Service Manual

86
SERVICING
2. Check for 24 volts from W1 to C terminal on the igni-
tion control. No voltage. Check transformer, room ther-
mostat, and wiring.
If you have 24 volts coming off the transformer but
receive approximately 13 volts on the terminal board
between (C) and (R), check for blown fuse.
3. Check for 120 volts to the induced draft blower (low-
stage) by measuring voltage between Pin 3 and Pin 4
(on the 5-pin connector) located on circuit board. No
voltage, check for loose connection in the 5-pin con-
nector or replace ignition control.
Check for 120 volts to the induced draft blower (high-
stage) by measuring voltage between Pin 2 and Pin 4
(on the 5-pin connector) located on circuit board. No
voltage, check for loose connection in the 5-pin con-
nector, no call for high stage heat or replace ignition
control.
2 Y2
HUM
OG Y1C W1 R
DE
W2
1
4. If voltage is present in Steps 1 through 3 and the in-
duced draft blower is operating, check for 120 volts to
the ignitor during the preheat cycle. Measure voltage
between Pin 1 and Pin 5 (on the 5-pin connector) lo-
cated on ignition control. No voltage, check low stage
and high stage pressure switches or replace the igni-
tion control board.
5. After the ignitor warmup time, begin checking for 24
volts to the gas valve. Voltage will be present for seven
seconds only if proof of flame has been established.
6. If proof of flame was established voltage will be provided
to the air circulation blower following the heat on delay
period.
a. BEFORE replacing the ECM motor assembly or the
end bell, first check the motor with an Emerson
UltraCheck-EZ
TM
diagnostic tool . If the motor runs
with the diagnostic tool, the motor is good. To check
the end bell, see the previous variable speed testing
section of this manual before replacing the end bell.
b. The two-stage variable speed furnaces should have
120 Volts at the motor at all times, even without a call
for cooling or heating. These motors receive their
operational signals (5 Volts dc) through the 4-pin
wiring harness, connected between the motor and
integrated control board. NOTE: For complete
troubleshooting information on units using the ECM
blower motors, refer to the servicing section - Check-
ing Air Circulator Blowers (S-16B) in this service
manual.
S-314 CHECKING FLAME SENSOR
A flame sensing device is used in conjunction with the
ignition control module to prove combustion. If proof of
flame is not present the control will de-energize the gas
valve and "retry" for ignition or lockout.
WARNING
HIGH VOLTAGE
D
ISCONNECT ALL POWER BEFORE SERVICING OR
INSTALLING THIS UNIT.
MULTIPLE POWER SOURCES MAY
BE PRESENT.
FAILURE TO DO SO MAY CAUSE PROPERTY
DAMAGE, PERSONAL INJURY OR DEATH.
1. Connect a micro-amp meter in series with this wire and
the sensor terminal.
L
INE VOLTAGE NOW PRESENT
WARNING
2. Place the unit into a heating cycle.
3. As soon as flame is established a micro-amp reading
should be evident once proof of flame (micro-amp read-
ing) is established, the hot surface ignitor will be de-
energized.
4. The Integrated Ignition controls will have 1 to 4 micro-
amps. If the micro-amp reading is less than the mini-
mum specified, check for high resistance wiring con-
nections, sensor to burner gap, dirty flame sensor, or
poor grounding.
5. If absolutely no reading, check for continuity on all com-
ponents and if good - replace ignition control module.
NOTE: Contaminated fuel or combustion air can create a
nearly invisible coating on the flame sensor. This coating
works as an insulator causing a loss in the flame sense
signal. If this situation occurs the flame sensor must be
cleaned with steel wool.