Rotary Encoder Failure: How to Test and Replace

Quick diagnosis table

Symptom	Likely cause	Action
A/B counts drift under load	Coupling slip or loose encoder mounting	Re-torque coupling, remount encoder
Zero pulse missing once per rev	Index sensor fault in encoder	Replace encoder
Counts jump randomly	Shield grounding or cable interference	Re-route cable, check shield
Encoder very hot to touch	Over-voltage supply or wrong variant	Verify 5V/24V supply and variant
Drive shows encoder alarm only at speed	Signal amplitude loss at high RPM	Replace encoder or cable

Step-by-step procedure

1. Continuity test the encoder cable pin-by-pin with the connector wiggled — the most common 'encoder fault' is a broken wire.
2. Check the shield termination at both ends: the shield must land on the dedicated shield clamp, never on signal pins.
3. With the drive powered and axis disabled, rotate the shaft by hand and watch the position counter — counts should be smooth and repeatable.
4. Measure the supply voltage at the encoder connector under load — a 5V encoder fed 4.6V at speed is a cable or drive supply fault.
5. Compare A/B pulse timing with an oscilloscope if available; amplitude loss at speed points to the encoder or cable, not the drive.
6. Only if all above pass, replace the encoder — matched by motor MPN and variant, then recalibrate the axis.

Replacement parts & selection notes

Replacement encoders are selected by motor MPN, resolution (PPR), output type (TTL/HTL/push-pull) and interface (incremental/absolute). Send us the motor nameplate or the old encoder label and we confirm the exact match — free of charge.

Frequently asked questions

Related pages

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• Servo encoder replacement guide
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