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Old 10-19-2008, 02:41 PM   #1
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carrier furnace...no heat blower won't stop


hi new guy looking for some help with my carrier model 58gp100-8a. all help welcomed.

we turned on the heat and only the fan runs at low-no heat-no flames at burners with pilot light on. the fan keeps running on low unless i turn the power off. it will run no matter how the thermostat is set.so i changed the thermostat..same thing. decided to check the voltage at the thermostat. zero.

so i check things at the furnace. no juice at the terminals for the thermostat. i put in a new transformer that didn't change anything either. i firgure the only thing left was the curcuit board controler..so i replaced that and the same thing. nothing changed with any of the above.

thank you for any help
kevinp

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Last edited by kevinp; 10-19-2008 at 03:42 PM.
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Old 10-19-2008, 04:18 PM   #2
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carrier furnace...no heat blower won't stop


Usually a blown transformer can cause the problem you are having. IF a new transformer did not solve the problem, then look for a loose wire or connection in the control wiring. Even a blown control fuse.

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Old 10-19-2008, 05:48 PM   #3
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carrier furnace...no heat blower won't stop


Just a few questions about your problem,are you sure its the low speed thats running,how old is the furnace(approx.)? the reason i ask is that if a safety high limit opens(trips) it will enable constant fan to cool down the heat exchanger reguardless of the thermostat setting,usually high speed
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Old 10-19-2008, 05:51 PM   #4
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carrier furnace...no heat blower won't stop


thanks for the help. there was a new fuse on the new control board. just checked it and it is ok. triple checked all the wiring AGAIN and still the same thing.

the installation manual said to check for 24 volts between R and C terminals for the thermastat located on the control panel. i got no voltage reading there, but the installation guide didn;t say what to do if that was the case. the new control panel is icm-217

thanks again for any help and /or suggestions
kevin
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Old 10-19-2008, 05:54 PM   #5
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pretty sure it is low because i jumped 24 volts to the terminals for the thermastat and the one switch would click and the blower would go to high speed.
i think it is late 80's early 90's.
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Old 10-19-2008, 06:05 PM   #6
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where are you picking up the 24v to change the speeds of the fan if r-c shows no 24v?
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Old 10-19-2008, 06:17 PM   #7
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im gonna assume you meant icm 271 fan center check your limits i can almost guarentee one of the is open,are you comfortable reading a wiring shematic?
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Old 10-19-2008, 07:38 PM   #8
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i picked up the 24 volts from an external plug in adapter for a phone. i checked what it was putting out it was about the same so i tried it. i am alittle ok with schematics. how would i check the limits and would they be bad /wrong on a new fan center control card (icm 271) ?

and i really appreciate your suggestions..thanks

Last edited by kevinp; 10-19-2008 at 07:43 PM.
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Old 10-14-2009, 10:51 PM   #9
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carrier furnace...no heat blower won't stop


I'm posting this response almost a year after the initial thread was started, primarily to give anyone doing a search a solution to the problem, which I did not see a final resolution to. The same/similar problem was occuring with my own furnace. The thermostat seemed like it was working okay, but the fan blower would stay on even with the thermostat set to off. The AC compressor outside would not go on in cool mode, and no flames in heat mode. Just a constantly running blower.

Lots of searching on Google but no luck. Then I came upon a discussion about someone asserting the contactor coil on the compressor outside could cause a problem with the ICM 271 fuse. But my board was the original board, not a newer ICM replacement. My board is a Carrier 302075-304 part, which the ICM 271 (or 271C) is a replacement for.

That didn't make sense to me because if I removed the 2 low voltage wires (from C and Y) coming from the outside compressor, there would be no connection from the compressor to the control board, yet I still had the same problem.

I checked the voltage coming out of the transformer (blue and red wires going to SEC1 and SEC2 on my board) and I had voltage. But I was in luck, as my downstairs unit was an exact duplicate of the upstairs unit. So I took the board form the downstairs unit and put it in the upstairs unit. Same problem, so I know the board is not the issue.

I narrowed it down by looking at what was coming from the board to various components of the furnace (I am not an HVAC person, nor even someone who understands a circuit board). There are 2 wires going to the Gas control valve, but they don't connect to anything on the blower. There are 3 wires going to the blower itself, but they are just Common, and 2 line wires (1 red and 1 black), so no control wires there.

But low and behold there are 2 wires going to something just above the Gas control valve. A little more searching and I discovered this is the Fan Limit switch that was discussed in the earlier post I found on Google.

