Lennox MPB35ST-NM-B Care And Operation Instructions Manual page 28

TROUBLESHOOTING THE ELECTRONIC IGNITION SYSTEM
START
• Turn Off Gas Supply.
• Ensure Valve Switch Is In ON Position.
• Disconnect Control Harness.
Insert A
End View of Control Harness Connector
24 Volt Hot
24 Volt
Switched
Check For Damaged or Missing
Terminals in Connector
Insert B
Igniter Terminals
Igniter will cycle off and back
1
on once during the 90 second
ignition trial. All voltage mea-
surements must be taken while
the igniter is powered.
28
• Check for proper volt-
age at control harness
(see insert A). Voltage
2
should be 24V between
pressure switch and 24V
common and 24V hot.
YES
• Plug control harness into
valve. Wait for internal check
delay.
24 Volt
Common
• Igniter warms up and glows
red.
YES
1
• Turn on gas supply.
• Pilot burner lights.
YES
• Main valve opens and main
burner lights.
YES
• System Is okay.
When measuring voltage
2
at connections, use care
to ensure terminals are
not damaged.
NOTE: DIAGRAMS & ILLUSTRATIONS ARE NOT TO SCALE.
CHECK
• Line voltage power
• Low voltage transformer
NO
• Limit controller
• Wiring
• With pilot burner cable con-
1
nected, measure voltage at
NO
valve HSI element output. 24V
2
nominal. (See insert B)
YES
• Replace igniter/flame rod
assembly.
YES
• Check that pilot gas is flowing. Wait
to ensure pilot gas tubing is purged. NO
NO
YES
• Measure voltage between 24V hot and
24V common leads to valve control.
Must measure at least 19.5 VAC with
igniter powered (see insert A). To
identify proper lead, this check must
be done with the valve control con-
nected and igniter powered.
YES
• Replace Pilot Assembly.
• Check that pilot flame makes good
contact with pilot burner flame rod.
• Check for good electrical connection
NO
through the pilot tubing.
• If both of the above are good, replace
igniter/flame rod assembly.
YES
• Cycle on/off switch off and back on.
• Main burner lights.
NO
• Replace valve.
• Replace valve.
• Check transformer and
NO
line volt supply.
NO
• Replace valve.