SERVICING
34
CHECKING TRANSFORMER AND
CONTROL CIRCUIT
A step-down transformer 120 volt primary to 24 volt second-
ary, 40 VA (Heating and Cooling Models) supplies ample ca-
pacity of power for either operation.
WARNING
HIGH VOLTAGE
Disconnect ALL power before servicing or
changing any electrical wiring. Multiple pow
-
er sources may be present. Failure to do so
may cause property damage, personal injury
or death.
1. Remove blower compartment door to gain access to
the thermostat low voltage wires located at the fur-
nace integrated control module.
2. Remove the thermostat low voltage wires at the fur-
nace integrated control module terminals.
With Power On (and Door Interlock Switch closed):
WARNING
Line Voltage now present.
3. Use a voltmeter, check voltage across terminals R
and C. Must read 24 VAC.
4. No voltage indicates faulty transformer, open fuse,
bad wiring, bad splice, or open door interlock switch.
5. Check transformer primary voltage at incoming line
voltage connections, fuse, splices, and blower door
interlock switch.
6. If line voltage is available to the primary side of trans-
former and not at secondary side, the transformer is
inoperative. Replace.
7. After completing check and/or replacement of trans-
former and check and/or repair of control circuit, rein-
stall blower compartment door.
8. Turn on electrical power and verify proper unit operation.
CHECKING AIR CIRCULATOR BLOWER MOTOR
(MULTI-SPEED ECM MOTOR)
WARNING
Disconnect ALL power before servicing.
1. Remove blower compartment door to gain access
to the circulator blower motor and integrated ignition
control.
2. Check for any obstruction that would keep the fan
wheel / fan motor from turning.
3. Check wiring, the motor has two wiring harnesses,
a main harness and a control harness. The main pin
harness has: White neutral wire connected to the
Neutral terminal on the control board. Black wire con-
nected to the CIRC H terminal on the control board.
Red wire connected to the COM terminal, which is a
female spade connection next to the T1 – T5.
Green ground wire connected to cabinet ground.
The control harness has:
Blue wire connected to T1 on the control board.
Red wire connected to T2 on the control board.
Orange wire connected to T3 on the control board.
Black wire connected to T4 on the control board.
Yellow wire connects to T5 on control board.
The multi-speed ECM motor requires a line voltage
power supply (black connected to CIRC H and white
connected to neutral on the control board) as well as
a signal on one of the speed taps (T1-T5).
The speed tap voltage is A.C. and can vary which tap
is energized depending on DIP switch selection. The
voltage reading from any one of the speed taps is ref-
erenced between the female COM terminal next to the
speed taps on the control board. From COM to T1 or
T2, T3, T4, T5 you should read 24 VAC on the low
voltage speed taps.
CONNECTOR ID
DESCRIPTION
CONNECTOR VOLTAGE
L
LINE, L1
LINE, L1
G
GROUND
CHASSIS GROUND
N
LINE, L2
LINE, L2
C
SIGNAL COMMON 24VAC COMMON
1
TAP 1
24VAC
2
TAP 2
24VAC
3
TAP 3
24VAC
4
TAP 4
24VAC
5
TAP 5
24VAC
Motor Tap Identification
CHECKING DUCT STATIC
The maximum and minimum allowable external static pres-
sures 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 insuffi
-
cient air that can cause excessive temperature rise, result-
ing in limit tripping, etc. Whereas not enough static may re-
sult in motor overloading.
