How to Troubleshoot a Furnace Blower That Does Not Kick On
A furnace blower motor that doesn't kick on when it should is usually a simple problem to troubleshoot. Blower motors are thermostatically controlled to turn on after the air in the fire box has reached operating temperature and to remain on after the air in the fire box drops below a preset temperature after the burner goes out. The basic circuitry is the same for gas-fired, oil-fired or full electric furnaces. The most valuable tool when troubleshooting any furnace problem is an electric wiring schematic, and there should be one on the inside of your furnace's door or service access panel. Does this Spark an idea?
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Things You'll Need
- DMM (digital multimeter)
- Test leads with insulated alligator clips
- Electrician's screwdrivers
- Diagonal pliers/wire cutters
- Wire strippers
- Lineman's pliers/electrician's pliers
- Needle-nose pliers
- 1/4-inch drive socket set
- Flashlight or work light
Instructions
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1
Switch your furnace disconnect switch to the Off position.
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2
Remove the fuse, and check for continuity through it using your DMM. If it reads open, replace the fuse with a new one of the same amperage rating.
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3
Remove one wire from your blower motor's circuit breaker, and check for continuity through it. If it reads "open," push the reset button and check for continuity again. If it still reads open, the circuit breaker is bad and needs to be replaced.
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4
Locate the blower motor thermostat and jumper across it with one of the alligator-clip leads, being careful that the clips don't short out against the furnace frame.
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5
Turn your furnace disconnect switch back on. The blower motor will start immediately if the thermostat is bad. If the blower starts right up, shut off your furnace's main disconnect switch, remove the old thermostat, and replace it with a new one.
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6
If the blower doesn't start with the thermostat bypassed, use your DMM and checked for line voltage at the motor's terminals.
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If there's no voltage between the terminals, check for a line voltage reading between the hot side and ground. A reading there will indicate that you have an open neutral, and you will need to trace out the neutral side of the motor wiring until you find the break. With line voltage at the blower motor's terminals, the problem lies with the motor itself.
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8
Turn off your furnace disconnect switch again before proceeding further. Before condemning the motor, check to see if the squirrel cage will turn freely by hand.
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If the squirrel cage turns freely, the motor is the culprit and needs to be replaced.
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Tips & Warnings
Always move from the simple to the most difficult when troubleshooting any electrical problem.
Squirrel cage blowers can become jammed or partially jammed by an accumulation of dirt in their bearings, and it doesn't take much to keep one of these low-powered motors from starting.
Furnace motors are relatively inexpensive and not worth the time and effort it takes to disassemble and rebuild a defective one.
