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Gulf Coast

Technical Service Manager

Troubleshooting PWM Furnace Motors

The blower motor has a 115 VAC applied to it anytime the furnace is powered. First manually close the blower door switch then verify the 115 VAC power is between L1 and Neutral (L2) Wires.(L1 and Neutral (L2) are removed for clarity in this photo!

  • 3/4 hp & 1 hp PWM blower motors have a power choke located on the blower housing
  • Power chokes are used to filter line power and to reduce current draw of the motor. Power choke may be bypassed for troubleshooting purposes.
  • Open the blower door switch
  • Disconnect L1 feed to power choke
  • Insert meter probe into the L1 connector from the furnace control
  • Manually close blower door switch and verify 115 VAC power
  • Release blower door switch
  • Reconnect L1 to power choke
  • Then open the blower door switch
  • Disconnect black lead between power choke and motor
  • Insert meter lead into disconnected power choke lead
  • Manually close blower door switch and verify 115 VAC power from power choke
  • Release blower door switch
  • Reconnect power choke lead
  • Power choke may be temporarily bypassed for troubleshooting
  • Open blower door switch.
  • Resistance should be greater than 0  and under 1 with no resistance to ground

Checking for Secondary Voltage

  • Manually close blower door switch and verify 24 VAC between R and Com at thermostat terminals of furnace control
  • The PWM blower motor used in the current furnaces is controlled by switching OFF and ON a DC voltage (and resultant current) through the motor controller

Checking 15 VDC Power to Blower

  • Verify all harnesses are connected and 115 VAC power is ON Close Blower door switch
  • Set meter to volts DC (VDC)
  • Insert meter probe into the back of brown lead of PL16
  • Touch other meter probe to COM on furnace control
  • Voltage from furnace control should be about 15 VDC at the brown wire
  • 15 VDC is nominal
  • May be slightly higher or lower.   Actual line voltage affects low voltage.
    • Meter type affects reading.
  • If reading is unstable or scrolling
    • Check harness connectors and pins Clean meter probes
    • Try a probe with a finer point to improve  contact inside the connector.

Checking Motor Torque Control

  • So that there is no call at the thermostat terminals – Remove R thermostat lead and communication plug
  • 115 VAC power should be ON.
  • Close Blower door switch.
  • With meter set to volts DC (VDC), insert meter probe into the Molex connector with the yellow wire of PL16.
    • This Plug located on the PCB has Yellow and Brown wires going to the Motor Plug.
  • Touch other meter probe to COM on furnace control With no motor operation, voltage will be between 10 to 12 VDC.
  • Voltage for the speed control is fed from the furnace control at the brown wire, through the motor circuitry, and back on the yellow wire.
  • If voltage is 0 VDC or significantly below 10VDC then this indicates a likely issue with the connections at  the molex plugs at either PL13 or PL16.

Dip Switch Nomenclature

SW1

  • 1 RCL
  • 2 LHT = Low Heat Operation
  • 3 LHR = Low Heat Rise
  • 4 C/E = Comfort Efficiency Adjustment
  • 5 CFM = CMF per ton Adjust
  • 6 TST = Component Self Test
  • 7 DLY OFF = Blower Off Delay 90 Seconds
  • 8 DLY OFF = Blower Off Delay 180 Seconds

SW2

  • 1 TWN = Allows for selection of furnace Main (OFF) or secondary (on) when Twinned furnace setup is required.
  • 2 CFM See Page 37 of 58CTW Installation Manual for adjustments.
  • 3 CF
  • 4 CF
  • 5 CF
  • 6 A/C
  • 7 A/C
  • 8 A/C

Lastly

  • Check motor winding Resistance
  • All windings should have resistance values less than 20 Ohms
  • Winding resistance should be equal +/- 10%
  • We checked the voltage at the plug  connecting the power to the choke from Printed Circuit Board P.C.B.).
    • Then we bypassed it to verify if the power choke is good or bad.
    • Next we verified 15 vdc on the brown wire on PL16.
    •  We then verified 10-12 vdc on yellow wire on  PL16 when the blower motor not running and  6 to 8 vdc when the blower motor was running.

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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