Troubleshooting Fanuc Servo Devices


Troubleshooting Fanuc Servo Devices

Recently we had a customer that we helped with troubleshooting a FANUC servo A20B-1003-0090 board that he was installing into an A06B-6058-H005 drive. A handful FANUC troubleshooting options are listed below.

Troubleshooting for the DCAI alarm:

[check items]

  1. Setting S2 for the S series
  2. Machine load
  3. Check connection of separate discharge unit

[Adjustment procedure]

A. Check amplifier setting S2. If the setting is incorrect, go to Cause 1. If the setting is correct, go to A-0.

A-0: Check whether a separate discharge unit is being used. If it is being used, go to A-1. If not being used, go to A-2.

A-1: Check the connection of the separate discharge unit. If the connection is incorrect, go to Cause 2. If connection is correct, go to A-2.

A-2: Check the acceleration/deceleration frequency. If the frequency is too high, go to Cause 3. If the frequency is low enough, go to A-3.

A-3: Replace the servo amplifier. If a DCAL alarm no longer occurs, go to Cause 4. If a DCAL alarm still occurs, go to Cause 3.


1). If the setting S2 of the S series servo amplifier is incorrect, a DC alarm is caused.

2). If the separate discharge unit is connected incorrectly, a DC alarm occurs.

3). Compared to the regenerative power of the amplifier, the regenerative energy of the motor is too large. (The inertia is too large or the acceleration/deceleration frequency is too high.) In this case, try to decrease the acceleration/deceleration frequency or install a separate discharge unit.

4). The discharge transistor (Q1) in the servo amplifier is defective.

MRO Electric and Supply has new and refurbished FANUC CNC products available, including FANUC Servo parts. We also offer repair pricing. For more information, please call 800-691-8511 or email

Rebuild a Fanuc Motor – Model A06B-0131-B075 Process

Gallery of pictures showing the process that it takes to rebuild a Fanuc motor, part number A06B-0128-B675#7008.

Original pictures of the motor. Fan is dirty, all sides show significant use.

fanuc motor repair
fanuc motor repair

Upon disassembling the FANUC servo motor, the residue was found in the stator prompting the removal of the motor shaft. This would have caused a short in the stator once running.

The finished product, after the motor is rebuilt and fully tested.

rebuild a fanuc motor
fanuc servo motor repair
MRO Electric stocks new and refurbished FANUC CNC Motors. We also provide pre-priced Control Techniques repairs. For more information or to request a quote, please call 800-691-8511 or email


We recently shipped out two Fanuc A16B-3200-0040 boards to a customer and upon installation, they received a Fanuc CHECKSUM error on their control. Here are the notes that we use to troubleshoot this Fanuc error which can be used to try and solve the problem.

There are two common causes for this problem.  The first and most likely is TP communications.  If you see 3-5 “_” appear under the checksum number, and no red LED’s on the CPU, then this is the problem.  This means that 1/2 the communications signal is missing.  Swap the Pendant and cable (Cable is probably crushed or cut), and everything should hopefully be great.  There is a possibility of damage on the CPU side, however.

MRO Electric and Supply has new and refurbished FANUC CNC and FANUC Robotics parts available. We also offer repair pricing. For more information, please call 800-691-8511 or email

The second is a corruption of software on the CPU.  This will result in the middle red LED (going from memory, so it might be a different LED) on the CPU turning on.  The only way to recover from this is to dump in an image, or reload core & put in your most recent backup.  Corruptions can occur from the backup battery being dead, noisy power being supplied, or any major power glitch while the system is on (power loss, lightning strike, etc.)