# Furnace fan/blower won't turn on in AUTO mode



## pwindre (Oct 29, 2016)

I have an old Sears furnace - Model No. 867763240. With the fan switch on AUTO, the blower works fine in the AC mode. In the heater mode, the burners fire but the fan doesn't come on. If I manually switch to the ON position, the fan runs.

I replaced the fan/limit switch and verified that it works correctly. About 90 seconds after burners ignite, there is power coming out of the fan/limit switch (yellow line.)

I also replaced the Fan Switch Relay, but that didn't solve the fan problem.

Faulty control board? The problem is this board - 96845 has been discontinued. Does anyone know any universal board that might work in my furnace?

For a temporary fix, can I connect the yellow wire (in FAN MOTOR - HEAT) to yellow one in FAN SWITCH?

Thanks!


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## BayouRunner (Feb 5, 2016)

Isn't there suppose to be a relay that plugs into that board? Also what kind of thermostat do you have? I don't think the board is where your problem is but I can't see everything on your furnace. Normally the fan in the heat cycle is controlled by a device that senses the heat of the air around the exchanger on the old furnaces.


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## BayouRunner (Feb 5, 2016)

If you turn the dial on the silver box behind the gas pipe does the blower come on? You may have to slide a switch, remember your old settings before you move anything


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## pwindre (Oct 29, 2016)

BayouRunner said:


> If you turn the dial on the silver box behind the gas pipe does the blower come on? You may have to slide a switch, remember your old settings before you move anything


No, it doesn't. As mentioned in the post, 90 seconds after burners ignite, there is power on yellow line.

Yes, there is a relay that plugs into the board. I took it off for taking the photos.


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## user_12345a (Nov 23, 2014)

This furnace is *very* old - i mean, the schematic is hand drawn/written. It could be from the early 1970s. the heat exchanger is suspect and if you're lucky it's 65% efficient, probably less. you won't get more than that with a standing pilot and draft hood.

remove the black metal piece covering the burners and shine a bright light into each heat exchanger cell looking for cracks, holes. remove the blower assembly and look at the heat exchanger from there.

no guaranty you'll see everything but better to inspect what u can then to not.

if u insist on keeping it running....

Jump out the terminals on the board where the fan limit connects,to rule that and the wiring to the limit.

If the fan doesn't come on you need to see if the board is passing power to the heating speed tap.

With the furnace warmed up or the terminals jumped, check for voltage between the heat and neutral fan taps on the board.

If it's not passing voltage, change the board if you can even find one.

If it is the heating speed tap on the motor is bad.

OR set the thermostat fuel switch to electric so it brings on the fan with a call for heat. This won't be perfect, you'll get a cold draft when the unit starts, heat left in the furnace when it shuts down, but at least it will heat without having the fan on all the time.
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Edit:

i didn't catch the empty socket.

The relay could very well not have anything to do with heating mode.

it could be double throw where the power for the heating tap simply passes through but for cooling it applies power to the high speed tap.

you could potentially ohm out the relay with it disconnected.

you'll have to know which connections do what though.

Could be the relay or a bad trace on the board. or a bad blower motor speed tap.


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## yuri (Nov 29, 2008)

Post a pic of the relay. It looks like it may be a Honeywell R8222D or K or variant which is still available.


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## pwindre (Oct 29, 2016)

Here is the picture of the old and new relay.


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## BayouRunner (Feb 5, 2016)

Well if your relay has been replaced, and your fan control is good, I'd say either the board is bad or the low speed on motor is out. You really don't need that board. You could mount that relay and just wire direct. I could do it in just a few minutes, but It would take me a long time to tell you how to do it over the Internet lol. You just have to determine if it's a connection in the board or a motor winding issue. Do you get 110 volts to motor at any point during heat cycle? If you do then it's a bad connection in board. They do go bad where the relay plugs in. Some of the guys on here are a lot better than me at explaining how you could do it without the board.


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## user_12345a (Nov 23, 2014)

okay, so you changed the relay and it didn't work?

as i suggested, the power to the heating tap passes through the normally closed section.

try my other suggestions.

If the board is bad there a certainly ways to work around it and get almost full functionality including a/c fan switching.

Would void the safety certifications though.

There are fan centers; you would have to re-wire the furnace to use one.

The humidifier would have to be wired a different way as well.

But honestly this unit is well past it's expected lifespan. Unless you're low on money, in a hot climate or selling the house soon, replacement is the best option at this point.

