What the A.C90 alarm means

Serial communication between the SERVOPACK and the encoder is impossible; the drive has no position feedback and refuses servo-on for that axis. This is one of the faults our repair team handles on Yaskawa DX100 / DX200 / YRC1000 systems — the cause is usually identifiable within the first on-site inspection.

Most common causes

  • Damaged encoder cable (broken wire, crushed by cable track)
  • Loose or oxidized encoder connectors at motor or SERVOPACK
  • Electrical noise on the encoder line
  • Faulty encoder or SERVOPACK communication circuit
Field note: the first item on this list is what we find most often on real machines. Check it before you go deeper.

Step-by-step field fix

  1. Reseat the encoder connectors at both ends
  2. Continuity-test the encoder cable end to end, especially where it flexes in the cable track
  3. Check encoder cable separation from motor power cables and verify shielding
  4. Swap-test with a known-good cable
  5. Replace encoder (motor) or SERVOPACK based on the swap result
Safety first: never bypass safety circuits or force contactors closed. If the alarm involves E-stop chains, brakes or drive power, and you are not a trained robot technician, contact a professional — wrong moves here can cost you a robot or an injury.

Still stuck? Get an engineer on the phone — right now

Tell us the alarm code, the robot model and what was happening when the alarm came up. Our engineers run these diagnostics every day, and remote diagnosis by phone or video call is free.

Get a Free Remote Diagnosis See Repair Services

Hotline: +86-20-39029800 · WhatsApp: +86-188-1886-2860

Related Encoder & Position alarms

A.810 — Encoder Backup Error — Position Lost A.830 — Absolute Encoder Battery Error
Other brands: we also keep field-fix libraries for Universal Robots · Kawasaki · Staubli and more — see the complete alarm code hub.

Frequently asked questions

QWhat does the Yaskawa A.C90 alarm mean?

Serial communication between the SERVOPACK and the encoder is impossible; the drive has no position feedback and refuses servo-on for that axis. In field practice it belongs to the encoder & position family of faults.

QCan I clear a Yaskawa A.C90 alarm myself?

Sometimes. Work through the step-by-step checks on this page — the first listed cause is what we find most often in the field. If the alarm returns after two attempts, or the fault involves power, brakes or safety circuits, stop and call a qualified robot service engineer.

QHow fast can a Henghuan engineer help?

On-site response in the Pearl River Delta within 30 minutes to 3 hours, across mainland China in 1-3 business days, and worldwide within 2-5 business days. Remote diagnosis by phone/WhatsApp/video is available immediately.

Written by the Henghuan Robotics repair team from field service experience. Alarm descriptions are our own summary for troubleshooting reference and may vary by controller and software version — when in doubt, check your Yaskawa documentation or contact us. Henghuan is an independent service provider, not affiliated with or endorsed by Yaskawa.