Table of Contents

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Table of Contents
Safety Circuits
Induced Draft Motors
Vent Pressure Safety Switches (3-11) High Limit
Controls (12-14) Flame Roll-out Limit Switches
(15-17) Induced Draft Blower Housing Limit
Switches (18-19) Measuring Flame Current (20-22)
Checking Shaded Pole Induced Draft Motor
Windings (35-36) Checking PSC Induced Draft Motor
Windings (37-38) Checking Earlier Variable Speed
Induced Draft (39-40) Motor Variable Speed 3
Phase Induced Draft Motors (41-42)
Gas Valves
ECM Blower Motors
ECM Blower Motor Quick Test (43-45) LitePort
Furnace Control Information (46-48)
Measuring Inlet Supply Gas Pressure (23) Manifold
Pressure Adjustment (24-30)
Hot Surface Ignitors
Checking Silicon Carbide Hot Surface Ignitors
(31-32) Checking Silicon Nitride Hot Surface
Ignitors (33-34)
3
Vent Pressure Safety Switches

• The vent pressure safety switch on 80 efficient
  furnaces is connected to the induced draft blower
  housing. The vent pressure safety switch
  monitors the negative pressure developed by the
  induced draft blower assembly. If the induced
  draft blower pressure falls below the vent
  pressure safety switch minimum closing pressure,
  the switch will open.
•  
• On 80 efficient gas furnaces, this switch (fig.
  1) is used to prove the presence of adequate
  combustion air. It does not prove any operation
  of the vent system, chimney, because 80 furnaces
  do not use the induced draft blower to vent
  products of combustion. (THRU CHIMNEY) These
  furnaces are Category 1 appliances. Category 1
  means these furnaces vent the products of
  combustion by natural draft.
•  
• The vent pressure safety switches on 90
  efficient gas furnaces (fig. 2) are connected to
  the secondary heat exchanger area. The vent
  pressure safety switch on these models monitor
  the secondary heat exchanger area to ensure there
  is adequate negative pressure present.
•  
• The induced draft blower assembly draws in
  combustion air and also vents the products of
  combustion.

4
Vent Pressure Safety Switches
These furnaces are Category 4 appliances.
Category 4 appliances use the induced draft
blower to create a positive pressure on the vent
system.   Vent pressure safety switches on
Category 4 appliances protect the furnace from
operating with a lack of combustion air and from
blockages or restrictions in the flue
system. The vent pressure safety switch circuit
is powered by 24 volts. The switch is a normally
open device that closes on an increase in
negative pressure. Modern furnace integrated
furnace controls (IFC) monitor the position of
this switch during a call for heat via a
connection to either a 9 pin or 12 pin plug
located on the IFC board. The 9 pin plugs (fig.
3) are for single stage furnaces (fig. 4) and
the 12 pin plugs are for 2 stage furnaces (fig.
5).
Continued on Next Slide
5
Vent Pressure Safety Switches
Continued The plugs have two pins for each vent
pressure safety switch. One pin sends 24 volts
potential to the switch and the other pin
receives the 24 volt potential back to the IFC.
If the furnace is a two stage model, there will
be one pin for voltage out to both switches and
then two pins to receive the 24 volt potential
back at the IFC board (fig. 6).   When a call for
heat is received by the IFC, the IFC sends 24
volts out to the vent pressure safety switch. If
the switch is open, the IFC will not receive 24
volts back. However, if the switch is shorted,
or stuck in the closed position, the IFC will
receive 24 volts back. The IFC will then lock
out and flash an appropriate fault code on its
LED. If the IFC determines the vent pressure
switch is open, it will start the induced draft
motor for a pre-purge cycle if all other safety
devices are in their proper position.
Notice Prior production variable speed vent
motor models had the pressure switches report to
the microprocessor on the vent motor assembly.
The status of the pressure switches was then
communicated by the microprocessor to the IFC
board.
6
Vent Pressure Safety Switches
Notice there are two vent pressure safety
switches shown (fig. 7), they are installed on a
two stage gas furnace. The front switch is for
second stage induced draft motor speed and the
back one is for first stage induced draft motor
speed. The first stage switch will have a closing
pressure that is lower than the second stage
switch (fig. 8). The switch has a lower close
pressure to match the slower speed of the induced
draft blower when on low fire. During low speed
operation the high speed switch should be open
and the low speed switch closed.  During a call
for second stage heat the induced draft motor
goes to high speed and generates an increased
amount of negative pressure. Both the low speed
switch and high speed switch should be closed
during both low and high speed induced draft
motor operation. The vent pressure switch must be
in the open position prior to induced draft motor
operation (fig. 9). If the furnace control senses
a closed vent pressure switch when the induced
draft motor is off, the furnace control will
lockout. The control will not allow heat
operation until it senses the switch has opened.
If a technician tries to bypass the switch with a
jumper, the furnace will lockout on the next call
for heat cycle.
