Trane ACONT900AC43UA Service Manual page 146

Induced Draft Motors
stage heat. If PS-2 does not transition open during
this learning routine, the IFC will shut the furnace
heating cycle down, send a three flash code, wait
thirty seconds and will restart the first stage heating
cycle again if there is still a call for heat.
Induced draft blower operation after learning
routine
The pressure switches are constantly monitored
during the furnace operation. If a pressure switch
should open for a period of three seconds, the IFC
will increase the RPM of the vent motor by 300 RPM
and wait for the switch to close. If the switch is still
open after three seconds, the IFC will again increase
the vent motor RPM speed and wait for the switch
to close. The IFC is programmed to repeat this
routine for a total of 12 seconds.
If PS-1 should open and remain open for more than
three seconds when the furnace is operating in
second stage operation the IFC is to stop the furnace
cycle and flash the three flash fault code. The IFC
will wait thirty seconds and restart the first stage cycle
if there is still a heat call. The IFC will stop sending
the three flash fault code when PS-1 closes or the
call for first stage heat is removed.
If PS-2 should open during second stage operation
and the IFC can not get it closed by increasing the
vent motor RPM the IFC will go back to first stage
operation. During this first stage operation the IFC
will continue to send a three flash fault code. The
IFC will continue to operate the furnace in first stage
operation for ten minutes if the thermostat is still
calling for heat. If the thermostat is still calling for
second stage operation the IFC will go back to
second stage operation and if PS-2 closes this time
the IFC will quit sending the three flash fault code.
GF - B 4
Troubleshooting
Step 1
If the induced draft blower motor does not run during a
call for heat, check to ensure the first stage pressure
switch is in the open position prior to the induced
draft blower motor operation (Figure 3.)
Figure 3
If the furnace is blinking a three flash diagnostic code
and the motor is off, check for 24 volt potential from
24 volt common to each terminal on the first stage
pressure switch. One terminal should have 24 volts
present and the other should have zero volts. If there
is voltage potential at both terminals, the pressure
switch is stuck closed. Replace the pressure switch.
If the switch checks out to be in the open position,
continue on to Step 2.
Step 2
Shut power to the furnace off. Unplug the induced
draft blower motor from the wiring harness (Figure 4.)
Figure 4