Fanuc Encoder: How It Works with Servo Motors, Common Alarms, Fault Diagnosis & Repair Guide
A Fanuc encoder (pulse coder) is the “eyes and memory” of a Fanuc servo motor. Without a working encoder, the CNC system cannot know the motor’s exact position or speed, leading to alarms, vibration, positioning errors, or complete machine stop. In this article, we explain how Fanuc encoders connect to motors, how to identify encoder faults, common alarm codes, and step-by-step repair & maintenance.
A Fanuc encoder is mounted directly on the rear shaft of the servo motor. It rotates together with the motor, converting mechanical rotation into electrical pulse signals.
Key functions:
- Position feedback: Tells the CNC where the motor is (0.001 mm precision).
- Speed feedback: Reports real-time RPM to the servo amplifier.
- Magnetic pole detection: Ensures correct motor phase control.
- Absolute position memory: Retains position after power off (absolute encoders).
How they work together:
Motor → Encoder (reads rotation) → Feedback cable → Servo amplifier → CNC system → Adjust motor movement.
This is a
closed-loop control system; if the encoder fails, the loop breaks, and the motor cannot run normally.
You can refer to our
Fanuc motor and
Encoder.
2. How to Tell If It’s an Encoder Fault (5 Clear Signs)
Most encoder issues show these symptoms. If you see any of the following, the encoder is highly suspect:
- Servo alarms: SV0301, SV0368, SV0369, SV0360, SRVO-067/SRVO-068
- Motor vibration or jerking during movement
- Positioning errors: The machine does not reach the correct coordinate
- Random alarms: Alarms appear and disappear, often when the cable moves
- No position data: CNC shows “0” or unstable position values
Quick test to confirm encoder fault:
- Swap test: Exchange the encoder with a known good one of the same model. If the alarm moves to the new axis, the original encoder is bad.
- Cable check: Inspect the feedback cable for cuts, oil contamination, or loose connectors (70% of faults are cable-related).
- Battery check (absolute encoders): Voltage below 2.7V causes position loss and alarms.
3. Most Common Fanuc Encoder Alarms & Meanings
Below are the most frequent encoder-related alarms on Fanuc systems and robots:
- SV0301 – APC communication error (feedback cable/encoder)
- SV0368 – Serial data error (encoder internal fault or cable noise)
- SV0369 – Serial data transmission error (shielding/interference)
- SV0360 – Pulse coder code check error (encoder damage)
- SRVO-067 – Encoder cable break or poor contact
- SRVO-068 – Encoder signal unstable (battery/connection)
Quick fixes for common alarms:
- Check cable: Replace damaged feedback cable; ensure good shielding.
- Check connectors: Clean and tighten motor-side and amplifier-side plugs.
- Replace battery: For absolute encoders, replace battery if <2.7V.
- Replace encoder: If swap test confirms encoder failure.
4. Fanuc Encoder Repair: Step-by-Step Guide
If your encoder is faulty, follow these steps for safe repair or replacement:
Step 1 – Power off & lockout
Disconnect all power to the servo motor and amplifier to avoid electric shock and damage.
Step 2 – Remove the encoder cover
Loosen the screws on the motor rear cover; be careful not to drop small parts.
Step 3 – Disconnect the feedback cable
Unplug the encoder cable from the encoder and the servo amplifier.
Step 4 – Remove the old encoder
Use a special puller tool to separate the encoder from the motor shaft. Do not pry—this will damage the motor shaft or encoder housing.
Step 5 – Clean & inspect
- Clean the motor shaft with a lint-free cloth.
- Check the encoder shaft and coupling for wear or damage.
Step 6 – Install the new encoder
- Align the encoder keyway with the motor shaft key.
- Push the encoder on firmly (do not hammer).
- Tighten mounting screws evenly.
Step 7 – Reconnect cable & power on
- Reconnect the feedback cable; ensure connectors are fully seated.
- Power on the system; reset alarms.
- Run a test move to verify normal operation.
5. Maintenance Tips to Extend Encoder Life
- Keep it dry: Avoid oil, coolant, or moisture entering the encoder (common failure cause).
- Secure cables: Fix cables properly to avoid bending or pulling during movement.
- Check battery regularly: Replace absolute encoder batteries every 1–2 years.
- Avoid vibration: Mount motors on rigid bases to reduce encoder vibration.
- Regular inspection: Check connectors and cables during monthly maintenance.
6. Conclusion
The Fanuc encoder is indispensable for servo motor precision and reliability. Understanding its relationship with the motor, recognizing common alarms, and following proper repair steps can save you costly downtime. If you need genuine Fanuc encoders (A860 series) or professional repair service, contact us today.
