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Old 08-25-2015, 06:41 PM   #1
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Blower runs but stops when AC comes on


Hi!

I have a Lennox ML193 furnace with a Lennox 5 ton a/c unit outside and a filtrete 'radio thermostat'. Everything's been working absolutely fine until today.

Came home and turned the A/C on and the furnace circuit breaker tripped. Went downstairs to fix it, came up stairs, and now the problem...

On the thermostat, when I turn the fan on, the blower motor starts. If I flip it to turn the a/c on, the furnace blower motor stops and the ac unit starts outside and I can feel coolant traveling through the lines. I'll turn everything off at the tstat and wait a few minutes. The I can then turn the furnace blower motor on again.

I was wondering if anybody had any suggestions on what to look for to get this thing back working again.

Thank you in advanced.

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Old 08-25-2015, 06:54 PM   #2
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Blower runs but stops when AC comes on


When you turn the fan to ON it starts the low speed. When you turn on the AC it energises the outdoor unit and cooling fan relay at the same time to switch the fan off low to the cooling speed. It should never trip that circuit breaker. Sounds like a problem with the circuit board cooling relay shorted. Should have a 5 yr warranty but you need a Lennox dealer to get it. Or buy the board from cozyparts.com and it may be cheaper than paying a dealer for a service call and labor and to get it.

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Old 08-25-2015, 07:22 PM   #3
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Blower runs but stops when AC comes on


Thanks for the response. I've been googling and I'm wondering if it's a problem with the relay as well.

I'll also add that it only tripped the breaker the first time and the on-board fuse is still in tact and works fine. The only thing now is the fan turns off when the ac turns on.

Is there an easy way to test the relay to see if that's the problem?

Also, unfortunately, I think the unit is about 6 years old.

Last edited by wiz561; 08-25-2015 at 07:23 PM. Reason: added warranty.
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Old 08-25-2015, 07:48 PM   #4
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Blower runs but stops when AC comes on


Another update. I turned the a/c breaker off so it doesn't get power, disconnected the thermostat and jumped R -> G (fan runs) then R -> Y and fan stops.

Interesting...when I did the R -> Y, the relay does click.... So I wonder if it's something else.

Last edited by wiz561; 08-25-2015 at 07:52 PM. Reason: added turned ac breaker off
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Old 08-25-2015, 10:53 PM   #5
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Blower runs but stops when AC comes on


When you went to "fix it" the first time, did you do anything other then reset the breaker? Which breaker, the furnace/ah or condenser unit?

My guess is that the motor shorted on the high speed tap, but the low speed is still OK for now. If so, the shorted wire may have only made momentary contact with ground and is now hanging free inside the motor. Try one of the other wire leads on the blower.

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Old 08-25-2015, 11:17 PM   #6
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Blower runs but stops when AC comes on


Check for voltage between the cooling blower tap and neutral on the circuit board with the unit calling for cooling and the blower access panel switch depressed.

Assuming that y is wired to the board, if you have voltage there, the high speed tap on the motor is bad.

Otherwise, you need a new circuit board.

To get cooling temporarily if the board is bad, swap the heating (or continuous if the board has a separate continuous tap) and cooling speed taps around, remove both low voltage wires from the Y terminal and Margette them. The board will see the call for cooling as a call for continuous fan, but it will be running and at the right speed.

Disconnect and cap off the wire connected to W to prevent the heat from running at the wrong fan speed and get a new board before fall.

It goes without saying that iff you don't understand this post or aren't very comfortable/safe testing live electrical circuits, it's time to call a pro. I assume no responsibility for personal injury or damage to equipment.

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Old 08-26-2015, 06:08 AM   #7
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Blower runs but stops when AC comes on


Thanks for the response. I actually ran through your test last night and saw that the a/c (or cool?) lug was putting out 120v when the a/c was switched on. When it was switched off, it was putting out 0v.

Also, when the ac was turned off and the blower was switched to 'on', the blower was off. About a minute or so after the ac was turned off, the blower started back up again (the ac cycle finished).

