Troubleshooting Danfoss Alarm 38 or AL38 or A38: Internal Fault Guide

Categories: Drives, PLC & HMI, Faults & Alarm

Dealing with Alarm 38 on your Danfoss VFD? Learn the causes of this internal communication fault and step-by-step solutions to reset your VLT drive.

Troubleshooting Danfoss Alarm 38: Internal Fault Guide

Danfoss Alarm 38 or AL38 or A38 indicates Internal Fault. this means the control card detects a communication failure or a checksum error within the drive’s internal hardware components.

Danfoss VLT Troubleshooting: Alarm 38

ALARM 38

Standard Message: Internal Fault

On Danfoss VLT drives like the AutomationDrive FC 302, AQUA Drive FC 202, or HVAC Drive FC 102, Alarm 38 is a general “Internal Fault” signal. This occurs when the control card detects a communication failure or a checksum error within the drive’s internal hardware components.

Pro Tip: Alarm 38 is often accompanied by a specific “Internal Fault Code” (e.g., 0, 512, or 1280). If you call technical support, having this sub-code ready is essential for a faster diagnosis.

Common Causes of Alarm 38

  • Electrical Noise (EMI): Heavy electromagnetic interference from nearby equipment disrupting internal data transfer.
  • Control Card Mismatch: After a replacement, the control card firmware may not be compatible with the power card.
  • Loose Connections: The physical ribbon cable or connection between the control card and the power card is loose.
  • Environmental Stress: Excessive vibration or high ambient temperatures causing temporary hardware glitches.
  • Hardware Failure: A genuine failure of the EEPROM or the processor on the control card.

Troubleshooting Steps

  • 1. Power Cycle the Drive: The first step for any Internal Fault is a “Hard Reset.” Turn off the main power, wait until the display goes completely dark, then wait another 2 minutes before powering back up. This clears temporary logic hangs.
  • 2. Check for External Interference: Ensure the drive is properly grounded. Check if the motor cable is shielded and that the shield is properly clamped to the metal plate. EMI is the #1 cause of “ghost” internal faults.
  • 3. Inspect Internal Ribbon Cables: (Authorized personnel only) Ensure the flat ribbon cables connecting the control card to the power board are seated firmly and haven’t vibrated loose.
  • 4. Check Option Modules: If you have fieldbus cards (Profinet, Ethernet/IP) installed, remove them one by one to see if a faulty option module is triggering the internal error.
  • 5. Update Firmware: In some cases, updating to the latest firmware version can resolve checksum errors and communication bugs between internal boards.

Is it time to replace the Control Card?

If you have reset the drive multiple times and Alarm 38 returns immediately upon power-up—and you have ruled out electrical noise—the internal processor is likely damaged. In this case, the Control Card usually requires replacement.

Applicable Models

This troubleshooting guide is relevant for the following Danfoss series:

  • VLT® AutomationDrive FC 301 / 302
  • VLT® AQUA Drive FC 202
  • VLT® HVAC Drive FC 101 / 102
  • VLT® Midi Drive FC 280