7
Vent Pressure Safety Switches
This safety feature prevents furnace operation
when an insufficient level of combustion air is
being drawn into the burners. The switch also
prevents operation of the burners on 90
efficient models when the flue is restricted. If
the furnace was allowed to operate with
inadequate combustion air, the furnace would
malfunction and produce carbon monoxide. If the
burners were allowed to operate with insufficient
vent, products of combustion would spill into the
home.
How to Check for a Shorted Vent Pressure Safety
Switch   1. Remove power to the furnace.   2.
Disconnect the wires from the vent pressure
switch terminals.   3. Using an ohmmeter, check
for continuity across both vent pressure switch
terminals (fig. 10) .   4. A shorted vent
pressure switch will measure resistance. Replace
the switch if it is shorted.   5. An open vent
pressure switch will measure infinite resistance.
The switch is not shorted.   6. When finished,
replace the wires and remember to restore power
to the furnace.
8
Vent Pressure Safety Switches
How to Check for Proper Pressure Switch Close
Operation
To determine if a vent pressure safety switch is
closing at the proper pressure you will need to
measure the negative pressure being sensed by the
switch. A good instrument for measuring induced
draft negative pressure is a Magnehelic
gauge. For 80 efficient gas furnace induced
draft pressure, the gauge scale should be capable
of reading accurately down to as low as .05
inches of water column and as high as 3 inch of
water column. For 90 efficient gas furnace
induced draft pressure, the scale should be
capable of reading accurately down to as low as
.05 inches of water column and as high as 2 to 3
inches of water column. To sense the pressure at
the back of the pressure switch, a tee is placed
in the tubing between the switch and inducer
housing (fig. 11). One opening in the tee is
connected to the induced draft blower housing,
one opening to the LOW PORT of the Magnehelic
gauge, and the other tee port is connected to the
vent pressure switch. The HIGH PORT of the
Magnehelic is left open to atmosphere. If the
furnace is a sealed combustion model, the high
port of the Magnehelic is connected to the tube
leading to the burner enclosure.
Continued on Next Slide
9
Vent Pressure Safety Switches
Continued When the induced draft motor runs, the
pressure being sensed by the vent pressure safety
switch is displayed on the Magnehelic gauge fig.
12). If the pressure is greater than the closing
set point of the switch, yet the switch does not
close, replace the switch. If the pressure is not
high enough to close the switch, there is a
problem with the induced draft blower assembly or
the vent/combustion air system.
Switching Function Check 1. Connect a Magnehelic
gauge between the induced draft motor housing and
the pressure switch (fig. 13). 2. With the
induced draft motor running, note the negative
pressure reading on the Magnehelic gauge. The
pressure must be greater than the close pressure
of the pressure switch being tested. (continued)
Continued on Next Slide
10
Vent Pressure Safety Switches
Continued
3. Use a voltmeter to check for 24 volts at the
pressure switch VOLTAGE IN terminal. Check by
placing one voltmeter lead to ground and the
other to the VOLTAGE IN terminal (fig. 14).
(MOTOR RUNNING) You should measure 24 volts. If
the furnace is a two stage model, check both the
low and high fire pressure switches to ensure you
have 24 volts at the VOLTAGE IN terminals. (MOTOR
RUNNING) Next, check for 24 volts at the VOLTAGE
OUT terminal. Place one voltmeter lead to ground
and the other to the VOLTAGE OUT terminal (fig.
15). You should measure 24 volts. If you do not
measure 24 volts, the pressure switch is open.
Replace the switch. If you measure 24 volts, the
switch is in the closed position. (MOTOR RUNNING)
11
Vent Pressure Safety Switches
Causes of Low Induced Draft Blower Housing
Pressure 80 Efficient Models
Seized induced draft blower wheel. Failed
induced draft motor, or capacitor. A lack of
combustion air supply. Kinked or plugged tube
from induced draft blower to pressure switch.
Causes of Low Vent Pressure on 90 Efficient
Models.
Plugged vent or induced draft assembly.
Failed vent motor or capacitor. Plugged heat
exchanger. Improper sized vent piping.
Blocked condensate drain.
LED Flash Code Response (40 Furnace Models)
If a White Rodgers IFC board has detected a vent
pressure safety switch fault, the diagnostic LED
will flash 3 times and lock the furnace out until
the pressure switch fault has been corrected.
12
High Limit Controls
High limit controls protect against abnormally
high air temperature in the heat exchanger
airstream area. These limit switches (fig. 1)
are 24 volt operated automatic reset devices that
open when the air temperature they sense exceeds
the switches open setting. The open setting of
the limit switch is usually not found on the
switch itself, but rather on the furnace
nameplate. The high limit is wired in series
with other furnace limit switches (fig. 2).
Modern furnace integrated furnace controls (IFC)
monitor the position of limit switches during a
call for heat via a connection to either a 9 pin
or 12 pin plug located on the IFC board. The 9
pin plugs (fig. 3) are for single stage furnaces
and the 12 pin plugs (fig. 4) are for 2 stage
furnaces. The plugs have two pins that connect
to the furnace limit devices.