One wire has some sort of circuit at the end, just before the connector to the fan limit switch, and the other is just a straight wire. So I took the fan limit switch from the downstairs unit and put it in the upstairs unit, no luck. But could the switch in the upstairs unit have failed, and also blown this odd looking circuit on the wire that comes from the control board. Maybe it's an inline fuse of some sort.

So I took the wire from the downstairs unit and put it in the upstairs unit, with the good limit switch from downstairs, and the whole thing works like a charm. I then swapped the old upstairs limit switch into the downstairs unit with the old upstairs wire and now I have the same problem in the downstairs unit. So it seems the switch went bad and blew the inline fuse on the wire, or vise versa. Regardless, my upstairs unit now works and tomorrow I'm on my way to Johnstone supply for a new limit switch and the fuse on the wire (or whatever the heck it is).

Based on my other searching, seems like this limit switch is a $10 part and the inline fuse is probably less than a $1. If I had the above information ahead of time, it would take less than 10 minutes to fix the problem, hence the reason for writing this post in case anyone else has the same problem.

Happy DIY!

Last edited by SchnabelJS; 10-14-2009 at 11:04 PM.
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Old 10-24-2009, 12:23 AM   #10
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carrier furnace...no heat blower won't stop


Thanks for posting the late follow up, SchnabelJS. Im having this exact problem right now. I swapped out the controller board and limit switch and have been confounded. I will try replacing that little fuse or whatever it is. I thought it was just some sort of in line resistor. Thanks again for taking the extra time.
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Old 10-24-2009, 12:42 AM   #11
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carrier furnace...no heat blower won't stop


my in line fuse thing says MICROTEMP 4283A NZGT 141C on it. cant find any such component on line except under the microtemp site but no comparable device. did you have any luck finding a replacement?
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Old 10-24-2009, 12:42 AM   #12
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carrier furnace...no heat blower won't stop


Hey, good to know someone else is still reading this thread. I'm now more informed than when I wrote the rather detailed analysis you replied to.

The "thing" at the end of the wire, closest to the right lead of the high temperature limit switch, is called a "Fusable link". If you buy a replacement "kit" for your furnace, you'll probably get quoted prices ranging from $15 to $50. But I talked to a local HVAC supply here in Austin and they told me they have fusable link parts at various temperature ratings and they all cost less than $5. The one on my unit apparently has a limit of 280 degrees fahrenheit.

That is, if you have a flame rollout of from the heat exchanger (where something like a crack in the heat exchanger causes the flames to come out the front of the furnace), the fusable link will overheat and fail.

I tinkered a bit and found out that you can simply jumper the two wires going to the high temperature limit switch together and if your blower turns off, then the switch or the fusable link, or as in my case, both, are the culprits.

I'm still in the AC season, so I've got my leads jumpered and my HVAC system works fine. But I caution anyone reading this, the issue that caused the fusable link and the high temp limit switch to fail in the first place, needs to be discovered.

In my case, it turns out that the pilot flame had become too strong on my upstairs unit and was basically about 3 inches high, going up the side of the furnace. It should be about an inch high. I think I have to replace the gas control valve to solve that problem.

Note, if you go buy a new limit switch, you should be able to find it on the web for less than $20, not including shipping. When I called my local HVAC shop, they wanted to charge me $45. When I asked them for a price on a fusable link "kit" they wanted to charge me $20, but they said I could just buy a fusable link and solder it myself (careful, you need a pencil solder so you don't overheat the fusable link, which is temperature sensitive in the first place.

Hope this helps!
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Old 10-24-2009, 12:47 AM   #13
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carrier furnace...no heat blower won't stop


we135B,

I think your fusable link operates at 141 degrees centigrade. So just convert that to fahrenheit and call your local HVAC supply store to see if they have a similar one in stock. But, try jumpering the leads going to the high temp limit switch first. Make sure you attach the jumper on the fusable link wire so that the fusable link is not in the circuit, otherwise you haven't isolated it out.
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Old 10-24-2009, 01:21 AM   #14
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carrier furnace...no heat blower won't stop


i bypassed the fuseable link and it works! thanks for your help. you are a hero in the HVAC world! I was ready to throw in the towel and call the HVAC guy. hmmm. where shall i spend the $200 i saved??
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Old 10-24-2009, 06:35 AM   #15
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carrier furnace...no heat blower won't stop


Quote:
Originally Posted by we135b View Post
i bypassed the fuseable link and it works! thanks for your help. you are a hero in the HVAC world! I was ready to throw in the towel and call the HVAC guy. hmmm. where shall i spend the $200 i saved??
Finding out why it blew in the first place.

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