Mitsubishi MDS alarm 25 means the absolute position data was lost — the backup battery voltage in the absolute position encoder dropped too low to hold it. The servo axis no longer knows where it is, and after the cause is fixed the zero point must be re-established (our zero point guide covers that procedure — alarm 25 is the drive-side sibling of the Z71 story). Mitsubishi's five-step investigation is full of practical intelligence — including exact numbers most people have never seen.
Step 1 — Is warning 9F present at the same time?
Warning 9F is the low-battery warning — its presence splits the diagnosis:
- 9F present → the battery is genuinely low → Step 2.
- 9F absent → the battery may be fine and the loss happened another way → Step 3.
Step 2 — Measure the battery (threshold: 3 V)
- Less than 3 V → replace the battery, then establish the zero point. Replace with the control powered ON where the configuration allows, so the encoder keeps its supply during the swap — see our battery replacement guide.
- 3 V or more → the battery is fine but its voltage isn't reaching the encoder → check the NC bus cable connection (the battery feed path).
Step 3 — Did alarm 18 occur at the last power-ON?
If alarm 18 (encoder initial communication error) fired last time the machine powered up, the encoder conversation was interrupted — the position loss is a consequence, not a battery problem: cycle the drive control power and establish the zero point.
Step 4 — Was the encoder or battery cable left disconnected?
The step with the numbers nobody knows: the encoder's internal backup only holds position for a limited time once its cable is off. Mitsubishi's guide: about 20 hours or more disconnected on a new unit — only about 10 hours once the unit is 5 years old — and the position is gone.
- Left disconnected that long (motor sent for rewind, cable off during repairs) → no fault at all: power the drive control back ON and establish the zero point.
- Cables were never disconnected → Step 5.
Step 5 — The cable, then the drive
- Check the encoder cable and battery cable with a tester — faulty connection → replace the cable (a broken battery conductor drains position memory silently every power-off).
- Connections normal → the drive unit is the remaining suspect — its battery/backup circuit; bench repair or exchange.
Diagnosis checklist (in order)
- Warning 9F present? → measure the battery: <3 V → replace + zero point; ≥3 V → NC bus cable.
- Alarm 18 at last power-on? → power cycle + zero point.
- Cables off for >20 h (new) / >10 h (5 years old)? → no fault: zero point.
- Meter the encoder/battery cables → replace if faulty.
- All normal → drive unit backup circuit: bench repair or exchange.
- ALWAYS finish by establishing the zero point — and verify soft limits before production.
Related guides
- Setting the Mitsubishi zero point (absolute position initialization)
- Mitsubishi CNC battery replacement
- Mitsubishi MDS alarm 18 (encoder communication)