Need to test blower motor, how?


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Old 04-10-10, 04:26 PM
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Need to test blower motor, how?

I have a old (25+ years) lennox furnace. About a week ago we had a power outage and later in the week when we tried to turn the furnace on it would not come on. When I set the fan in either heat or cool the blower does not turn on. I started by testing the thermostat by connecting red to white, red to green, etc the blower still did not come on also the compressor does not turn on when the thermostat calls for it either. So I decided it was not the thermostat.

I have checked all the breakers and fuses and they are all fine. I opened the panel where the blower is and cant find a reset button on the blower motor but I don't think that the motor overheated because it was not in use when it broke. I also pulled the panel for the blower control off looking for a control board but there is no control board! All I can see is a variable potentiometer (A variable resistor, i have no idea on spelling?) a transformer, a relay, and a safety switch for the door. All of this is just fastened to a large cut in type box. There is a switch, a light switch type switch, but as far as I know it only controls the pilot light.

I have a simple multimeter but don't know where to start. I really don't want my grandma to have to pay someone to fix this. I have pretty good experience with DC but not so much with AC. This thing is in a cramped crawl space (with some nice dead kitties ) and is giving me headaches! I'm sure I can replace a capacitor or motor myself but I need to know how to test it. A wiring diagram or something similar would be PERFECT! It is a Lennox gas fired forced air furnace model number GS18Q3/ 4-100-3.

Thank you for any help! I'm stuck and don't know what to do!!! I can get pictures if it would help.
 
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Old 04-10-10, 05:06 PM
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I would start at your furnace ( indoor unit) Turn the Fan switch on your thermostat from Auto to ON.
If you are comfortable with using the meter,If your indoor unit has a wiring terminal strip check power with your meter one lead on C the other on R you should have 24vac present If so so check one lead on C and the other on G you should have 24vac
Post the results so we can proceed in the right direction.
 
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Old 04-10-10, 05:58 PM
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Are you sure about that model number? The closest thing I can find at Lennox is a GSR14-4,5 Series (CGA) (4/1/1985).

However, since nothing works, check the transformer first. You should have 120 going in and 24 volts (There abouts) coming out.

Does this link work without needing to log in?

http://tech.lennoxintl.com/PDFs/527855w.pdf
 
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Old 04-10-10, 06:20 PM
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None of the wires going to my thermostat have power, is that normal? I've tried measuring voltage across white to red and green to yellow and about any other combination I could think of and nothing worked. I thought the controller might just read resistance or something but I'm thinking it should have some voltage but I might be checking it wrong also.

I'm pretty sure about the model number, that's whats engraved in the panel that covers the blower control.

I have a 24V 4A AC transformer here, tomorrow (not enough daylight now, couldn't see what I was doing) I think I'm going to try to replace the transformer if I cant read any voltage across it. Should a 4A transformer be enough?

I also found that my motor is a 3 speed motor with white being common, red being low, blue is medium, and black is high. The thing that I thought was a potentiometer is a fan and main limit, to let the condenser and burner get to temperature before turning the fan on. (In my defense, It says potentiometer on it )

If I test the transformer and it reads good I think I will wire the motor to a extension cord and just plug it in using the red and white wires for a low speed. I think that would be a better idea since I don't think a extension cord could handle the amps this motor would pull on full speed.

I would like to get a list of stuff to check that I could follow tomorrow, I appreciate all the help and welcome any more advice. This furnace is all under the house in a crawl space, there is no indoors unit but the thermostat. To get to the furnace I have to crawl through a tiny hole. I hope I don't have to replace the motor, I don't think I can get it through the hole to get it out

Thanks again!

Edit: I can view that link, but I don't think that matches my furnace.
 

Last edited by red07g5; 04-10-10 at 06:23 PM. Reason: .
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Old 04-10-10, 06:50 PM
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Do the burners come on: if yes, time to test motor & fan switch
If no: forget the motor for now. Do you have power coming to furnace? Sounds from your post nothing is happening, time to recheck breakers. You said they were good. How by looks? Try turning the breaker off, actually past off and then to on position. Don't just use another transformer. Test for voltage with your meter.
 
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Old 04-10-10, 07:05 PM
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I do think the furnace has power, the switch that turns the pilot light on and off works although I'm not sure if the burners come on. I know when I set the thermostat to cool the compressor does not come on.
 
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Old 04-10-10, 07:35 PM
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That is the closest model I could find, no 18's at all. Look inside the furnace, might me a sticker in there with more info, other numbers. Last resort is to call Lennox and have them email a diagram.

Sometimes we can't see the forest for the trees. Other times we are so busy looking at the forest we don't notice the trees.

Your first post tells the tale. Everything worked. Power goes out, power comes back. Nothing works.

If there is no common terminal at the thermostat you can't check for 24 volt control voltage.

Back under the house you go my friend.

First things first.

There is a terminal block in the unit where the 120 volts power is connected to the unit. 120 there?

Next, is there 120 going to the door safety?

Close the switch, use electrical tape to keep it closed. Is there 120 coming out?

