Payne, Carrier Blower fan will not shut off

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  #1  
Old 06-30-15, 07:50 PM
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Payne, Carrier Blower fan will not shut off

Model: 394GAW024050
year: 1979

Blower fan will not shut off when switched to Auto (or ON)
When it ON mode it runs faster than in auto.

I have browsed these forums and have a basic idea of what to look for but still unable to identify my problem.

what I do know is this:
1) tstat wires are good - no shorts
2) transformer is outing 28.8 V

I began testing the limit switch but inconclusive.
attached is an image of the control board - any ideas on my next step?

many thanks.

image to follow -
circuit board: 302075-302
* having problems uploading - I will supply soon
 
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Old 06-30-15, 08:59 PM
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The board only allows 50kb picture files. It will resize large pictures but not LARGE pictures like from a cell/iphone. Those pics are 2-3mb in size.

The following should be your board.

Name:  $_57.jpg
Views: 381
Size:  44.9 KB

Why is testing the switches inconclusive ? They are either open or closed(shorted). They should be shorted in the normal state.

Since your blower speeds up when set to FAN ON that means it's the heating system that's keeping the blower on. Your control board has a time delay built into it for the heating blower. That means that board is most likely defective.

I'm seeing 1978 as a production date for this series furnace.That's over 35 years old. Make sure you have the heat exchanger checked for cracks. It would be a good idea to have a pro check out the furnace for integrity. If you don't have a service company.... try your gas supplier.
 
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Old 06-30-15, 09:38 PM
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yes, that looks like it.

testing was inclusive as I used an ohm meter set to Continuity/beep mode.
I tested across both sides of the diodes but never heard a beep - maybe I am doing it wrong?
I tested D2 and others but not really sure what is the limit switch and fuse link.

yes old but serviced a couple (5) years ago when the igniter was replaced.
 
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