When you can't get a board any more and the schematic is hand written, you know it's really obsolete.


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## BayouRunner (Feb 5, 2016)

Meant to say if you don't get 110 to the motor then the board is bad. The only thing I don't like about this site is I can't edit my post, always says I don't have permission


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## pwindre (Oct 29, 2016)

Thank you very much for a very detailed post. Reading your post a few times, but there are somethings that I don't understand.



user_12345a said:


> Jump out the terminals on the board where the fan limit connects,to rule that and the wiring to the limit.


Could you please explain how to do that?



user_12345a said:


> With the furnace warmed up or the terminals jumped, check for voltage between the heat and neutral fan taps on the board.


Do you know where are the neutral fan taps on this board?


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## pwindre (Oct 29, 2016)

user_12345a said:


> okay, so you changed the relay and it didn't work?
> 
> as i suggested, the power to the heating tap passes through the normally closed section.
> 
> ...


We are planning to replace it, but right now it's best if I could get it working for a year or 2. This furnace doesn't have a humidifier anymore, so there is no need to wire that. Thanks


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## BayouRunner (Feb 5, 2016)

Don't know where or even if the neutral goes to the board. It doesn't consume any electric so it's not necessary. Although your neutral is good, because the motor works on high speed. Try measuring the motor voltage in the heat cycle, once your yellow wire powers up, that will tell you where the problem is. Also most likely one side of that relay is most likely not being used. It's just a standard plug in and all connections are not used


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## user_12345a (Nov 23, 2014)

Do you have a/c? 

----explaining the post-------

on the schematic i can see the fan switch wires going to two tabs on the board.

A black and yellow going to two tabs, labelled fan.

Disconnect the black and yellow and jumper them with a wire with alligator clips. 

Restore power and see if the fan comes on. If it does, there's an issue with the wiring to the fan/limit switch. otherwise proceed with troubleshooting the board.



2. The fan is connected to terminals cool, heat and white.

When the furnace is warmed up, fan part of fan/limit switch closed, the board should be applying power to the heat terminal.

With the power on, door switch depressed you can very carefully check for 120v ac between terminals "heat" and "white".


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## user_12345a (Nov 23, 2014)

BayouRunner said:


> Don't know where or even if the neutral goes to the board. It doesn't consume any electric so it's not necessary. Although your neutral is good, because the motor works on high speed. Try measuring the motor voltage in the heat cycle, once your yellow wire powers up, that will tell you where the problem is. Also most likely one side of that relay is most likely not being used. It's just a standard plug in and all connections are not used


power goes to the board, neutral passes through it and to the fan motor, eac terminals.

the relay has power applied and cuts the connection to the heating fan tap on the board, applies power to the cooling tap.


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## pwindre (Oct 29, 2016)

BayouRunner said:


> Don't know where or even if the neutral goes to the board. It doesn't consume any electric so it's not necessary. Although your neutral is good, because the motor works on high speed. Try measuring the motor voltage in the heat cycle, once your yellow wire powers up, that will tell you where the problem is. Also most likely one side of that relay is most likely not being used. It's just a standard plug in and all connections are not used


When the yellow wire (from fan limit control to fan switch relay) powers up, there is no power from the heat tap to the motor. I'm using the Klein Tools Voltage Tester, so I don't know if that is accurate.  Thanks


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## pwindre (Oct 29, 2016)

user_12345a said:


> Do you have a/c?
> 
> ----explaining the post-------
> 
> ...


Yes, I have a/c and it works fine.

Thanks for the explanation. I'm going to get a Multimeter from Home Depot.


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## user_12345a (Nov 23, 2014)

where are u checking for power?

if it's already at the board from fan limit, u can ignore my instructions.

is it one of those non-contact testers? u can do a much better diagnosis with a voltage meter.

--------
I asked about a/c because you'll need a relay to keep on using it.

You can't just go through the fan limit and bypass the board.

Make sure it's not the motor or other wiring first.

But if it's the board you can bypass it with a single pole, double throw relay. we can help you deal with that.


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## pwindre (Oct 29, 2016)

user_12345a said:


> where are u checking for power?
> 
> if it's already at the board from fan limit, u can ignore my instructions.
> 
> is it one of those non-contact testers? u can do a much better diagnosis with a voltage meter.


