position detection system is enabled, the servo motor remembers its multi-turn count across power cycles via a backup battery riding on the encoder line. That subsystem drives the most frequent class of MR-J alarms: encoder communication faults and absolute-position / battery warnings (AL.16, AL.20, AL.1E, AL.25, AL.92, AL.9F, AL.E3, AL.E5, AL.EA).
If you are building a replacement stock or troubleshooting an axis that will not clear AL.25 after a battery swap, please check our Mitsubishi servo and encoder collection for tested units. A widely deployed J4 platform drive such as the Mitsubishi MR-J4-20A runs $357.49 with confirmed USD pricing.
The MR-J amplifier reads a 17-bit (MR-J4 standard) or 22-bit (MR-J5 / HG-SR current generation) absolute encoder. While the drive is powered, the encoder's multi-turn SRAM is sustained by the amplifier's internal 5 V supply. When the drive is powered off, a 6 V lithium battery pack — MR-BAT6V1SET for MR-J4 single-axis, MR-BAT6V1SET-A for J4W multi-axis or J5-G, A6BAT (3.6 V) for legacy MR-J2S — preserves the multi-turn count and home position. The motor used most often is the Mitsubishi HC-KFS43 17-bit absolute unit, listed at $114.14.
On power-up the amplifier polls the encoder on CN2, checks the battery on CN4, and reads absolute position through the ABS handshake (ABSM / ABSR / ABST) on CN1. When AL.25, AL.92, AL.9F, or AL.E3 fires, always re-execute home-position return before resuming production — skipping it can cause an unintended move.
Fault manifestation: Seven-segment display shows AL.16 (encoder error 1 at startup), AL.20 (encoder error 2 after a working link), or AL.1E (serial encoder CRC error during a cycle). The amplifier typically refuses servo-on until the alarm is cleared. MR-J3 shares the same numbering.
Root causes: Encoder cable disconnected at the motor or amplifier end with bent or oxidized pins; flex-track cable crushed, kinked, or saturated with coolant causing intermittent opens on +5 V, GND, or data pairs; cable shielding broken exposing the serial pair to VFD or welding-inverter noise; failed encoder inside the motor (most common after 30,000 hours on HC-KFS / HC-SFS units); damaged CN2 connector inside the amplifier.
Repair steps:
Fault manifestation: Seven-segment shows AL.25 on every cold start. The drive comes up, refuses servo-on, and the host reports a position-loss fault (Mazak 212 / 302 on the same axis). Multi-axis machines often log two paired alarms when the battery banks deplete together.
Root causes: MR-BAT6V1SET voltage dropped below 4 V (warning threshold is AL.92) and the drive was powered off before replacement, emptying SRAM; battery unplugged while the drive was unpowered; short in the battery harness or loose CN4 connector; first power-up after the absolute detection system was enabled (Pr.PA03 = 1) — normal until homing; amplifier was swapped but encoder never re-registered, leaving SRAM contents incompatible with the new drive.
Repair steps:
Fault manifestation: AL.92 flashes or stays on (battery cable disconnected or degraded); AL.9F stays on once battery voltage crosses the low threshold. Position is still retained — one power-down away from AL.25.
Root causes: Battery at end of life (most common at the 3–5 year mark); CN4 not fully seated after maintenance; broken harness between MR-BT6VCASE and CN4; ambient temperature outside the rated 0–55 °C accelerating self-discharge.
Repair steps:
Fault manifestation: AL.E3 (absolute position counter warning) appears after a high-travel or vibration event; AL.E5 (ABS time-out) appears only while the controller is reading absolute data in transfer mode; AL.EA (ABS servo-on warning) appears within 1 s of starting ABS data transfer if the host has not yet issued servo-on.
Root causes: AL.E3 stems from the multi-turn counter exceeding its recommended travel window between homing events. AL.E5 means the controller missed the ABSM / ABSR handshake on a 5 s timeout — check ladder logic, scan time, and the ABS sequence. AL.EA is usually a PLC sequence error: the SON signal is off when ABS transfer starts.
Repair steps:
| Interval | Action |
|---|---|
| Every 6 months | Wipe encoder connectors, verify CN4 battery voltage (>5.5 V) |
| Every 12 months | Inspect flex-track encoder cables; replace any with jacket damage or conductor drift |
| Every 3 years | Replace MR-BAT6V1SET batteries on all J4 axes regardless of warning status |
| Every 5 years | Re-baseline home position; log before/after to detect mechanical creep |
| After every E-stop trip or lightning strike | Inspect the absolute battery subsystem before restart |
Pairing J4 amplifiers with current-generation 22-bit motors such as the Mitsubishi HG-SR202 ($331.26) gives you a 4,194,304 ppr feedback path and tighter multi-turn tolerances, which reduces AL.E3 frequency on high-resolution axes.
Q: Can I replace the absolute battery with the drive powered off?
A: You can, but AL.25 will fire on the next power-up and you'll need to re-execute home-position return. The MR-J4 manual and the Mazak field service note both recommend keeping control power ON during a hot-swap to preserve the multi-turn count.
Q: AL.25 cleared after homing but came back within a week. What next?
A: Suspect parasitic drain. Measure current between CN4 pin 1 and ground with the drive powered off — anything above ~30 µA indicates a leak. Inspect the junction cable (MR-BAT6V1BJ) on multi-axis drives.
Q: Will an MR-J4 drive work with an HC-KFS series motor (J2S-era)?
A: Yes, in J3 compatibility mode. Use a J3-spec encoder cable and expect slightly lower loop bandwidth than a native HG-KR / HG-SR pairing.
Q: How long does MR-BAT6V1SET last in a typical factory?
A: Mitsubishi rates it at about 5 years of shelf life and roughly 3 years once installed. Replacements are cheap insurance against an unexpected AL.25 event.
