Servo Encoder Replacement: Complete Tutorial

Quick diagnosis table

Symptom	Likely cause	Action
Position drift while running	Encoder glass scale contamination or coupling slip	Clean or replace encoder; check coupling
Alarm A.870 / AL.37 (encoder fault)	Encoder signal loss — cable or encoder	Test cable first, then encoder
Motor runs away at speed	Encoder A/B channel fault or loose mounting	Check connector, remount encoder
Encoder counts only in one direction	One channel dead — usually cable or encoder	Replace encoder or cable
Battery alarm after power loss	Absolute encoder backup battery dead	Replace battery, then re-home axis

Step-by-step procedure

Lock out and tag out all power, and wait for the DC bus to discharge (5+ minutes on drives).
Mark the encoder shaft position and the motor endbell orientation before removing anything — this preserves commutation alignment on absolute motors.
Disconnect the encoder connector at the drive, not at the motor, and test continuity pin-by-pin first. Most 'encoder faults' are cable faults.
If the encoder is confirmed faulty: remove the encoder cover, loosen the shaft coupling screw, and withdraw the encoder straight out — do not pry on the glass scale.
Fit the new encoder with the marks aligned, torque the coupling screw to spec, and reconnect.
Power up and run the drive's encoder calibration / commutation offset routine (e.g. p0432/p0437 on Siemens, AL.24 sequence on Yaskawa) before enabling the axis.
Jog the axis slowly and verify feedback sign and counts against the expected direction and travel.

Replacement parts & selection notes

Replacement encoders are matched by motor MPN, not drive model — the encoder variant differs by motor frame and option (battery-backed absolute, resolver, incremental 2048/4096 PPR). Send us the motor nameplate and we confirm the exact encoder, including coupling and screw kits.

Frequently asked questions

Can I replace just the encoder, not the whole motor?

In most cases yes — the encoder is a separate assembly. Send the motor MPN and we confirm the encoder variant and supply it with mounting hardware.

Why does the axis need recalibration after encoder replacement?

The commutation offset (absolute position reference) is stored relative to the encoder zero. A new encoder means a new zero — the calibration routine re-learns it.

The new encoder alarms immediately — did I get a wrong part?

Check the connector seating and cable first (80% of cases), then confirm the encoder variant against the motor serial. We verify compatibility free of charge before shipping.