1989 RUUD fan limit switch

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  #1  
Old 02-08-19, 08:42 AM
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1989 RUUD fan limit switch

i guys! I have what I think is a 1989 model of RUUD furnace, gas fired, and it has worked great for a billion years. Yesterday the fan decided that it liked to LIVE and didn't want to cut off. Here's the furnace. I can't find a model number plate on it anywhere.









So I replaced the batteries in the thermostat. Still fan is always on. Then I checked the fan switch, it's on AUTO. So I swapped it with the old thermostat and it still stuck on.
Finally I removed the green wire altogether and the fan STILL stayed on. This tells me that it's the fan limit switch, from what I can see. Now my problem is - where is my limit switch?

I found the burner and where the connectors from the thermostat all connect in here:

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Here's the connector. Is this my limit switch?













And the fan itself was in the top half here with a capacitor and a bunch of wires. There are two wired mounts on the fan itself. The capacitor one that (I think) controls fan speeds, and the other one closer to the camera here that I don't know what it does.






Finally, there's this one hidden on the bottom left INSIDE corner of the fan with 2 wires running to it. It was harder to take this picture, but the wires running through the dust on the floor and past the fan motor are the ones showing the target. Not the ones in the foreground:








Can someone point out the switch that controls the fan so I can swap it out? Thanks!
 

Last edited by PJmax; 02-08-19 at 06:18 PM.
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  #2  
Old 02-08-19, 03:26 PM
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The limit switch won't keep the fan running. Try removing stat and see if the fan shuts down.
 
  #3  
Old 02-08-19, 04:07 PM
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Thanks Skaggsje, but that was the first thing I did
I even swapped it for the old thermostat and it ran the whole time.
 
  #4  
Old 02-08-19, 05:29 PM
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The model number should be in the burner section. I can't see the hi-limit switch in the pics.
 
  #5  
Old 02-08-19, 06:24 PM
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In a newer furnace.... an open limit could keep the blower running. Not in that dinosaur.

I labeled one of your pictures.....
The fan center is where the thermostat and 120v lines meet.
There is a relay in there that could possibly be stuck but doubtful.

The blue arrow is where the ID tag may be.

The pink arrows are where the fan/limit switch would be.

The orange arrow points to a connection that may be an issue. Those two wires are twisted together. They may have been connected to one half of the fan/limit switch. They are definitely in the gas valve line.

The limit switch on the blower housing is mostly there if the furnace is used horizontally. It's not likely trip in your configuration.

Keep looking for the model number. It's on an ID tag there somewhere.
Check the back of the blower door for a wiring diagram.
 
  #6  
Old 02-08-19, 06:58 PM
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Thanks Pete, but your pic didn't come through. Can you post it again?
There's a diagram on the blower door, but it doesn't seem to have the model number.
 
  #7  
Old 02-08-19, 07:29 PM
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I downloaded, labeled and reloaded one of your pictures. (The second one)
Post a picture of the diagram.
 
  #8  
Old 02-09-19, 07:48 AM
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Ahh! I didn't know you could change my post. Ok, so the orange arrow points to the twisted wires that were on the anti carbon monoxide switch that tripped every time we had high wind. we installed a CO detector and bypassed the switch.

The pink arrow going up is nothing but wires running along the roof there.

The blue arrow points to a tag I'll post a pic of in a minute, but I couldn't see a model number on it.

When I get it open I'll re-connect the CO thingy and see if that fixes it, but if I remember right that killed the fan, not kept it on.

Thanks for all the help PJ! I really appreciate it. I'll post the pictures in a few minutes after I get the kids fed.

MK
 
  #9  
Old 02-09-19, 09:52 AM
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The orange arrow was just highlighting an issue I spotted. It's not the problem with the blower.
The fan/limit switch would usually sit above the burners in the front panel.
 
  #10  
Old 02-09-19, 10:59 AM
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I bet that's it! I didn't see it before because it was covered up by a metal cover. Here's a pic of that thing











And my two cover panels:





















And that sticker on the right of the burners on the bottom:

 
  #11  
Old 02-09-19, 12:06 PM
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You're hired...... great pictures.

That model number on that tag is for the furnace base. Not a complete model. We're getting closer.

Yes..... you've spotted the fan/limit control. The wiring diagram shows two of the four wires going in to it are the red fan wires. Rather than try to explain how it works..... the grayfurnaceman has a good video showing it. More than likely yours will need replacement. Not really a DIY but watch the video and get a better idea. These switches run roughly $125-$150.

Fan/limit video



Thanks Gray..... good video.
 
  #12  
Old 02-09-19, 02:49 PM
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That's exactly what I needed. Thanks PJMax! now to pull it and get a replacement installed (or clean it up and see if it works!)
 
  #13  
Old 02-09-19, 09:25 PM
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Well I got the cover off the timer and confirmed there is a "click" for all of the marks until I get to the 80 degree mark, that one doesn't click at all (to turn it off).
I think that's the problem.

Now I see that my gas line is in front of it. is it long enough that I need to remove the gas line to get into it? Or can I just pull it off and be OK? The big ring connector was almost touching the cover, so much so that I had to bend the clips on the sides to slide it off.





If I do have to remove the gas line, I'm very much going to re-route that line away from the fan limit switch. The way it is now is just stupid.
 
  #14  
Old 02-09-19, 10:47 PM
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Wow.... that is miserable. You will have to remove the gas line as that switch has an 8-12" shaft that is inside the furnace. It will never get by that pipe.
 
