Lennox 80mgf blower fan not working

10 years ago

Hello,

I am having problems with the Lennox 80mgf gas furnace. The unit fires up but shuts down after approximately 1 minute. I think the blower motor is not coming on (I hear a click, possibly power rely to the blower motor, about 10 seconds before the gas shuts down). After it shuts off, one of the diag lights stays on and one blinks, I don't recall which is which.

I suspect the blower motor is not turning on, but do not know how to diagnose the problem. Can I feed power directly to the blower motor to check if it is working? If so how? The system is not wired for continuous fan operation and so I could not use that avenue.

Also when is the gas exhaust fan supposed to come on? I think in one try it did not turn on. Later I fed power directly to the motor but it did not help.

Any help would be greatly appreciated. Thanks.

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Comments (4)

klem1
10 years ago
Your problem is typical of what hvac technicains spend 40 hours a week diagonoising and repairing. I don't know what your profession is but put your situation in context with some asking a barber how to give themselves a hair cut or a doctor how to give themselves an electrocardigram.
The inquirer needs to already know the drill and only have an unusual glitz or perhaps need a sematic for what they are dealing with. Call a repairman before you make it worse,set the place on fire or electrocute yourself.
hkky
Original Author
10 years ago
I am an engineer by training. If you can't be constructive why respond?
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klem1
10 years ago
I do feel it's best to let somone fix it for you but maybe not.
Anyhow,here's where the problem seems to be.
YOU STATED "Also when is the gas exhaust fan supposed to come on? I think in one try it did not turn on. Later I fed power directly to the motor but it did not help."
The "gas exhust fan" is known as induced draft. If it didn't run when "fed power directly" the motor should be replaced.
My reluctance was for good cause which is recconizable to one who works in the trade. Far too much can be lost in translation while dealing with flamable gas or electricty. Dealing with both at once is invitation for disaster. Here is an example. When you said "Later I fed power directly to the motor but it did not help." That could mean the draft motor still didn't run,,,, OR,,,,,it could mean the blower motor still didn't come on. No harm,no foul but such way of comunicating is not good while working on gas and electrical. Engineers pride themselves on being exact so you should understand where I'm coming from.
hkky
Original Author
10 years ago
Thanks for your input. I certainly take my safety very seriously.

Later on last night I found the power venter circuit diagram and determined the motor did not turn because I was too lazy to crawl out of the crawl space to turn on the heater at the thermostat (I jumped the terminals at the heater) so the relay never received activation power.

Anyways, the problem is now fixed. The blower motor capacitor was bad, it only measured 1.5 microfarad instead of 5.