689670-BIM-A-0311
Johnson Controls Unitary Products
53
contactor is pulled in. Check for loose wiring between the
contactor and the supply air blower motor.
3.
If M3 is pulled in and voltage is supplied at M3, lightly touch
the supply air blower motor housing. If it is hot, the motor
may be off on inherent protection. Cancel any thermostat
calls and set the fan switch to “AUTO”, wait for the internal
overload to reset. Test again when cool.
4.
If M3 is not pulled in, check for 24 volts at the M3 coil. If 24
volts is present at M3 but M3 is not pulled in, replace the
contactor.
5.
Failing the above, if there is line voltage supplied at M3, M3
is pulled in, and the supply air blower motor still does not
operate, replace the motor.
6.
If 24 volts is not present at M3, check that 24 volts is
present at the supply air blower motor terminal on the UCB.
If 24 volts is present at the UCB terminal, check for loose
wiring between the UCB and M3.
a. If 24 volts is not present at the UCB supply air blower
motor terminal, check for 24 volts from the room
thermostat. If 24 volts is not present from the room
thermostat, check for the following:
• Proper operation of the room thermostat (contact
between R and G with the fan switch in the “ON” position
and in the “AUTO” position during operation calls.)
• Proper wiring between the room thermostat and the
UCB, and
• Loose wiring from the room thermostat to the UCB
7.
If 24 volts is present at the room thermostat but not at the
UCB, check for proper wiring between the thermostat and
the UCB, i.e. that the thermostat G terminal is connected to
the G terminal of the UCB, and for loose wiring.
8.
If the thermostat and UCB are properly wired, replace the
UCB.
9.
If the blower motor runs with the fan switch in the “ON”
position but does not run shortly after the furnace has
ignited when the fan switch is in the “AUTO” position,
check the room thermostat for contact between R and G
during “W1” calls.
On calls for heating, the supply air blower operates but the draft
motor does not (the room thermostat fan switch is in the
“AUTO” position).
1.
The draft motor has inherent protection. If the motor shell is
hot to the touch, wait for the internal overload to reset.
2.
If the motor shell is cold with the room thermostat calling
for heat, check for line voltage at the motor leads. If line
voltage is present, replace the draft motor.
3.
If line voltage is not present, check for line voltage on the
ignition control at the “inducer” terminal draft motor relay
(DMR or DMC) contacts in the main control box and check
to see if the (DMR or DMC) is pulled in.
The draft motor runs but the furnace does not light and the
spark ignitor does not spark.
1.
Check for 24 volts at the spark ignitor from the ignition
control board (ICB). Check the 24-volt wiring from the ICB
to the spark ignitor. Check for 24 volts at the ICB spark
ignitor terminal.
2.
Check the ground wiring for the ICB and the gas valve is
intact and making good electrical connection. Check the
ceramic insulator on the spark ignitor for breaks or cracks.
Replace the spark ignitor if damaged.
3.
With the draft motor running, check for 24 volts at the
pressure switch terminal on the ICB. If not present, check
for 24 volts on the terminal from the pressure switch. If
present, go to step 4. If 24 volts is not present, the either
pressure or rollout switch is not closed. Or the draft motor
is not sufficiently evacuating the heat exchanger tubes or
the pressure switch has failed. Check the operation of the
pressure switch. Check the line voltage to the unit; if line
voltage is low, call the local power company. If the problem
persists, the draft motor may need replacement.
4.
If the furnace is hot, it may be out on a high temperature
limit open; wait for limit reset.
5.
If all are intact replace the ICB.
The draft motor runs and the spark ignitor sparks at the burner,
but the burner does not ignite and a gas odor is not detected at
the draft motor outlet.
1.
Check to ensure gas is being supplied to the unit. Confirm
that the gas pressure to the unit is within the proper limits
as described in the “POST START CHECKLIST”.
2.
Check the voltage at the gas valve and at the gas valve
terminals on the ICB. Check all wiring between the ICB and
the gas valve. Check to make sure the ground connections
are intact.
3.
If 24 volts is present, remove the pilot burner and the
orifice. The removal procedure is described in “BURNER/
ORIFICE INSPECTION/SERVICING.” Inspect the orifice
for obstruction. If it is clear, replace the gas valve.
Main burners light but exhibit erratic flame characteristics.
1.
Check the main burner orifices for obstruction and
alignment. The removal procedure is described in
“BURNER/ORIFICE INSPECTION/SERVICING”. Clean or
replace burner orifices and burners as needed.
