SERVICING
38
Electrical Checks - Low Voltage Control Circuits
1. Turn on power to air handler or modular.
2. Check voltage between pins on the 4-wire motor control
harness between the motor and control board.
3. Voltage on pins should read:
• Pins 1 to 4 = 3.3vdc
• Pins 1 to 2 = 3.3vdc
• Pins 3 to 4 = 15vdc
Motor Control/End Bell Checks
1. Disconnect power to air handler or modular blower.
2. Disconnect the motor control harness and motor power
harness.
3. Remove the blower assembly from the air handler or
modular blower.
4. Remove the (3) screws securing the control/end bell to
the motor. Separate the control/end bell. Disconnect
the 3-circuit harness from the control/end bell to remove
the control/end bell from the motor.
5. Inspect the NTC thermistor inside the control/end
bell (see gure below). Replace control/end bell if
thermistor is cracked or broken.
6. Inspect the large capacitors inside the control/end bell
(see gure below). Replace the control/end bell if any
of the capacitors are bulging or swollen.
7. Locate the 3-circuit connector in the control/end bell.
Using an ohmmeter, check the resistance between each
terminal in the connector. If the resistance is 100 W
or greater, the control/end bell is functioning properly.
Replace the control/end bell if the resistance is lower
than 100 W.
8. Reassemble motor and control/end bell in reverse of
disassembly. Replace blower assembly into air handler
or modular blower.
Motor Checks
1. Disconnect power to air handler or modular blower.
2. Disassemble motor as described in steps 2 through 4
above.
3. Locate the 3-circuit harness from the motor. Using an
ohmmeter, measure the resistance between each motor
phase winding. The resistance levels should be equal.
Replace the motor if the resistance levels are unequal,
open circuited or short circuited.
4. Measure the resistance between each motor phase
winding and the motor shell. Replace the motor if any
phase winding is short circuited to the motor shell.
5. Reassemble motor and control/end bell in reverse of
disassembly. Replace blower assembly into air handler
or modular blower.
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