  #15  
Old 02-10-19, 05:04 PM
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Just a thought, PJ, but can I replace it with one of those flexible plastic gas lines instead? That would make it even easier to move if I have to do this again and i wouldn't have to plan out a metal pipe route.

 
  #16  
Old 02-10-19, 05:45 PM
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That would not be an approved method for inside a furnace. The gas valve must be hard piped thru the furnace case. Once outside you could use flex.
 
  #17  
Old 02-10-19, 06:12 PM
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Ok, well it was a thought. I'll stick with the rigid pipe and put a few more elbows in it to run it around the switch. I'll post up how it goes just in case anyone else gets into this pickle.
 
  #18  
Old 02-13-19, 01:50 PM
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PJ if you're still around, I replaced that fan switch (I remembered to remove the jumper) and rerouted the gas line (it looks a lot better) but my fan is still running. I'm not sure what to do at this point.


I removed the thermostat and all the wires again to once again make sure it's not a faulty thermostat, but it's still running constantly, even when I switch the new fan limit switch to "manual". There's no way to turn it off short of opening the fan door, which trips a switch on the door and cuts power to the fan.

Any more ideas you have would be great. I'll post pics of the stuff I did (limit switch, gas re-route) tonight. It's going to be 70 today and tomorrow so we're safe for now.
 
  #19  
Old 02-13-19, 02:06 PM
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Oh, and guess what else I found?

 
  #20  
Old 02-13-19, 04:46 PM
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Ok, I found out the reason the fan wasn't turning off. It was a relay switch on the left wall that you labeled "fan center" in my second pic. A quick rap with a screwdriver got it flipping right again. But....now the fan won't turn on when the furnace kicks on.

I'm thinking it may be that wart looking thing on the back of the blower that's not letting the fan kick on because when I took it off to look at it, the center was crusty and burned.

It has two red wires going in, two black coming out.


I'm going to go into the limit switch with a voltmeter and see if I can trace the 24 volt (black) line and see where it's acting stupid. Wish me luck!
 
  #21  
Old 02-13-19, 05:47 PM
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Wow.... that furnace is over 28 years old. Make sure to have it occasionally check for internal rust/rust thru in the heat exchanger.

You shouldn't need to post any more pictures. Go to the fan center. I pointed it out in the labeled picture. Look for the G terminal and remove any wires on it. See if the fan still stays on.

Ok.... it's the relay. Do you have A/C in that furnace ?
 
  #22  
Old 02-13-19, 06:18 PM
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The fan works properly now that I flipped the relay with a tap. However, it doesn't work when the furnace is on. It stays OFF now.

Yes, we have AC. When the AC kicks on the fan also works fine, so it's just the heat that comes on without the fan. Also, if I switch the thermostat fan to "on" it works just fine too.

The only way it doesn't work right is if I turn on the heat. It kicks on the burners and the new limit switch twists like it should, then it clicks to turn on the fan, but nothing turns on. I turn it off before it gets to the 200 mark to trip the limit.

By the way, I'm giggling over this. At first I couldn't get it to turn off, now I can't get it to turn on (with the burners on anyway)!


Edited to add more info so we're on the same page.
 
  #23  
Old 02-13-19, 06:51 PM
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Here's the problem..... the relay is a form C or has a normally open and normally closed set of contacts. When the relay is resting...... the NC contacts are (or should be closed) to allow the fan/limit to control the blower. When you use the A/C.... the relay switches to the NO contacts for the blower. If you've jammed the relay.... it won't work for A/C either.

You're going to have to change the fan center too.
 
  #24  
Old 02-13-19, 07:33 PM
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Scratch previous reply. I'm looking for a fan center now but haven't found one yet.

Another update: I found the relay itself! Part number 42-21571-01 if anyone happens on this thread 500 years from now with a 530 year old furnace

Ordered and waiting for delivery on friday!
 

Last edited by MissKittKatt; 02-13-19 at 08:04 PM.
  #25  
Old 02-13-19, 08:01 PM
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Third one down.... White Rodgers is a good choice.
Fan centers - at large

You can temporarily bypass the relay for now if you want. Let me know.
 
  #26  
Old 02-13-19, 08:27 PM
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Thanks PJ! The White Rogers part number is ASIN B000LER8LO, Manufacturer's part number 90-118E. (again for future searches)

How do I bypass that relay?
 
  #27  
Old 02-13-19, 09:10 PM
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What I see on the wiring diagram. There will be five wires on the back of the fan center. The white and black together are 120v. The other three are the relay.

Red - NC - heat
Yellow - C
Black - NO - cool
I would turn power off. Cut all three wires off the old relay.
Cap off the black. Combine the red and yellow.

The new relay will have different colors but the functions will be the same and are listed.
 
  #28  
Old 02-15-19, 04:37 PM
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New relay put in, fan stays on all the time again. Arrgh!

What is that wart on the front of the blower? Could that be the problem?
 
  #29  
Old 02-15-19, 05:03 PM
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I don't know what that wart is. I'd guess it's a temperature switch for when the furnace is used horizontally. I can't get enough detail off the wiring diagram and I haven't found a manual for that furnace.
 
  #30  
Old 02-15-19, 09:45 PM
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If it blew, would that cause power to run to the blower? I pulled the green wire off entirely and it's still on, and if I disconnect the fan limit switch entirely the fan stays on. It's the only other place i see 2 red and 2 black wires on a device.
 
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