13
High Limit Controls
One pin sends 24 volts potential to the limit
switches and the other pin receives the 24 volt
potential back to the IFC. When a call for heat
is received by the IFC, the IFC sends 24 volts
out to the high limit circuit. If the high limit
switch is open, the IFC will not receive 24 volts
back. If the high limit or any of the other
limits open, the furnace will shut down burner
operation and run the furnace blower and induced
draft blower in an attempt to cool down the
furnace.
Check Procedure with Voltmeter 1. Call for
heat 2. Place your voltmeter leads across the
high limit switch terminals (fig 5). 3. You
should measure 0 volts if the limit is closed.
If the limit is open you will measure 24
volts. 4. If you measure 0 volts across the high
limit, yet the furnace is indicating an open
limit fault, check for 24 volts to ground at each
side of the high limit switch. If you do not
measure voltage, another limit switch is open.
14
High Limit Controls
Check Procedure with Ohmmeter 1. If the furnace
is hot, allow it to cool down, and then remove
power to the furnace. 2. Remove the two wires
from the high limit switch. 3. Using an ohmmeter,
check for continuity across the switch (fig.
6). 4. You should measure 0 ohms if the switch is
closed. If the high limit is open, replace it.
Causes of High Limit Trips
Dirty filters Dirty A/C Coil Undersized
ducting Blower motor failure Dirty blower
assembly Over-firing of the furnace burners
15
Flame Roll-out Limit Switches
Flame roll-out limit controls (fig. 1) protect
against abnormally high air temperature in the
furnace burner area. These limit switches are 24
volt operated manual reset or one time failure
devices that open when the air temperature they
sense exceeds the switches open setting. The
switches can be either snap disc or fuse link
devices. The open temperature of the switch is
normally not listed on the part. The flame
roll-out switch circuit is powered by 24 volts.
The flame roll-out switch is wired in series
with other furnace limit switches (fig. 2).
(TCO) Modern furnace integrated furnace controls
(IFC) monitor the position of limit switches
during a call for heat via a connection to either
a 9 pin or 12 pin plug located on the IFC board.
Continued on Next Slide
16
Flame Roll-out Limit Switches
Continued The 9 pin plugs (fig. 3) are for
single stage furnaces and the 12 pin plugs (fig.
4) are for 2 stage furnaces. The plugs have two
pins that connect to the furnace limit devices.
One pin sends 24 volts potential to the limit
switches and the other pin receives the 24 volt
potential back to the IFC.  If the limit opens,
the furnace will shut down burner operation and
run the furnace blower and induced draft blower
in an attempt to cool down the furnace.
Check Procedure with Ohmmeter 1. If the furnace
is hot, allow it to cool down, and then remove
power to the furnace. 2. Remove the two wires
from the flame roll-out switch. 3. Using an
ohmmeter, check for continuity across the switch
(fig. 5). 4. You should measure 0 ohms if the
switch is closed. If the switch is open, you will
measure infinite ohms. If you measure infinite
ohms reset the switch if it is a reset type, if
it cannot be reset, replace the switch.
17
Flame Roll-out Limit Switches
Check Procedure with Voltmeter 1. Call for
heat. 2. Place your voltmeter leads across the
limit switch terminals (fig. 6). 3. You should
measure 0 volts if the limit is closed. If the
limit is open you will measure 24 volts. 4. If
you measure 0 volts across the high limit, yet
the furnace is indicating an open limit fault,
check for 24 volts to ground at each side of the
switch (fig. 7 ) (fig. 8). If you do not measure
voltage, another limit switch is open.
Causes of Flame Roll-out Trips
Low gas pressure A lack of combustion air
Plugged burner orifice Plugged heat exchanger
18
Induced Draft Blower Housing Limit Switches
The induced draft blower housing limit control
(fig. 1) protects against abnormally high flue
temperature in the induced draft blower housing.
This limit is only present on 90 efficient
models.  These switches are 24 volt or 115 volt
operated auto reset devices. The switches are
mounted to the plastic induced draft blower
housing. The open temperature of the switch is
normally not listed on the part. Furnaces wired
to break 115 volt power to the induced draft
blower, report a pressure switch error when this
switch opens because the vent motor never comes
on during a call for induced draft blower
operation. (See fig. 2) Furnace models may use a
24 volt switch that is wired directly to the
furnace IFC board 9 or 12 pin plug. Furnaces
wired in this manner have the capability to
report an open induced draft blower limit switch
on the diagnostic LED. (See fig. 3)
Continued on Next Slide
19
Induced Draft Blower Housing Limit Switches
Continued The IFC board plugs have two pins
that connect to the furnace limit devices. One
pin sends 24 volts potential to the limit
switches and the other pin receives the 24 volt
potential back to the IFC. When a call for heat
is received by the IFC, the IFC sends 24 volts
out to the induced draft blower limit circuit.
