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    litefog's Avatar
    litefog Posts: 4, Reputation: 1
    New Member
    		  
    #1

    Nov 19, 2008, 04:16 PM
    Another Blowing Furnace Transformer !
    Furnace: Carrier Downflow-Gas 58DRC, circa 1988

    First symptom of problem: Blower would not shut off after heating cycle

    What has been done so far:

    Bypassed the limit switch and a fusible link to see if that was what kept the blower on. Blower stays on, so they appear OK.

    I then test the control board transformer's primary and secondary leads with a multimeter. I find there is an open winding on the primary side. So I replace the transformer (120v-24v) and turn back on the power. When I set the thermostat to call for heat, I hear the transformer sizzle. It's dead.

    I have now replaced with another new transformer with an inline 3 amp fuse on one of the secondary leads.

    I have also checked for loose & broken wires, no luck.

    What should I do now to isolate the problem?
    juggallojed's Avatar
    juggallojed Posts: 165, Reputation: 10
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    #2

    Nov 19, 2008, 06:39 PM

    Well disconnect everything and try one circuit at a time.

    Having no 24 volt will cause the fan to run so that is normal. Check the ohms on the gas control and the rest of that circuit.
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
    Uber Member
    		  
    #3

    Nov 19, 2008, 07:09 PM

    If you want to get fancy. Get two 12 V brake lite bulbs and wire in series and place in series with secondary. Pull wires until the light goes out.
    litefog's Avatar
    litefog Posts: 4, Reputation: 1
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    #4

    Nov 20, 2008, 06:01 PM
    Guys, thanks for your replies!

    Conclusion:

    It dawned on me that when I bypassed the fan limit switch to see if it would shut off the blower, the transformer was already blown. I never checked the switch with a "good" transformer. Anyhow, replaced the fan limit and all is working well!

    Question:

    Why would a bad limit switch cause a transformer burn out like that?
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
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    #5

    Nov 20, 2008, 06:17 PM

    Depends on how it's wired.
    mygirlsdad77's Avatar
    mygirlsdad77 Posts: 5,713, Reputation: 339
    Plumbing Expert
    		  
    #6

    Nov 20, 2008, 06:19 PM
    Quote Originally Posted by KeepItSimpleStupid View Post
    If you want to get fancy. Get two 12 V brake lite bulbs and wire in series and place in series with secondary. Pull wires until the light goes out.
    What ever happened to keep it simple stupid.lol
    mygirlsdad77's Avatar
    mygirlsdad77 Posts: 5,713, Reputation: 339
    Plumbing Expert
    		  
    #7

    Nov 20, 2008, 06:21 PM

    Bad limit most likely didn't cause the transformer to burn up.
    litefog's Avatar
    litefog Posts: 4, Reputation: 1
    New Member
    		  
    #8

    Nov 20, 2008, 06:33 PM
    Quote Originally Posted by mygirlsdad77 View Post
    bad limit most likely didnt cause the transformer to burn up.
    I would agree... though the transformer fuses stopped blowing once the switch was changed.
    mygirlsdad77's Avatar
    mygirlsdad77 Posts: 5,713, Reputation: 339
    Plumbing Expert
    		  
    #9

    Nov 20, 2008, 06:35 PM

    Have seen stranger things. Hope all works well for years to come. Take care

    Wouldn't mind if you rated me
    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 435
    Heating & Air Conditioning Expert
    		  
    #10

    Nov 20, 2008, 07:42 PM
    Some fan and limit controls have both the 120 volt blower circuit and the 24 volt low voltage limit circuit in the same control. It could be possible to jump the wrong wires and toast the transformer or the internal switching created a short taking out the transformer..
    T-Top's Avatar
    T-Top Posts: 1,871, Reputation: 100
    Ultra Member
    		  
    #11

    Nov 20, 2008, 08:19 PM
    Did you remember to cut the jumper on the new fan limit control if the control is using 24v limit for one side and 115v for blower operation. If not your sending a 115v to the 24v side.
    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 435
    Heating & Air Conditioning Expert
    		  
    #12

    Nov 20, 2008, 08:55 PM
    They are supposed to read the directions that come with the part. Woops I forgot no body reads the directions. LOL I am sure by now if it was a problem there would be another post but it is a very good reminder about that jumper. I missed that part.
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