GF - I 4
Main Limit Switch
Figure 7
Step 4
A zero volt reading indicates a closed switch (proper
condition.)
Step 5
A 24-volt reading indicates the switch is open.
Step 6
If the air temperature is below the switch open set-
ting, yet the switch remains open, replace the switch.
Causes of main limit opening during
furnace operation
• Low indoor air volume
• Undersized ducting
• Dirty return air filter
• Dirty evaporator coil
• Dirty blower wheel
• Failed indoor blower motor
• Failed run capacitor on indoor blower motor
• Improper operation of ECM variable speed
motor/wiring harness
• Faulty main limit switch
• A rare case would be over firing of the furnace
burners
Step 3
Place both meter leads across the main limit electri-
cal terminals (Figure 7).
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