If the limit switch is open, the IFC will not
receive 24 volts back. If this limit is open, the
IFC will flash a fault code. The induced draft
blower and the indoor blower motor will run until
the switch re-closes.
Check Procedure with Ohmmeter 1. If the furnace
is hot, allow it to cool down, and then remove
power to the furnace. 2. Separate the plug
assembly located near the limit switch (fig.
5). 3. Using an ohmmeter, check for continuity
across the two plug pins (fig. 6).
Causes of Flame Roll-out Trips
Overfiring of the furnace. Excessive supply
air temperature due to a lack of indoor air
volume (Plugged secondary.)
20
Measuring Flame Current
The flame sensing circuit (fig. 1) proves the
presence of flame. Without this circuit the
furnace would not know when flame is present. The
circuit consists of a flame sensing rod (fig. 2)
located at the opposite end of the burner
assembly from the furnace gas manifold
connection. The flame sensing rod is positioned
in front of the last burner. The flame rod has an
electrical terminal connection with one wire
(fig. 3).
The wire connects the flame sensing rod to the
furnace 9 or 12 pin plug located on the IFC board
(fig. 4). On some models of furnaces, the flame
sensing wire is connected to a terminal on the
IFC board. The voltage to the flame sensing rod
is typically around 80 volts AC when measured
with a digital voltmeter.
21
Measuring Flame Current
When a call for heat takes place and the burners
ignite, flame envelopes the flame rod. The flame
is a conductor and can conduct a very small
amount of current. The voltage potential at the
flame rod now has a conductor path through the
flame. Since the flame rod is much smaller than
the burner ground area, more current flows in one
direction than in the other. This is called
flame rectification. The current flowing in one
direction is now greater than in the other
direction so it is DC current. The amount of
current that actually flows is in the microamp
range. Typically from 2 to 4 DC microamps. The
minimum flame current necessary to maintain
ignition on White Rodgers ignition controls is 1
DC microamp.
How to Measure Flame Current with a Digital
Multimeter Capable of Reading DC Microamp
Range 1. Disconnect power to the furnace. 2. Set
the multimeter to the DC microamp scale. 3.
Remove the wire from the spade terminal on the
flame sensing rod. 4. Connect one end of a jumper
wire to the flame sensing rod spade terminal
(fig. 5). 5. Connect the other end of the jumper
wire to one lead of the multimeter (fig. 6).
Continued on Next Slide
22
Measuring Flame Current
Continued
6. Connect the other lead of the multimeter to
the wire removed from the flame sensing rod (fig.
7). 7. The meter is now wired in series between
the flame sensing rod and the IFC board (fig.
8). 8. Restore power to the furnace and call for
heat. 9. When flame is established the actual
flame current will be displayed on the
multimeter. 10. When you are finished with your
measurements, remove the call for heat. Allow
the furnace to run through the complete sequence
of operation and then remove power to the
furnace. Reconnect the wire to the flame sensing
rod and restore power.
Causes of Low or No Flame Current
Dirty flame sensing rod Cracked ceramic
insulator on flame sensing rod Cracked or
broken flame sense rod wire Poor furnace
ground Reverse polarity of 115 volt power
23
Measuring Inlet Supply Gas Pressure
Inlet supply gas pressure requirements for single
stage natural gas models is 5 to 7 inches water
column. LP models require an inlet supply
pressure of 11 to 13 inches water column. The
gas valve has a pressure port tap from which to
measure the inlet gas pressure. Inlet supply gas
pressure is measured with a manometer or
Magnehelic gauge.
Procedure for Reading Inlet Gas Pressure 1. Turn
off power to the furnace and shut off the gas
supply to the furnace. 2. Remove the pressure
port plug on the inlet side of the gas valve body
(fig. 1). 3. Insert a hose barb fitting into the
port and tighten till snug (fig. 2). 4. Connect a
manometer to the hose barb (fig. 3). 5. Restore
power to the furnace and call for heat. If the
furnace is two stage, make sure both stages of
heat are calling. HAVE ALL GAS APPLIANCES ON WHEN
CHECKING INLET GAS PRESSURE. 6. Once the gas
valve has energized, the inlet pressure will be
displayed in inches of water column on the
manometer. If the inlet pressure falls above or
below the inlet requirement, contact the gas
company for correction. 7. When finished replace
screw cover. End call for heat. Remove power.
Disconnect hose barb and replace the pressure
port plug (fig. 4). 8. Restore power.
Fig. 4
24
Measuring Manifold Gas Pressure
Manifold Pressure Adjustment
Manifold pressure requirements for single stage
natural gas models is 3 to 3.5 inches water
column. LP models require a manifold pressure of
10.5 to 11 inches water column. The gas valve
has a pressure port tap from which to measure the
manifold pressure (fig. 1) and has an adjustment
screw to raise or lower the manifold pressure
(fig. 2). Manifold pressure is measured with a
manometer or Magnehelic gauge.
