Single Stage Multi Position Furnace
Service Manual
21
440 08 2001 02
If a flame outage (I.E. loss of gas supply, blown out, etc.)
should occur during a run cycle (Main burner operation), the
ignition system immediately de--energize the main valve and
re--power the ignitor circuit placing the system back in to the
“Trial For Ignition” mode.
As previously, it will remain in this “Trial For Ignition” mode
(Ignitor powered and pilot valve open) either until the pilot
lights and flame is sensed, or until the call for heat ends.
The SV9541M system
is not sensitive
to furnace grounding
or line voltage polarity. Accordingly, you cannot experience
a lockout due to those reasons.
Assuming that the main burner did not experience any prob-
lems during the run cycle (normal operation) it would contin-
ue to operate as long as the call for heat remained.
Once the call for heat ended, the ignition system control
would immediately close the main and pilot valves, and de--
energize the electronic fan timer output.
De--energizing the electronic fan timer output causes the
“OFF” delay timing to begin, and when the pre--selected time
(90, 100, 140, or 180 seconds) expires, the blower will turn
off.
TROUBLESHOOTING
Malfunctions of the HONEYWELL SV9541M “Smart Pilot”
system may be easily diagnosed using a voltmeter and a
spare igniter/flame rod assembly.
The igniter itself can also be checked using an Ohmmeter.
Resistance of a “Good” igniter should be 10 Ohms or less.
See trouble shooting flow chart and the sequence of opera-
tion flow chart on pages 36 and 37 of this manual for addition-
al information on the operation and troubleshooting of this
system.
26. CHECKING FLAME CURRENT
The Honeywell SV9541Q Ignition system used in this furnace
series proves (verifies) flame via the Flame Rectification
method.
Flame Rectification is a process of converting Alternating
Current (A.C.) into Direct Current (D.C.) During the ignition
sequence, an alternating current (A.C.) Voltage is applied to
the Flame probe.
When the burner lights the flame conducts an electrical cur-
rent between the flame probe and the burner ground. Due to
the difference in size between the flame probe and the burn-
er ground area this current flows mostly in one direction. This
creates a pulsating Direct Current that flows back to the igni-
tion control proving flame.
This flame current (D.C. Microamps) may be checked (while
flame is present) using a D.C. Flame Sensor kit is available
from outside vendors.
27. CAPACITORS
Figure 31
Checking Capacitor
100
m¦
Microfarads
+
5
m
p
10
m¦
1000
m¦
10000
m¦
Capacitors are used for both the circulating (conditioned air)
blower motor and the exhaust (combustion) blower. Before
replacing one of these motors (assumed to be bad) the con-
dition of its capacitor should be verified, since it, and not the
motor, may be the source of the problem.
Before checking
any
capacitor, the supply power to the unit
should be turned “OFF”. The capacitor should then be dis-
charged (through a resistor) before testing. A 20,000 Ohm
2 Watt resistor can be used for this purpose.
The condition of the capacitor should then be verified with a
capacitor analyzer (one that indicated the capacitor’s value
in microfarads) rather than with an Ohmmeter. The reason
for this, is that an Ohmmeter test can only indicate if a capac-
itor is “OPEN’, or “SHORTED”, it cannot verify if its value (mi-
crofarads) is within an acceptable range.
Capacitor should test to within 10% of its rated value. Capac-
itors testing outside this range should be replaced. A weak
capacitor can be the cause of a motor failing to start.