Yes, I used Klein Tools non-contact tester. I checked for power at the yellow wire from the heat tap down to the motor. I don't have voltage meter atm.


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## BayouRunner (Feb 5, 2016)

user_12345a said:


> power goes to the board, neutral passes through it and to the fan motor, eac terminals.
> 
> the relay has power applied and cuts the connection to the heating fan tap on the board, applies power to the cooling tap.


I'm good user lol. I just didn't blow up the picture and look at it too hard. I've been doing this stuff for over 40 years now. Just knew he didn't have neutral troubles. 

I'm guessing that one of those relay sockets are not making contact or the back of that board is burnt since your not getting voltage to the motor on low speed. You have everything you need with the exception of spades to get you by until you replace your furnace. Take a good look at that exchanger of what you can see and watch your flames when the blower starts up. That is an old one.


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## supers05 (May 23, 2015)

Swap the "cool" and "heat" taps, (purple and yellow wires). If the fan runs in heat, albeit on high speed, then your motor is shot. 

You'll have to switch the wires back for AC, but the motor may have one more tap available to try. 

Cheers!


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## user_12345a (Nov 23, 2014)

it's tough to know what background/experience people have online.

---------
You have everything you need with the exception of spades to get you by until you replace your furnace.

-----------

Are you suggesting re-using the relay but connecting wires to it?

i was going to suggest a full size relay with a mounting bracket.

===============
just swapping the leads first is a good idea actually. u don't need a meter for that


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## BayouRunner (Feb 5, 2016)

user_12345a said:


> it's tough to know what background/experience people have online.
> 
> ---------
> 
> ...


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## pwindre (Oct 29, 2016)

supers05 said:


> Swap the "cool" and "heat" taps, (purple and yellow wires). If the fan runs in heat, albeit on high speed, then your motor is shot.
> 
> You'll have to switch the wires back for AC, but the motor may have one more tap available to try.
> 
> Cheers!


I just did that - Swapping the "cool" and "heat" taps, but the fan still didn't run in heat. It ran when switching to Fan On mode.

Also, I just noticed that the old relay doesn't have solder joints on the bottom row. Is it normal? They both have the same part # White Rodgers 90-340 Type 91 DPDT Relay.


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## supers05 (May 23, 2015)

pwindre said:


> ...
> Also, I just noticed that the old relay doesn't have solder joints on the bottom row. Is it normal? They both have the same part # White Rodgers 90-340 Type 91 DPDT Relay.


Doesn't matter. 

As for the fan part. There's clearly something missing from the circuit diagram and the wiring diagram. I guess back then they only needed the heating half on the diagram. How did you wire up the relay? Any pictures of it actually in your furnace would help. 


Cheers!


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## supers05 (May 23, 2015)

Pull the board out as best as you can and take a picture of the back. You'll probably find a burnt wire or trace. Looks like some water got into that area. 

Cheers!


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## user_12345a (Nov 23, 2014)

supers05 said:


> Doesn't matter.
> 
> As for the fan part. There's clearly something missing from the circuit diagram and the wiring diagram. I guess back then they only needed the heating half on the diagram. How did you wire up the relay? Any pictures of it actually in your furnace would help.
> 
> ...


i don't see a problem. everything is there.

the relay plugs into the control board.


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## pwindre (Oct 29, 2016)

supers05 said:


> Doesn't matter.
> 
> How did you wire up the relay? Any pictures of it actually in your furnace would help.
> 
> ...


I just plug it in the plastic thing. The row without solder joints is at the bottom (actually, that 2 taps don't connect to anything.) Thanks


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## supers05 (May 23, 2015)

user_12345a said:


> i don't see a problem. everything is there.
> 
> the relay plugs into the control board.


I think i was looking at it at a weird angle. I see it now..... Doh. <smacks forehead> 

Electrical diagram still doesn't include g.  

Cheers!


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## user_12345a (Nov 23, 2014)

if you click to enlarge the picture g is there.


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## supers05 (May 23, 2015)

pwindre said:


> I just plug it in the plastic thing. The row without solder joints is at the bottom (actually, that 2 taps don't connect to anything.) Thanks


Yeah.... It's been one of those days..... 
Anyways, that board is dirt simple. It's just point to point connections on the back. I'm guessing some water or bug caused a short and killed a connection. 

Cheers!


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## supers05 (May 23, 2015)

user_12345a said:


> if you click to enlarge the picture g is there.