Before making an adjustment to the manifold
pressure, make sure the inlet gas pressure is
within acceptable limits.
Procedure for Reading Inlet Gas Pressure 1. Turn
off power to the furnace. 2. Remove the pressure
port plug on the outlet side of the gas valve
body (fig. 3).
Fig. 1
Fig. 2
Fig. 3
Continued on Next Slide
25
Manifold Pressure Adjustment
Continued
3. Insert a hose barb fitting into the port and
tighten till snug (fig. 4). 4. Connect a
manometer to the hose barb (fig. 5). 5. Restore
power to the furnace and call for heat. 6. Once
the gas valve has energized, the manifold
pressure will be displayed in inches of water
column on the manometer. 7. If the manifold
pressure must be adjusted, remove the slotted
cover to gain access to the manifold pressure
adjustment screw (fig. 6). Turn the screw
clockwise to increase pressure and
counterclockwise to decrease pressure (fig. 7).
8. When finished replace screw cover. End call
for heat. Remove power. Disconnect hose barb and
replace the pressure port plug. 9. Restore power.
Fig. 4
Fig. 5
Fig. 6
Fig. 7
26
Manifold Pressure Adjustment
Gas Pressure Manifold Adjustment Two Stage
Furnaces
Before making an adjustment to the manifold
pressure, make sure the inlet gas pressure is
within acceptable limits.  Manifold pressure
requirements for two stage natural gas models is
1.4 to 1.7 inches water column first stage and 3
to 3.5 inches water column second stage. LP
models require a manifold pressure of 6 to 6.2
inches water column first stage and 10.5 to 11
inches water column second stage.  The gas valve
has a pressure port tap to measure the manifold
pressure. There are two adjustment screws to
raise and lower manifold pressure for first and
second stage fire rates. The first stage
adjustment screw is on top of the gas valve (fig.
8) . The second stage adjustment screw is next to
the pressure port plug on the outlet side of the
gas valve (fig. 9). Manifold pressure is measured
with a manometer or Magnehelic gauge.
Continued on Next Slide
27
Manifold Pressure Adjustment
Continued
1. Turn off power to the furnace. 2. Remove the
pressure port plug on the outlet side of the gas
valve body (fig. 10). 3. Insert a hose barb
fitting into the port and tighten till snug (fig.
11). 4. Connect a manometer to the hose barb
(fig. 12). 5. Restore power to the furnace and
call for first and second stage heat. 6. Once
the gas valve has energized, the second stage
high fire manifold pressure will be displayed in
inches of water column on the manometer.
Continued on Next Slide
28
Manifold Pressure Adjustment
Continued
7. If the manifold pressure must be adjusted,
remove the slotted cover on the outlet side of
the gas valve to gain access to the manifold
pressure adjustment screw (fig. 13). Turn the
screw clockwise to increase pressure and
counterclockwise to decrease pressure. 8. Once
the second stage manifold pressure has been set,
replace the high fire slotted cover. 9. Remove
the low fire slotted cover (fig. 14) and reduce
the call for heat to first stage only. Note the
drop in gas pressure on the Magnehelic gauge. If
the pressure needs adjustment, turn the
adjustment screw to get the proper pressure.
10. When finished replace screw cover. End
call for heat. Remove power. Disconnect hose
barb and replace the pressure port plug. 11.
Restore power.
NOTE about LP gas pressure adjustment When
adjusting the gas pressure on LP equipped
furnaces, first turn the high fire adjustment
screw clockwise until it bottoms out before
setting the low fire propane fire rate. If you
do not do this, you will not be able to set the
low fire rate to its proper level.
29
Manifold Pressure Adjustment
Gas Pressure Manifold Adjustment Sealed
Combustion Furnace
Sealed combustion burners (fig. 15) are not at
atmospheric pressure. The burners are actually
at a negative pressure compared to atmosphere.
Therefore, you must reference the burner box
pressure when measuring manifold pressure.   To
measure manifold pressure on a system that has
sealed combustion you must have a manometer or
magnehelic gauge with a high and low pressure
connection. To measure gas pressure with
sealed combustion (fig. 16), first, connect a tee
into the hose connecting the gas valve to the
burner box. Next, connect one end of the tee to
the tube leading to the burner enclosure. Next,
connect the last tee opening to the low port of
the magnehelic gauge or manometer. Connect high
port of the magnehelic gauge or manometer to the
pressure port tap on the gas valve. The pressure
you read will be the manifold pressure referenced
to the burner box pressure.
30
Manifold Pressure Adjustment
Pressure Measurement New 36G Gas Valves
The new White Rodgers 36G gas valves (fig. 17)
require a special kit to measure the inlet and
outlet gas pressure. The kit number is KIT07611.
