12
Midco
International
Inc.
8470 70
Service & Trouble Chart
The Thermal Switch is a heat sensitive single pole double throw snap switch that energizes the
blower motor to provide cooling air if, because of a heat back up through the burner when the motor
is off , the temperature of the blower housing reaches 160° F. The thermal switch is not wired into the
gas valve circuit and does not turn valve off when activated by heat back up.
The switch mechanism is a bi-metallic disc that snaps over when it is heated to the set temperature.
The switch is mounted so that the disc bears against the side plate of the blower housing.
Testing the switch to prove that it is functional can be done by removing the ignitor assembly and
blowing hot air through the opening toward the blower inlet. If hot air is not available, remove the
switch and check for continuity. With the switch cool, there should be continuity between terminals 1
(black wire) and 3 (red/white). To simulate the “hot” condition, place the switch on a fi rm surface and
apply fi nger pressure to the switch disc. When the disc “snaps”, there should be continuity between
terminals 1 and 2 (yellow/black wire).
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Make sure the switch or timer is calling for heat. Defective wiring or loose connections can simulate
outlined below. Check associated wiring before replacing a component.
ELECTRICAL AND FLAME CHECKS MUST BE MADE IN THE ORDER LISTED.
A . Confi rm 120 volts between strip terminals.
1. L2 and L1: No voltage, fused disconnect off or fuse open.
2. L2 and 3: No voltage, safety controls open.
3. L2 and 10: No voltage, burner switch or timer open and/or remote switch or timer open.
4. L2 and 4: No voltage, optional high or low gas pressure switch is tripped.
5. L2 and Thermal switch terminal 1 (black wire):
a. No voltage, thermal switch is defective.
b. Voltage present, defective Motor.
A. Make certain Flame Safeguard lockout switch is not tripped.
CAUTION: Tests are valid only during the 4 or 10 second trial for ignition. Main Manual
Shut-Off Valve must be closed and Manual Ignitor Valve open.
1. Confi rm 120 volts on strip terminals L2 and 5.
a. Voltage present: Continue to Step 2.
b. No voltage:
J81A - Motor interlock switch defective.
J121A - Blower air switch defective.
2. Confi rm 120 volts between strip terminal L2 and terminal 6.
a. Voltage present, continue to Step 3.
b. No voltage, fl ame safeguard defective.
3. Confi rm 120 volts on strip terminal L2 and 7.
a. Voltage present, continue to Step 4.
b. No voltage, fl ame safeguard defective.
4. Check for ignition spark (spark length approximately 1/8″).
a. Between spark generator hi-voltage terminal and ground: No spark, defective spark
generator.
b. Between spark electrode wire and chassis ground: No spark, spark generator defective.
c. For checking spark gap between spark electrode and ground barrier, refer to Part 2
Service VIII Ignitor and Regulator Assembly, Spark Test.
5. Connect manometer to ignitor gas pressure test tap, and check pressure during trial for
ignition period.
a. No pressure, confi rm that pressure of at least 5.5″ W.C. is available at the inlet of the
ignitor gas valve, and repeat Step 5. If still no ignitor gas pressure, ignitor gas valve
defective.
b. Ignitor gas pressure 3.0″ to 4.0″ W.C., continue with Step 6.
c. Adjust the ignitor regulator to 3.5″ W.C. while gas is fl owing.
XIV Thermal Switch
Trouble Chart
1 Motor Will Not Run
2 Motor Runs
Continuously But No Flame
and Burner Housing Cool
!
