WARNING: Troubleshooting of components necessarily re-
quires opening the electrical control box with the
power connected to the unit. Use extreme care
when working with live circuits! Check the unit
nameplate for the correct line voltage and set the
volt meter to the correct range before making any
connections with line terminals.
WARNING: Prior to any of the following maintenance proce-
dures, shut off all electric power to the unit to
prevent personal injury.
CAUTION: Label all wires prior to disconnection when servic-
ing controls. Wiring errors can cause improper and
dangerous operation. Verify proper operation after
servicing.
NOTE To find the Btu input, multiply the number of cubic feet of
gas consumed per hour by the Btu content of the gas in
your particular locality (contact your gas company for this
information - it varies widely from city to city.)
1. Draft motor operates and furnace lights but supply air
blower does not start after a short time delay with room
thermostat fan switch set to “AUTO”.
a. Set fan switch to “ON”. If blower motor runs, go to Step f.
If it does not, check to see if line voltage is being
supplied to the contacts of the contactor (M3), and if the
contactor is pulled in. Check for loose wiring.
b. If (M3) is pulled in, touch the supply air blower motor
housing. If it is hot, the motor may be off on inherent
protection. Disconnect power to the unit and check the
blower motor capacitor. If it is defective, replace it with
one of equal capacitance and voltage.
c. If (M3) is pulled in and the blower motor still does not
run, replace the blower motor.
d. If (M3) is not pulled in, check for 24 volts at the (M3) coil.
If 24 volts is present, replace the (M3) contactor.
e. If 24 volts is not present at the (M3) coil, check for loose
24 volt wiring back to the relay board. Check control
wiring to the room thermostat. If all is fine, replace the
relay board.
f. If the blower motor runs with the fan switch in the “ON”
position but does not run soon after the furnace has
ignited with the fan switch in the “AUTO” position, check
for loose 24 volt wiring between the relay board in the
main control box, the Mate-N-Lok connector in the
partition between the evaporator and gas heat sections
and the time delay (BT) or (ETD).
g. If all control wiring is fine, check for 24 volts at the relay
board. If 24 volts is present, replace the relay board. If
24 volts is not present, replace the (BT) or (ETD).
NOTE: The furnace may shut itself down on a high tempera-
ture condition during the procedure, but this will not
effect the test if it is done within 5 minutes of furnace
shut-down.
2. The supply air blower operates but the draft motor does not
when the room thermostat is set to call for heat and the fan
switch in the “ON” position.
a. The draft motor has inherent protection. If the motor shell
is hot to the touch, wait for the internal overload to reset.
b. If the motor shell is cold with the room thermostat calling
for heat, check for line voltage at the motor’s Mate-N-
Lok connector attached to the evaporator partition. If
line voltage is present, replace the draft motor.
c. If line voltage is not present, check for line voltage at
the draft motor relay (DMR or DMC) contacts in the main
control box and check to see if the (DMR or DMC) is
pulled in.
d. If the (DMR or DMC) relay is pulled in, check for a loose
line voltage connection.
e. If the (DMR or DMC) relay is not pulled in, check for 24
volts at the (DMC) coil or 24 V at the DMC-6 terminal.
If 24 volts is present, replace the (DMR or DMC) relay.
If 24 volts is not present, check for a loose 24 volt
connection back to the relay board and check the
connections from the room thermostat to the relay
board. If all connections are correct, replace the relay
board.
3. The draft motor runs but the furnace does not light and the
sparker does not spark.
a. Check all 24 volt connections from the relay board to
and in the gas heat section. Check low voltage connec-
tions to the(BT) & (DMC) or (ETD) located in the control
box.
b. If the furnace is hot, it may be out on an over tempera-
ture condition, wait for limit reset.
c. With the draft motor running, check for 24 volts at
terminal 25V on the ignitor control (IC) where the red
lead from the draft motor attaches. If 24 volts is not
present, the centrifugal switch (CS) has not closed or
has gone bad. Check the line voltage to the unit, if it is
correct replace the draft motor. If line voltage is low call
the power company.
d. If 24 volts is present at the ignitor control, check all
control wiring at the ignitor control and the high tension
wire to the ignitor. Check that the green ground wires
from the ignitor control, the gas valve and pilot burner
are all intact and making good electrical connection.
Check to make sure that the ceramic insulator on the
pilot ignitor or sensor is not broken or cracked, if all are
intact replace the ignition control “IC”.
4. The draft motor runs and the sparker sparks at the pilot
burner but the pilot does not ignite and a gas odor is not
detected at the draft motor outlet.
a. Check to make sure gas is being supplied to the unit.
Make sure that the gas pressure to the unit is within the
proper limits as described in the “POST START CHECK
TROUBLESHOOTING
530.18-N10Y
Unitary Products Group
19
