Trane ACONT900AC43UA Service Manual page 179

If for some reason flame is still not proven on this retry
for ignition, the IFC will then raise the AC sine wave
cycles four steps up (Figure 5.)
Figure 5
If the flame is proven the IFC will keep this step set-
ting. If flame is not proven the IFC locks out for one
hour; this lockout can be reset at the thermostat.
On a power interruption, the furnace control starts
the ignitor power at the high side of its range and re-
peats the learning routine.
The furnace IFC will call for heat and a trial for ignition
is initiated. If flame is not detected, the IFC will re-
cycle the furnace two more times before lockout.
This learning process is employed to provide the most
reliable ignition process and to extend the service life
of the ignitor. This control requires the correct polarity
of the 120 V.A.C. wiring and a ground connection. If
the line voltage is low, or the line polarity is reversed,
the IFC control will lock out the heating cycle and
cause the Red Fault LED to flash six times.
Troubleshooting
The ignitor operates at voltages below line level. Typi-
cally in the 70 to 80 volt range. These ignitors cannot
operate at 120 volts. If 120 volts is applied to the
ignitor over an extended period of time, the ignitor will
fail.
When a call for heat occurs, the furnace goes through
a pre-purge cycle. After the pre-purge cycle is com-
pleted, the trial for ignition cycle is started. The ignitor
Silicon Nitride Ignitor
goes through a warm-up period and then gas valve is
energized. If the ignitor fails to glow, the trial for igni-
tion period will fail.
Step 1
If the ignitor fails to glow, disconnect power to the fur-
nace.
Step 2
Unplug the ignitor wiring harness and access the pins
in the plug assembly leading to the ignitor.
Step 3
Using an ohmmeter, measure the resistance through
the ignitor (Figure 6) At room temperature, you should
measure between 11 and 18 ohms. If the resistance
is much higher or zero, replace the ignitor.
Figure 6
Step 4
If the ignitor ohms are in range, check all connections
before replacing the IFC board.
Note:
Ignitor Malfunction
Check voltage from ground to neutral. It should never measure
more than 10 volts.
Cause
Loose neutral wire connection
GF - E 3