The kit consists of a 5/16 tube, a connector, and
a 3/32 hex wrench (fig. 18). The kit is used to
connect a gas pressure manometer to pressure
bosses located on the gas valve body. The kits
5/16 inch tubing fits over the inlet and outlet
pressure bosses of the gas valve. The kits
connector connects the manometer to the 5/16 inch
tubing. The hex wrench opens the port inside of
the pressure bosses. Rotating the screw inside of
the boss counter clockwise one turn will open the
pressure port. To close the port, rotate the
screw clockwise one turn (fig. 19). Always follow
the safety instructions included with the
pressure tap adaptor kit. New production units
will have new easy to service White Rodgers 36
series gas valves. The valves feature easy to
access pressure adjustment screws and easy ON/OFF
pressure access bosses on the gas valve body
(fig. 20 and 21). The small size of the valve
makes service a breeze.  LP conversion kits are
available. The two stage valve shares the same
spring as single stage models.
31
Checking Silicon Carbide Hot Surface Ignitors
Silicon carbide hot surface ignitors (fig. 1) are
used as the ignition source on some furnace
models. The igniters are 120 volt operated
devices that are energized by the IFC board
during a trial for ignition period. The igniters
are connected to the IFC board at the igniter HOT
and igniter neutral terminals (fig. 2). When a
call for heat occurs, the furnace goes through a
pre-purge cycle. After the pre-purge cycle is
completed, the trial for ignition cycle is
started. The igniter goes through a warm-up
period and the gas valve is energized. If the
igniter fails to glow, the trial for ignition
will fail.
Checking the Igniter 1. Turn off power to the
furnace. 2. Let the furnace go through the
pre-purge cycle and enter the trial for ignition
period.
32
Checking Silicon Carbide Hot Surface Ignitors
Measure the voltage at the plug (fig. 4). There
should be 120 volts. If 120 volts is present and
the igniter does not glow, replace the igniter.
  If there is no voltage, check for 120 volts at
the IGNITER HOT and IGNITER NEUTRAL terminals on
the IFC (fig. 5). If voltage is present, repair
the wiring between the IFC and the plug. If there
is no voltage at the IFC board, replace the IFC
board.
33
Checking Silicon Nitride Hot Surface Ignitors
Silicon nitride igniters (fig.1) are used as the
ignition source on some furnace models. The
igniters operate at a voltages below line level.
Typically in the 70 to 80 volt range. These
igniters cannot operate at 120 volts. If 120
volts is applied to the igniter over an extended
period of time, the igniter will fail. Furnaces
that have silicon nitride igniters have IFC
boards that monitor furnace line voltage levels.
The IFC will regulate the voltage to the igniter
based upon the line voltage level (fig. 2). By
reducing voltage to the igniter, the life-span of
the igniter is extended. When a call for heat
occurs, the furnace goes through a pre-purge
cycle. After the pre-purge cycle is completed,
the trial for ignition cycle is started. The
igniter goes through a warm-up period and the gas
valve is energized. If the igniter fails to
glow, the trial for ignition will fail.
Fig. 1
34
Checking Silicon Nitride Hot Surface Ignitors
Check Procedure 1. If the igniter fails to glow,
disconnect power to the furnace. 2. Unplug the
igniter wiring harness and access the pins in the
plug assembly leading to the igniter (fig. 3). 3.
Using an ohmmeter, measure the resistance through
the igniter (fig. 4). At room temperature you
should measure between 11 and 17 ohms. If the
resistance is infinite or zero, replace the
igniter.  4. If the igniter ohms out OK, replace
the IFC board.
Ignitor Malfunction Check voltage from ground to
neutral. It should never measure more than 10
volts. Cause Loose neutral wire connection.
35
Checking Induced Draft Blower Motor
Checking Shaded Pole Induced Draft Motor Winding
80 efficient gas furnace models use shaded pole
motors to turn the induced draft blower assembly
(fig. 1). These motors are 115 volt operated
motors that have an internal overload
switch.  When a call for heat occurs, the motor
is energized by the IFC board. The IFC has a HOT
INDUCER terminal that has 115 volts present when
motor operation is called for. The IFC also has
a terminal connection for NEUTRAL.
Check Procedure If the motor does not run when it
should, the motor winding may be open, the motor
may be seized, the internal overload may be open,
or the IFC board may not be switching 115 volts
to the motor.   1. Disconnect power to the
furnace. 2. Unplug the motor from the wiring
harness (fig. 2).
Continued on Next Slide
36
Checking Induced Draft Blower Motor
Continued
3. Using an ohmmeter, check for continuity
between the two pins on the plug (fig. 3). You
should measure a small resistance. If you measure
infinite resistance, the Internal overload may be
open, or the motor winding may be open. If the
motor is cool, replace the motor. If the motor is
hot, allow time for it to cool and recheck to see
if the internal overload resets. Check for a
seized motor.   4. If the motor windings are OK,
connect the motor plug back into the wiring
harness.   5. Restore power to the furnace and
call for heat.   6. Check for 115 volts at the
IFC board terminals that power the induced draft
blower (fig. 4). There should be 115 volts
present. If there is no voltage, replace the IFC.
  7. If voltage is present, check the wiring
harness between the IFC and the induced draft
blower motor.
