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?

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.

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.

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?

Depends on how it's wired.

Quote:

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Originally Posted by KeepItSimpleStupid

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.

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What ever happened to keep it simple stupid.lol

Bad limit most likely didn't cause the transformer to burn up.

Quote:

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Originally Posted by mygirlsdad77

bad limit most likely didnt cause the transformer to burn up.

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I would agree... though the transformer fuses stopped blowing once the switch was changed.

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

Wouldn't mind if you rated me

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..

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.

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.