Troubleshooting the VFD Blower Drive (Video)

This tech tip video runs through how to trouble-shoot the variable frequency drive (VFD) on a rooftop unit when the blower is not running.

Here are some basic things to check:

Is the wild leg on L3?

Is there an alarm?

  • Green LED Flashing
    • Alarm condition has happened but the drive will continue to operate.
    • Plug in keypad to diagnose alarm.

Is there a fault?

  • Red LED flashing
    • Fault condition has occurred and the power must turned off for 5 min to reset.
    • Plug in keypad to diagnose fault.

Is there a call for the blower?

  • Check at thermostat terminals.
    • R to G, low speed.
    • R to W and R to Y2 high speed.
  • Check the low speed and high speed inputs on the drive low voltage terminals.
    • Yellow to Blue (DI2 to GND) 24 volts DC low speed call.
    • White to Blue (DI3 to GND) 24 volts DC high speed call.
  • If no call check the replay board outputs.
    • Replace the relay board.

Is there power to the drive?

  • Main U1,V1 and W1 should be line voltage.
  • If there is no power, check the inline fuses on the 580J/558J 07 to 12.

Is there power to the motor?

Is there power to all legs of the motor?

To bypass the drive, power the unit down.

  • Unplug the connectors coming from the control box and plug them directly into the motor leads. 
  • Make sure to bypass the inline fuses (07 to 12 Units).

Figure 24 is mentioned at the 1:22 mark and figures 25 and 26 are mentioned at the 2:49 mark.

VFD wiring diagram

VFD wiring diagram

VFD wiring diagram

Contact the experts at B-Y with any questions!

Are you a homeowner or own a commercial property? Check out to find a Bryant dealer near you!