I'm thinking that you are right in that it's the high speed tap on the motor that's bad. If that is the case, what is the fix? A new motor?
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Old 08-26-2015, 06:57 AM   #8
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Blower runs but stops when AC comes on


Yes, new motor. It's only a matter of time before the rest of the windings go. I assume that it's a psc motor. No heat sink design on the end, and capacitor mounted to its side? They are relatively cheap, and the can find universal versions. (They are sometimes not as easy to fit as the oem versions, but are cheaper and more available) Just match up the shaft diameter (probably 1/2"), the HP can be a bit higher but not lower, and the RPM should be fairly close. (Within 10%)

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Old 08-26-2015, 07:08 AM   #9
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Blower runs but stops when AC comes on


Thanks for the help and response. After looking it up, it looks like the control board may had been a cheaper and easier thing to change than the motor.

I didn't examine it, but I do know there's a capacitor on the side, like you mentioned.

If I called a hvac place to do this, can you guesstimate how much I should be paying? It looks like the motors online are a couple of hundred dollars, which kind of surprises me. I'll have to do a bit more research online too to see what's all involved with this...

Thanks again for your help.
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Old 08-26-2015, 07:54 AM   #10
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Blower runs but stops when AC comes on


Sounds like the motor is failing. Problem with diagnosing Lennox over the net is they have a mini breaker instead of a fuse for the circuit board on some models. I thought you reset the mini-breaker but you have a fuse on the board. Good thing we have a few more smart techs here. Very rare the motors fail like that.

cozyparts.com for Lennox OEM parts. OEM is much better IMO and it fits perfectly w/o adaptor bands. Get a new capacitor also.
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Old 08-26-2015, 02:03 PM   #11
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Blower runs but stops when AC comes on


If you have a smaller a/c like 1.5 or 2 tons, it can probably run just fine on the medium-high tap, especially if the blower is a 3+ ton drive. High is way too much for 1.5 btw.

If your ductwork is properly sized with the 3 ton drive even a 2.5 ton will be okay on medium-high.

Larger with lower airflow and the coil could freeze up.
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Old 08-26-2015, 02:24 PM   #12
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Blower runs but stops when AC comes on


Unfortunately, I have a 5 ton ac unit outside, and I doubt that the ductwork is properly sized.

I checked the prices of a new motor online and it looks like they hover around $300-$350. I called a local place up and he said if it's the motor, it would probably be about $450 to do it. I ended up accepting and he's suppose to swing by later today to take a look at it. If you throw overnight shipping in there, it might add another $100 bucks to it.

The local has the part in stock and ready to go.... I figured that I'll have him come out and worse case, if it is the motor, it will be about the same price as if I did it myself and overnighted the part.
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Old 08-26-2015, 03:25 PM   #13
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Blower runs but stops when AC comes on


Why not a universal motor and braket? would have set you back under 200.
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Old 08-27-2015, 11:03 AM   #14
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Blower runs but stops when AC comes on


In the end, I probably should had done this, but I've been wanting somebody to come over to look over everything to make sure it looks OK.

He came last night and looked over stuff and determined it was the motor. One test he did that wasn't suggested here was a continuity test between the neutral and the different speeds of the motor. He saw that the black (high) speed wasn't giving a reading while all the other colors (speeds) had some kind of continuity. Believe me too, I'm not knocking anybody on the forum because without your help, I wouldn't have been able to determine what the issue was. I'm just saying this to help somebody in the future.

So yes, it is going to cost more than a universal motor, but it was a gamble and if it goes out again in the future, he'll replace it for free for a year or so. I also don't have to worry about it fitting, the hp, balancing it, etc.... Also, I want to say that a OEM motor from grainger was about 100 bucks less than what he's charging me, and that wasn't including shipping. This guy has the part in stock and will install it for me, so all in all, I'm OK with it.

Thank you all again for your help. It was also great to know what the problem was so that I wouldn't be taken for a ride.

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