Alarm guide · Mitsubishi · Updated October 2026
Mitsubishi AL.51 alarm: overload 2
Mitsubishi AL.51 is overload 2: a very high current flowed for several seconds, often from a machine collision, a jam, a held brake or a wiring or encoder fault.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
What the alarm means
Overload 2 is the short, high-load alarm. Overload 1 (AL.50) is the longer, lower-load alarm. Alarm numbers and wording can differ between series, so confirm with the manual for your unit.
Common causes
- Machine collision or mechanical jam
- Brake not released
- Motor wires in the wrong phase order
- Encoder fault causing wrong commutation
- Gains or acceleration far too aggressive
Safe first checks
General guidance only. Follow the drive manual and your site's electrical safety rules. Power off, and let the DC bus discharge, before touching connectors.
- Note the exact alarm code and when it appears: at power-up, at start, or under load.
- Power off and turn the shaft by hand if safe: it should turn smoothly.
- Check the motor cable, encoder cable and connectors for damage and loose contacts.
- Check what changed recently: load, speed, settings, cables or a replaced part.
When to send it for repair
If the mechanism turns freely and the wiring is correct, send the motor: a winding, bearing or encoder fault is likely.
We test every motor on our Universal Servo Motor Repair Equipment, which reads and aligns the feedback and runs the motor under load without the original drive.
See Mitsubishi servo motor repair, Mitsubishi encoder repair and Mitsubishi servo drive repair.
Questions
Is AL.51 worse than AL.50?
It indicates a sharper overload. The checks are similar: mechanism, brake, wiring, motor.
How long does repair take?
Emergency repair is done the same day where the fault and parts allow, urgent in 3 working days and standard in 7 working days, counted from when the unit reaches our bench and the quote is approved. Every unit returns with a printed test report.
Can I get an exchange unit?
If we hold a matching unit, Servo Swap ships a tested exchange unit while yours is repaired. Mark "line down" on the request form and add the part number.
Related: AL.25 · AL.50 · AL.16 and AL.20
Brand names identify the units we repair. Star Automations is an independent multi-brand repair company and is not an OEM service centre.