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    Blower will not come on with A/C

    Asked Jun 2, 2009, 07:58 AM ó 3 Answers
    The blower will not come on when A/C is set to on. The compressor runs and I hear clicks from the relays but nothing happens. I jumpered the motor and it ran fine (it is bran new). By the way, the heater works just fine. I discovered a possibly burnt out diode with a 37 on it. Could this be the problem or simply a symptom of something larger?
    This is a Lennox heat pump hybrid M/N 80MGF 3/4-100A-5 single phase 13 years old with new compressor unit outside and new blower motor.
    Pic attached of possible burnt diode on control board.
    Thank you in advance,
    Grant

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    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
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    #2

    Jun 2, 2009, 08:03 AM
    I can't say whether the diode is causing the blower not to come on or not, but a burned out diode can definitely cause some problem, somewhere. Get another diode, 1N4005 would be a good one that is available everywhere, and jumper the faulty one. Make sure the orientation of the "band" is the same as the existing diode. If the motor comes on, you've found your problem. If it doesn't, I'd suspect something else on the circuit board that caused the diode to fail. There are a couple of relays on that board that could be the blower relays. The diode might be a protective diode.
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    grant newcastle's Avatar
    grant newcastle Posts: 2, Reputation: 1
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    #3

    Jun 2, 2009, 08:12 AM
    Quote Originally Posted by Perito View Post
    I can't say whether the diode is causing the blower not to come on or not, but a burned out diode can definitely cause some problem, somewhere. Get another diode, 1N4005 would be a good one that is available everywhere, and jumper the faulty one. Make sure the orientation of the "band" is the same as the existing diode. If the motor comes on, you've found your problem. If it doesn't, I'd suspect something else on the circuit board that caused the diode to fail. There are a couple of relays on that board that could be the blower relays. The diode might be a protective diode.
    Thank you Perito.
    I will try to find that diode today and pick it up on my lunch. I will jumper it tonight and let you know what I find out. I am hopeful to avoid buying a new control board. If anyone out there has more suggestions or things to try please do.
    Thanks again,
    Grant
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    Perito's Avatar
    Perito Posts: 3,139, Reputation: 150
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    #4

    Jun 2, 2009, 08:14 AM

    A 1N400# will also work. The last digit is the current rating. Any in that series should work.

    If you're good, you can check for an input signal to one or both of those relays. The signal should close the relays when calling for the blower. If the signal is there, but if the contacts don't closed (voltmeter check), the relay is faulty. If you can determine which are the output pins, you can jumper them. If the blower comes on, you know which relay is responsible for it, and you probably also know what the problem is.

    Be careful. It looks like the relays are rated for high voltage (there's a 240 VAC rating at 10-20 amps stamped on them).
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