37
Checking PSC Induced Draft Motor Windings
90 efficient gas furnace models use PSC motors
to turn the induced draft blower assembly. These
motors are 115 volt operated motors that have an
internal overload switch. The motors have start
and run windings along with a run capacitor. If
the furnace is a single stage model, the motor
will have one run winding speed tap if the
furnace is a two stage model, there will be two
run winding speed taps.   When a call for heat
occurs, the motor is energized by the IFC board
(fig. 1). The IFC has a HOT INDUCER terminal
that has 115 volts present when motor operation
is called for. The IFC also has a terminal
connection for NEUTRAL. If the furnace is a two
stage model, there will be a low and a high speed
terminal on the IFC board.
Check Procedure If the motor does not run when it
should, the motor winding may be open, the motor
may be seized, the internal overload may be open,
or the IFC board may not be switching 115 volts
to the motor, or the run capacitor may be
bad.   1. Disconnect power to the furnace. 2.
Unplug the motor from the wiring harness.
Continued on Next Slide
38
Checking PSC Induced Draft Motor Windings
Continued
3. Using an ohmmeter, check for continuity
between the white wire pin on the plug and both
black high speed pin and the red low speed pin
(fig. 2). You should measure a small resistance
between white and black and white and red. If
you measure infinite resistance, the Internal
overload may be open, or the motor winding may be
open. If the motor is cool, replace the motor.
If the motor is hot, allow time for it to cool
and recheck to see if the internal overload
resets. Check for a seized motor. If the motor
is not seized, make sure the run capacitor is not
bad.   4. If the motor windings are OK, connect
the motor plug back into the wiring harness.
  5. Restore power to the furnace and call for
heat.   6. Check for 115 volts at the IFC board
terminals that power the induced draft blower
(fig. 3). Make sure you check the proper speed
tap. There should be 115 volts present. If
there is no voltage, replace the IFC. If voltage
is present, check the wiring harness between the
IFC and the induced draft blower motor.
Remember to check the run capacitor!
39
Checking Earlier Variable Speed Induced Draft
Motor
Some variable speed gas furnaces use a DC drive
induced draft motor that has an on board
microprocessor control (fig. 1). The vent
pressure switches report directly to the vent
motor assembly. Pressure switch status is
reported to the IFC board on a communication bus
between the IFC board and the microprocessor
board on the vent motor. There are status LED
indicator lights on the IFC that communicate
diagnostic information received from the vent
motor microprocessor.   The vent motor has a 12
pin plug that has a line voltage connection (fig.
2). The 115 volt power must be present for the
vent motor to run. If the power is not present
at the vent motor, the induced draft blower
housing limit switch may be open.   The 12 pin
plug also has a 13 volt DC pin that must have
power present for the vent motor microprocessor
to function. The 13 volt DC power is received
from the IFC boards 5 pin plug. The IFC tells
the vent motor whether to operate at first stage
speed or second stage speed. The stage signal is
sent out on the 5 pin IFC plug (fig. 3). The
voltage levels are 1 volt DC on or 12 volts DC
off. For example, the first stage speed pin
would have 1 Volt DC if the IFC is calling for
first stage induced draft motor operation.
40
Checking Earlier Variable Speed Induced Draft
Motor
Checking Vent Motor Quick Test 1. If the induced
draft blower does not run, make sure there is
115 volts to the Vent motor 12 pin plug. Check
the voltage between NEUTRAL and pin 12 of the 12
pin plug on the vent motor (fig. 4). If voltage
is not present, check to make sure the induced
draft blower limit is closed. If it is open,
replace it. 2. If voltage is present, the IFC
board green LED labeled RPM should be flashing
slow. If it is not, check for 13 volts DC on the
IFC board. Measure at the 5 pin plug between
pins 2 and 3 (fig. 5). If voltage is not present,
replace the IFC board. 3. If 13 volts DC is
present at the 5 pin plug, jumper pin 4 on the 5
pin plug to ground (fig. 6). The motor should
start. If the motor starts, replace the IFC
board. 4. If the motor does not start, replace
the vent motor. NOTE To see if the motor will
change speeds, start the motor on low speed by
jumpering pin 4 to ground. Once the motor is
running remove the jumper from pin 4 and rapidly
jumper pin 5 to ground (fig. 7). If the motor
does not change speed, replace the motor.
41
Variable Speed 3 Phase Induced Draft Motors
Some variable speed furnace models feature
variable speed 3 phase induced draft motors
(fig. 1). The motor speed is adjusted to optimize
venting performance. The motor speed is
controlled by the furnace IFC board.  A 4 pin
plug connects the motor to the IFC board (fig.
2). The IFC board sends out three phase voltage
to change the speed of the motor. If the motor
fails to run, the problem is either with the
motor or the IFC board.
