Flame Sensor Tests
Component
Testing
Equipment R
Required:
Volt / Ohm Meter with a DC Scale
Signal TTransducer:
Part Number 120-08507
EXPLANATION
:
The flame sensor is responsible for monitoring burner flame
presence. It is used in conjunction with the ignition module.
Here is how it works. Flame sensing is the ability of a flame
to conduct current when current is supplied to the flame
sensor (from the module) and the flame sensor is immersed
in the burner flame. The alternating current received at the
flame sensor passes through the flame to the nearest
grounded surface, which is at the burner. At this point the
alternating current has been rectified or converted to direct
current. A green wire connected at the burner receives this
direct current and returns it back to the ignition module
thereby proving the presence of flame. The system control
allows the burner to operate as long as it receives this signal
from the flame sensor.
Checking ffor FFlame S
Sense:
With heater on check to make sure the black control module
(part number 120-09298) is sending approximately 100
V.A.C. from terminal “FSI” through the flame sensor wire to
the flame sensor. (For units with the white control module,
part number 120-08117, the flame sensor receives
approximately 24 V.A.C. from terminal FSI.) This can be
accomplished by disconnecting the flame sensor from its
power supply wire (from module) and performing the
following:
■
Shut the heater off.
■
Set your multimeter to A.C. Connect one probe to the
exposed terminal at the end of the wire from terminal
FSI and the other probe to ground (sheet metal screw
or cabinet).
■
Turn the heater on. As soon as there is a call for heat,
the control module will start sending approximately
100 V.A.C. (or 24 V.A.C.) to the sensor depending on
control module. If you do not see this voltage appear,
shut the heater off and check the wire for continuity. If
the wire is good, the control module is defective and
must be replaced. However, if the control module is
sending necessary voltage to the sensor, then
reconnect the sensor to its wire and recycle the heater.
If flame sense failure occurs, then proceed to check
these problem areas:
A. Extremely low gas pressure
B. Dirty flame sensor element. (Rub sensor element
briskly with steel wool or emery cloth.)
C. Cracked insulator base on sensor
D. Poor electrical connection
E. Flame sensor tip touching metal
F. Flame sensor tip not in burner flame. (See below for
proper positioning.)
Any of the preceding conditions allow flame current to be
“blocked” with subsequent extinguishing of the burner
flame. Therefore, make sure these conditions do not exist
before replacing the sensor.
If these areas have been checked, one final procedure is to
check for proper flame sense current. This procedure
requires the use of a signal transducer used in conjunction
with a digital meter.
August 1999
6.3-1
1
BURNER
FLAME
BURNER
CASTING
FLAME SENSOR
3/8 TO 1/2 IN.
1/2 TO 3/4 IN.