Move to the transformer. There are two sides, primary (120) and secondary (24). Check these voltages. If there is 24 on the secondary look for a fuse. Good fuse? Go to the board. Common and "R" on the output to the stat. 24 to the board no 24 out to the stat bad board.

Personally I think your transformer got whacked when the power went out or when it came back.
 
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Old 04-10-10, 08:22 PM
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I'm hoping it is the transformer, that would make it so much easier.

By common terminal on the thermostat you mean ground right? If so I think that white is common on the thermostat. Red calls for heat, yellow for AC, and green for fan. If all the above is true and the transformer is good but the blower is bad you should have a reading of 24V between white (common) and green (fan) right?

When you say board, what are you talking about? The fan control? This furnace doesn't really have a board, only a fan control, a black case with silver rotating wheel and white push in manual pull out auto switch in the lower left hand corner, a block that looks to be a series of relays, and a transformer. Unless I missed something, highly possible, I didn't see a fuse, I would think it would be a glass tube type most likely right?

Now as for checking for voltage I'm assuming the case is ground right. I should just be able to probe from the case to the points you mentioned right? Or do I need to use the common from the terminal block? (I hate AC, DC is so much easier )

Last but not least, I can't break anything by probing around right? I think this is only a 10 amp unit so I don't think I could hurt myself either right?

And why do I keep getting a message saying I have empty profile fields, I don't think I have any!
 
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Old 04-11-10, 06:51 AM
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Originally Posted by red07g5 View Post
By common terminal on the thermostat you mean ground right? If so I think that white is common on the thermostat. Red calls for heat, yellow for AC, and green for fan.
You don't mention a "C" terminal. So I assume your stat doesn't use one. Manual mercury bulb or battery operated I guess.

"R" is 24v hot.
"Y" is cooling
"W" is heat
"G" is fan
"C" is (would be) common. Or if you think in 120v terms neutral or ground.

Originally Posted by red07g5 View Post
When you say board, what are you talking about? The fan control? This furnace doesn't really have a board, only a fan control, a black case with silver rotating wheel and white push in manual pull out auto switch in the lower left hand corner, a block that looks to be a series of relays, and a transformer.
Sorry, thinking of a newer unit with a control board.
The fan control is used in heating mode. Actually called a fan/limit switch. The silver dial has 3 push in pins in it lined up to temps. Don't try to adjust if you have never done it, easy to break! From lowest to highest the temps are, fan off, fan on, high limit.

On a call for heat W and G are energized through R. "W" starts the furnace and "G" energizes the fan relay. The fan/limit hold the fan off. If you watch the silver dial will start to rotate. When the furnace heats up to "fan on" (the middle temp) the switch turns on the fan. You'll see the dial rotate back a little cooler.

Should something happen and the fan shut off, on air flow through the unit get cutoff, the dial will continue to rotate toward the highest temp (limit). If it gets this hot the furnace will cut out but it should continue to try running the fan.

With the furnace off (either normally or on a limit shutdown) the dial rotates toward cooler temp. When the lowest temp is reached (fan off) the switch cuts the fan off.

Originally Posted by red07g5 View Post
Now as for checking for voltage I'm assuming the case is ground right. I should just be able to probe from the case to the points you mentioned right? Or do I need to use the common from the terminal block? (I hate AC, DC is so much easier )
Here you do need to be careful. 120v kills more people than any other voltage out there!

Best thing to do is kill power and investigate what your looking at and where to take readings.

First thing to look for is 120v coming in. In a perfect world you'll have hot, neutral, and ground. Should go to some sort of terminal block in the unit.

From there the primary side of the transformer should be connected. Either trace from the block to the trans or from the trans to the block.

The other side of the trans is secondary 24v. If one of these wires is grounded, you just found "C". If not figure out which wire ends up connected to the "R" terminal at the stat.

There should be some sort of low voltage terminal block in there also. If not, you need to find out which secondary wire from the trans are "R" and "C".

At this point you should know where power is coming in.
Where the "R" and "C" terminals are.
Where the stat wires are.

Turn the power back on. Verify 120v in. Then check for 24v at "C" and the first measurable point for "R". No 24v buy a transformer. There may be a fuse in there, that old it would be a glass tube type.

If you do have 24v, we need to find where is going and where we loose it. You should be 24v between "C" and "R" going to the stat. With the stat in heat, you should have 24v between "C" and "R" going to the stat. Between "C" and "W" and "C" and "G" from the stat.

If 24v is going to the stat through "R" and not coming back through "W" and "G" it's the stat.

If it's not the trans, and not the stat, we need to start looking into the relays and other controls in the furnace.

Now we could use a diagram!
 
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Old 04-11-10, 06:55 AM
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Originally Posted by red07g5 View Post
I do think the furnace has power, the switch that turns the pilot light on and off works although I'm not sure if the burners come on. I know when I set the thermostat to cool the compressor does not come on.
You didn't say this before. If the compressor comes on the trans is good. Jumper "R" to "G" at the furnace and see if the fan starts.
 
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Old 04-11-10, 07:06 AM
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Found it! I was looking in wiring diagrams only.