I'm sorry, clearly, I wasn't specific enough. What on earth does a little letter do for us? I'm hoping all it does is go to the relay coil, which returns to common. That isn't shown on the ladder diagram. I hate connection diagrams. 

Cheers!


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## supers05 (May 23, 2015)

user_12345a said:


> This furnace is *very* old - i mean, the schematic is hand drawn/written. It could be from the early 1970s. the heat exchanger is suspect .... .....


So? 

My heat pump (which is only used as an AC now, also has a hand drawn diagram. It's still going strong. If it wasn't for our electricity rates, I'd still be using it for heat. Some of our RTUs have have drawn diagrams, and yes some are passing 35 years. (They are getting fairly rare though) They really don't make them like they used to. 

The only thing I'd be seriously worried about is the heat exchanger. It's a safety and a reliability aspect. After that, it's really up to the owner whether the ROI is there. (IE. in the south or on the coasts, where they only see 100 heating hours a year) 

Cheers!


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## user_12345a (Nov 23, 2014)

the ladder diagram is incomplete for sure.it's also missing the safety switchs. it didn't make sense so i just looked at the other diagram. i don't like ladder diagrams.

i'm sure the g terminal has a trace to the relay socket which would have a trace back to c.

for furnaces old means a lot -> heat exchanger and low efficiency. it makes no sense to replace something just because it's old, but if the parts are obsolete and it's wasting hundreds per year in fuel, or the heat exchanger is leaking it shouldn't be kept running.

Heatpumps and ac units are different; no monoxide risk. Want to keep an energy-pig running? go ahead. where we are the cooling season is short so efficiency isn't overly important. well, unless you're the type who needs it at 68f inside from may to october.


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## supers05 (May 23, 2015)

user_12345a said:


> .... well, unless you're the type who needs it at 68f inside from may to october.


Only outside on the roof.  wanna join us? 

The only safety in the furnace is there, the high limit. High voltage side, cuts out the transformer. Not the most safety devices..... But it met regulations at one time.... 

Ladders labeled well are the only way to work on larger appliances. A connection diagram would make you crossed eyed. 
Cheers!


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## supers05 (May 23, 2015)

Pwindre: we get a bit bored sometimes. Anyhoo, I still think you have a wire lose in the back of that board. (opposite of picture) 

The ladder wiring diagram makes it easy to re-wire if your really need to. You'll have heat, one way or another. 

I do believe you can go straight to the "fan switch" if that's the label there, as your original question asked. I 99% sure it's from fan control. It'll bring on your fan after your furnace heats up. 

You really need to look at that heat exchanger for cracks and holes. Do the best job that you can. It's your own heath on the line, not ours. I'd start budgeting for a new furnace sooner rather then later. 

Cheers!


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## user_12345a (Nov 23, 2014)

to use the fan switch without the board the power has to be fed through the relay for a/c and stopping more than one tap from being energized at once if the limit sticks closed.

kind of like this...


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## pwindre (Oct 29, 2016)

supers05 said:


> I still think you have a wire lose in the back of that board. (opposite of picture)


I'm afraid that I will cause more damage trying to pull that board out.



supers05 said:


> I do believe you can go straight to the "fan switch" if that's the label there, as your original question asked. I 99% sure it's from fan control. It'll bring on your fan after your furnace heats up.


Are there any safety concerns if I do that?



supers05 said:


> You really need to look at that heat exchanger for cracks and holes. Do the best job that you can...


I definitely will inspect the heat exchanger today.

Thanks


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## pwindre (Oct 29, 2016)

user_12345a said:


> to use the fan switch without the board the power has to be fed through the relay for a/c and stopping more than one tap from being energized at once if the limit sticks closed.


Thank you for the wiring instruction. It is not as simple as I thought.

Is there an easier way to wire it if I don't need the a/c? Your discussions scare me, I might have a new furnace installed before the summer.


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## supers05 (May 23, 2015)

pwindre said:


> I'm afraid that I will cause more damage trying to pull that board out.
> 
> 
> 
> ...


None that i can see from the diagram. 

Cheers!


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## supers05 (May 23, 2015)

As a test, after it's running. Pull off the wire to the limit. If the burners shut off, you're good. 

Cheers!


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## pwindre (Oct 29, 2016)

supers05 said:


> As a test, after it's running. Pull off the wire to the limit. If the burners shut off, you're good.
> 
> Cheers!


I'll try that. Thanks


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