SAIL SWITCH VOLTAGE CHECK
Hoping for help on troubleshooting a Furnace Sail Switch. Furnace was intermittently failing and now progressively worse. Blower would come on and after about 30 seconds would shut off - no audible at...
Forum Discussion
the proper way to check the sail switch is to measure the open and closed RESISTANCE. open resistance should be in the megohm range, closed resistance should be in the few ohms range.
Measuring the voltage at the sail switch with nothing for a load other than the DMM will NOT insure any validity to the switch. The DMM has to high of a resistance and won't show a voltage drop from a high resistance closed switch.
Measure the voltage across the switch when it is in the closed position and connected in the furnace when the blower engages is another valid way to check the switch. voltage across a closed switch should be millivolts.
Also the resistance of the sail switch in open position must be measured and it should be in the megohm range at least. Ideally it will be high enough that a DMM with a 20Mohm scale will read as open. It must be high enough that the board sees that the sail switch is open.
Measuring the voltage at the sail switch with nothing for a load other than the DMM will NOT insure any validity to the switch. The DMM has to high of a resistance and won't show a voltage drop from a high resistance closed switch.
Measure the voltage across the switch when it is in the closed position and connected in the furnace when the blower engages is another valid way to check the switch. voltage across a closed switch should be millivolts.
Also the resistance of the sail switch in open position must be measured and it should be in the megohm range at least. Ideally it will be high enough that a DMM with a 20Mohm scale will read as open. It must be high enough that the board sees that the sail switch is open.
