SERVICING
46
7. If no voltage is present, check supply voltage to the furnace.
See section S-1.
8. Disconnect power to the furnace. Reconnect the 5-circuit
power harness disconnected in step 2.
Electrical Checks - Low Voltage Control Circuits
1. Turn on power to the furnace.
L
IN
E
VOLTAG
E
NOW
P
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WARNING
2. Check voltage between pins 1 and 4 on the 4-wire motor
control harness between the motor and control board.
Voltage should be between 3 and 15 VDC.
3. If no voltage is present, check control board. See section
S-313.
Motor Control/End Bell Checks
WARNING
HIGH
VOLTAG
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D
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ER
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FO
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F
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DAMAG
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P
ER
SONAL
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.
1. Disconnect power to the furnace.
NOTE:
Motor contains capacitors that can hold a charge for
several minutes after disconnecting power. Wait 5 minutes
after removing power to allow capacitors to discharge.
2. Disconnect the motor control harness and motor power
harness.
3. Remove the blower assembly from the furnace.
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 figure below). Replace control/end bell if thermistor
is cracked or broken.
6. Inspect the large capacitors inside the control/end bell (see
figure 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 100k
Ω
or greater, the
control/end bell is functioning properly. Replace the control/
end bell if the resistance is lower than 100k
Ω
.
8. Reassemble motor and control/end bell in reverse of
disassembly. Replace blower assembly into the furnace.
Motor Checks
WARNING
HIGH
VOLTAG
E
D
IS
C
ONN
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T
ALL
POW
ER
B
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FO
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S
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VI
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HANGING
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M
ULTIPL
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POW
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SOU
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S
MAY
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.
F
AILU
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DO
SO
MAY
C
AUS
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P
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DAMAG
E
,
P
ER
SONAL
IN
J
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ATH
.
1. Disconnect power to the furnace.
NOTE:
Motor contains capacitors that can hold a charge for
several minutes after disconnecting power. Wait 5 minutes
after removing power to allow capacitors to discharge.
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 the furnace.
CHECKING DUCT STATIC
The maximum and minimum allowable external static
pressures are found in the specification section. These tables
also show the amount of air being delivered at a given static
by a given motor speed or pulley adjustment.
The furnace motor cannot deliver proper air quantities (CFM)
against statics other than those listed.
Too great of an external static pressure will result in insufficient
air that can cause excessive temperature rise, resulting in