42
Variable Speed 3 Phase Induced Draft Motors
Check Procedure 1. If the motor does not run,
make sure there is power to the furnace. Ensure
that a shorted vent pressure switch is not
present. 2. Disconnect power to the furnace and
unplug the vent motor from the wiring harness
(fig. 3). 3. Using an ohmmeter check the
resistance between all three motor leads (fig.
4). L1 to L2, L1 to L3, L2 to L3. You should
measure equal resistance through all winding
combinations. If any combination reads infinite,
the winding is open. Replace the motor. If all
combinations read infinite resistance, the
internal overload is open. If the motor is cool,
replace the motor. If the internal overload
resets, check for a seized motor.  4. If the
motor windings are OK, make sure the motor is not
seized.  5. If the motor is not seized, replace
the IFC board.
43
ECM Blower Motor Quick Test
ECM blower motors are used on some furnace
models. These motors are variable speed. They
will adjust their RPM in an attempt to deliver
the CFM that is programmed by the installing
technician. The motors operate on 115 volts if
installed on furnaces and 230 volts if installed
on air handlers.  The motor has two plugs (fig.
1). One plug is a five pin plug that connects
the line voltage to motor. The line voltage must
be present for the motor to operate.   The
second plug is a 16 pin plug that connects to the
furnace IFC board (fig. 2). This plug carries
control signals between the IFC and the ECM Motor.
44
ECM Blower Motor Quick Test
Check Procedure If the motor does not run, make
sure you have power to the IFC board and then
check the voltage between pins 4 and 5 of the 5
pin plug on the motor. You should read 115
volts. If voltage is not present, the motor will
not run. Check for a break in the wiring between
the IFC board and the motor. If the motor is
equipped with a choke coil, the choke coil may be
open.   If line voltage is present to the motor
and the motor does not run, jumper R to G on the
thermostat terminal strip (fig. 3). If the motor
runs the problem is in the low voltage thermostat
wire.   If the motor does not run, remove the 16
pin plug from the IFC board. Locate pins 12 and
15. Connect 24 volts from R to pin 12 and pin
15. Connect pins 1 and 3 to the common side of
the 24 volt transformer (fig. 4). The motor
should run.
Continued on Next Slide
45
ECM Blower Motor Quick Test
Continued
If the motor does not run, unplug the 16 pin
wiring harness from the motor (fig. 5). Put 24
volts to pins 12 and 15 and 24 volt common to
pins 1 and 3 at the motor (fig. 6). If the motor
starts, the fault is in the harness. If the
motor does not run, replace the motor module.
46
LitePort Furnace Control Information
Control Response
RED LED Fault Display   Past IFC boards lost all
fault information once power was removed to the
furnace. New production LitePort equipped boards
will retain fault information after power has
been removed.   If power is interrupted to a
furnace that has detected faults, the faults will
be retained in memory. When power is restored to
the furnace, the last four faults in the IFC
board memory will be displayed on the Red LED.
The faults are displayed in order last in first
out. The LED will not display these faults again,
until power is removed and then restored. If a
fault is present, the LED will display that fault
only.   The new LitePort series IFC boards store
event history in memory. The event history can be
recovered by using the LitePort optical coupler
and related software.

• Some of the events stored in IFC board memory
  include
• Number of recycles.
• Low flame current events.
• Line voltage irregularities.
• Pressure switch faults.
• Limit circuit faults.
• Calls for heat over a period of time.
• Number of calls for cooling over a period of
  time.
• Safety trips.

47
LitePort Furnace Control Information

• Why the RED diagnostic LED flashes every so
  often.
•  
• The furnace control board communicates diagnostic
  information at least every 20 seconds. The
  information is contained in the light emitted by
  the RED LED. This information can be read with a
  LitePort optical coupler system.
•  
• If information is being read by the optical
  coupler, the information could be delayed by as
  much as 60 seconds from the actual time the event
  occurred. If the IFC gathers information during
  the transmit period, the information you read may
  be delayed for a very short period of time.
•  
• How to clear out the fault history
•  
• Remove power to the furnace.
• Turn heat and cool demand off at the thermostat.
• Turn fan mode to ON mode. Not auto. (At
  thermostat) (OR JUMPER R to G at Furnace Board.)
• Turn the power back on.
• Jumper the following low voltage connections at
  the IFC thermostat terminal strip within 8
  seconds of power up
•  
• W1 on and then OFF three consecutive times. ( R
  to W1 at the low voltage terminal on the furnace
  IFC.)
• Control will blink the red and green LED for 2
  seconds to confirm fault history cleared.

48
LitePort Furnace Control Information

• To clear event history.
•  
• Turn power off.
• Fan to ON position at the thermostat. (OR JUMPER
  R to G at the furnace control board.)
• At the thermostat turn heat and cool demands
  off.
• Turn the power back on.
• Jumper the following low voltage connections at
  the IFC thermostat terminal strip within 8
  seconds of power up
• R to W1 On and then off.
• R to Y On then off.
• R to W1 On then off.
• Red and Green LEDs will blink for 2 seconds to
  confirm erase of history.