Alarm guide · Yaskawa · Updated October 2026
Yaskawa A.C90 alarm: Encoder Communications Error
Yaskawa A.C90 means the SERVOPACK cannot communicate with the motor encoder. The usual causes are a loose or damaged encoder connector or cable, water or corrosion in the connector, or electrical noise. If a known-good cable does not clear it, the encoder or the motor needs repair.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
What the alarm means
Yaskawa A.C90 means the SERVOPACK cannot communicate with the motor encoder. The usual causes are a loose or damaged encoder connector or cable, water or corrosion in the connector, or electrical noise. If a known-good cable does not clear it, the encoder or the motor needs repair.
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.
- Power off and wait for the charge lamp to go out.
- Reseat the encoder connector at the motor and at the SERVOPACK.
- Inspect the encoder cable along its length for crushing, cuts, oil or water.
- Check that the cable shield is earthed and the cable is routed away from power cables.
- Try a known-good encoder cable if you have one.
When to send it for repair
Send the motor when the alarm stays with a known-good cable, or follows the motor to another axis. We test the encoder on the bench, repair or replace it, and align it to the motor before the run test.
See Yaskawa servo motor repair, encoder repair and servo drive repair.
Questions
Can I clear the A.C90 alarm and keep running?
A reset may clear it for a while, but a feedback alarm usually comes back until the cable, connector or encoder is fixed. Running with an intermittent feedback fault can stop the axis without warning, so find the cause first.
Is it the motor, the cable or the drive?
Swap with a known-good cable first, then swap the motor to another axis if you can. If the alarm follows the motor, send the motor. If it stays on the axis with a good motor and cable, send the drive. We test both when you are not sure.
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.
Brand names identify the units we repair. Star Automations is an independent multi-brand repair company and is not an OEM service centre.