Suburban SF-25F Service Manual

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between electrode and ground to insure no sparking to sensor. Sparking
to sensor will damage the module board.
The electronic ignition system is made up of three main parts; the module
board, the electrode assembly, and the electrode wire. The module board
is the brain of the electronic ignition system and it has several functions.
1. When the blower reaches approximately 75% of the normal
r.p.m.and sufficient air flow is established, the sail switch engages and
completes a 24 volt circuit through the limit switch to the module board.
2. After a 12 - 18 second delay, 24 volt current will pass through
themodule board to the solenoid valve. The current to the valve opens it
and allows gas to the main burner; simultaneously, the module board
sends high voltage through the electrode wire to the electrode assembly.
The voltage seeks a ground between electrode and ground probe and a
spark occurs. The spark then ignites the main burner.
3. The module board also performs the lockout function in cases where the
spark fails to light the burner. When lockout occurs, the spark stops, the
voltage from the module board to the gas valve is discontinued, and the
valve closes. The unit will remain in lockout and the blower will continue to
run until the thermostat is turned off. Turning the thermostat off disengages
the lockout function of the module board. After the blower has stopped, the
ignition sequence can be started again. The module will try three times for
ignition before lockout.
It is important to determine the type problem being experienced, then
the proper checkout procedure can be made. The following is a list of
problems, how to identify in which area the problem is located, and
how to correct it.
1. Electrode not sparking - with blower running and micro switch
engaged, check the following:
a. Check for proper voltage at module board after the blower
motor reaches full r.p.m. If no voltage, check continuity through
micro switch and limit switch. Also check wiring and wire
connections.
b. Voltage is present but no spark at electrode after 12 - 18 second
delay, check electrode wire connections.
c. Wire connections OK, but electrode wire does not show
continuity through it - replace electrode wire.
d. Electrode wire does show continuity through it - check electrode
gap to be sure it is 1/8" maximum between electrode and
ground. NOTE: Gap between sensor and ground must be twice
electrode gap.
e. Electrode gap OK - check electrode assembly for possible
cracks or carbon on tip of electrode.
f. Electrode OK - replace module board.
2. Electrode sparking, but gas not coming through burner:
a. Check to see if voltage is coming out of module board to gas
valve after the 12 - 18 second delay. Check the wires in the
molex connector to be sure they are intact and making contact
with the module board. Check wire from the module board to
valve for continuity. Wire and connections check OK - replace
module board.
b. Voltage is coming out of module board to gas valve, but gas
valve does not open - replace gas valve.
3. Electrode sparking and gas valve opening, but burner will not
light:
a. Check to see if gas is coming through to the burner. This can
be accomplished by using a flow meter in the gas supply line. If
no gas is coming through the burner, check for obstruction in
gas line, in main burner orifice, or in main burner.
b. Gas is coming through burner, but spark will still not ignite
burner - check gas pressure.
Line Pressure - Min. 11" W.C.*, Max. 14" W.C.
To properly check pressure, first determine the line pressure, cycle
furnace and check pressure drop on demand. The drop in pressure
should not be more than ½" W.C. A drop of more than 1/2" would
indicate a faulty regulator, a restriction in the gas line, or a pinched
gas line. Excessive pressure drop could also be due to moisture
contamination.
c. Gas pressure OK - check for obstruction in main burner; check
to be sure electrode is positioned approximately 1/4" above and
directly over slots on the main burner - adjust electrode if
necessary.
d. Remove burner and check burner for obstructions. Clean as
required.
4. Burner ignites, but goes off and into lockout:
a. Check to be certain that flame sensor is over slots in the main
burner and that the main burner flame is burning against the tip
of the flame sensor - adjust by bending sensor probe. NOTE:
Sensor probe should be in the inner blue cone of the burner
flame (approximately 1/4" t o 5/16" above burner).
b. Burner still goes off and into lockout - check wire connections
at flame sensor and at module board.
c. Wire connections OK - check continuity through flame sensor
wire.
d. Continuity of flame sensor wire OK - check with micro amp
meter in series with flame sensor and flame sensor wire to be
certain that the flame sensor is generating at least seven micro
amps within seven seconds after the burner is ignited. Connect
meter as follows: (+) to sensor wire, (-) to sensor probe. Adjust
position of sensor probe, check for carbon deposits on sensor
probe if reading is less than seven micro amps.
e. Flame sensor circuit generating at least seven micro amps, but
burner still goes off and into lockout - replace module board.
5. Repeated module board failures:
a. Check to be certain that the electrode spark is not sparking
against the flame sensor portion of the electrode assembly.
b. Check to be sure module board or high voltage wires are not
shorted to the chamber wrapper or other furnace parts.
c. Be sure insulator covering the electrode wire connection on the
coil of the module board is in place and insulator behind module
board is in place.
d. Make certain that the transformer voltage is within 24 - 30 volts
A.C.
e. Be sure duct connections to furnace are airtight. Seal duct collar
connections to furnace cabinet with duct tape, if necessary to
prevent hot air leakage. No air leakage should exist anywhere
in the duct system, especially at connections on furnace
cabinet.
f. Be sure sensor wire terminal is tightly affixed to sensor probe.
g. Be sure high voltage electrode wire is in good condition and
properly positioned onto pierce point electrode.
6. Customer complains of unit going into lockout only once in a
while:
a. Thoroughly check electrode and burner relationship.
b. Lockout can occur if the gas pressure fluctuates at the time the
thermostat calls for heat. Pressure fluctuations can be caused
by a malfunctioning gas bottle regulator, an obstruction or a kink
in the gas line, or moisture in the gas bottle regulator or in the
gas lines. It is difficult to check for these fluctuations that will not
noticeably affect any other appliance in the coach. However,

Suburban SF-25F Specifications

General

Brand	Suburban
Model	SF-25F
Category	Furnace
Language	English