This is the installation manual. Wiring diagram starts on page 14.
http://tech.lennoxintl.com/PDFs/501942.pdf
 
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Old 04-11-10, 10:34 AM
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Awesome, Thank you!

Alright, now I think I can follow this thing pretty easy. I'll keep you posted of my results. Thank you so much for your help!!
 
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Old 04-11-10, 10:43 AM
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Originally Posted by red07g5 View Post
Last but not least, I can't break anything by probing around right? I think this is only a 10 amp unit so I don't think I could hurt myself either right?
It doesn't matter if your furnace draws 1 amp. If you have a 15 amp breaker at the box, that is what can bite you - just so you know, since the subject was raised.
 
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Old 04-11-10, 04:56 PM
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Well, I thought my furnace was getting power but it turns out it isn't. I feel stupid

Anyway, I figured I would check the breaker which I had flipped on and off a couple of times to check it. I tested across every breaker 1 at a time to the common strip and they all have voltage there so the breaker isn't bad as far as I know.

We just caught a mouse about the time I think the furnace quit. Is it probable or even common for a mouse to chew a wire without shorting or tripping a breaker? I'm running out of ideas again and my latest theory is a cut wire between the panel and furnace. It seems like the only possible way to have voltage at the breaker and none at the cut in box by the furnace.

I must seem like a idiot thanks for putting up with me!
 
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Old 04-11-10, 07:07 PM
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Originally Posted by red07g5 View Post
I must seem like a idiot thanks for putting up with me!
No, you don't seem like an idiot! I, at least, understand that you are a guy who probably works in computers and understands DC. Trying to help your grandmother save money and get this system working.

Originally Posted by red07g5 View Post
Well, I thought my furnace was getting power but it turns out it isn't. I feel stupid
Don't feel stupid. You came here to ask us, didn't you. But, you need to clarify here. You said in one post that if you turned on cooling the outdoor unit came on. In a single trans system the furnace must have power to turn on the a/c.

Are there separate cooling and heating transformers in this system? I don't think so but information provided does seem to indicate. If the furnace has no power and the outdoor unit starts, must be another trans to make this happen. Clarification?

Perhaps it is possible for a mouse to chomp through a wire, get a jolt and run off. Leaving you with a loose connection. To be sure you will need to inspect the entire wire run.

Do you have power to the furnace cut off switch, (disconnect) the light switch? perhaps this is the culprit.

A/C as with D/C you have 2 options. Load to source and find when the voltage (potential) starts. Or source to load and find where the voltage stops.

So, here comes Monday. Keep troubleshooting yourself or pay someone to do it for you?
 
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Old 04-12-10, 10:08 AM
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I don't think they are on a separate circuit. I thought the pilot might have power still because I could smell gas. I turned the valve from on to off while I was working on the unit just so I wouldn't gas myself out or blow myself up. Does the gas valve need electricity to keep the gas for the pilot running? If it doesn't that would explain why there could be no power and I would still smell gas.

I guess I'm going deeper under the house. I had this neat little tool I wish I could find now, it beeps if its anywhere near AC but you don't actually need to connect it to anything, just point it at the outlet breaker wire etc in question.
 
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Old 04-12-10, 02:57 PM
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Hmm. Your furnace has a standing piot. Meaning the pilot should be on 24/7/365. See page 11 of the pdf.

When you light the pilot, it heats the thermocouple. The thermocouple keeps the pilot valve open, and in turn keeps the pilot lit.

It's an old way to do it, but it's safe. If the pilot goes out, the thermocouple cools and shuts off the gas. No BOOM.

OH, let me back up. Your "gas valve" is actually two valves in one body. Pilot valve and main valve. Pilot is operated by pushing the know in to light and kept open by the thermo couple.

The main valve is operated by an electricity. Elec opens a solenoid and gas flows down the manifold to the burners, meets the pilot flame and poof, heat is on.

All that said, if you smell gas you either have a leak or the pilot valve is stuck open. On the extreme side the main valve maybe leaking by.

But what happened to the pilot. Did it get blown out? Did you turn off the gas valve and then back on?

This old of a furnace, it would be a good idea to inspect the heat exchanger for holes. If there are holes in the heatexchanger the fan can blow out the pilot. Hole will also send flue gas into the house, Carbonmonoxide which means headaches at least and death at worse.
 
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Old 01-05-12, 10:32 AM
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Lennox, GS18Q3 / 4 - 100, almost the same problem but...

Nothing works, AC, burner, fan, but the pilot light is on. I have power to the unit via a light switch and screw in fuse.

I have power to one side of the fan door switch but not the other; therefore, no 120VAC to the transformer and no 24 VAC.

Here is what bothers me: the fan does not work with the door closed but when I open the door the fan comes on. Letís start here before I continue.

I greatly appreciate any assistance.
Thanks
 
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Old 01-05-12, 10:57 AM
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Please don't piggyback on an old thread. It's confusing for those who would like to help you. Try reposting on a new thread.
 
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Old 01-05-12, 11:11 AM
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Thank you

I was worried about that and was just about to post a new thread.
Thanks again
 
 

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