Bad control board?


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Old 11-17-09, 08:28 AM
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Bad control board?

Have an old 1975 Payne forced air gas furnace model #394GAW000075 that the blower wont turn off.

The original problem came about when my wife went to turn the the T stat off and it continued to blow air with no heat. Still will heat like normal if you turn the T stat up. Its an old T stat with just a dial for temperature, no fan control. Only 2 wires R and W. The only way to get the blower to shut off is to unplug the unit or open the blower door tripping the blower door switch.

Unhooked the R wire on the T stat... blower still runs
Unhooked the W wire on the T stat... blower still runs
Unhooked both wires on the T stat... blower still runs
So I can conclude that the T stat is ok?

I have 24vac on the power leg of the limit switch (red wire) and 24vac post the fusible link on the other side of the switch (red wire) So can I conclude that the limit switch is good?
And can I assume that the heat exchanger is ok?

I push the blower door switch on and off to power up the unit(blower comes on immediately) and can not hear, feel or see the HFR relay closing. My trouble shooting and to the best of my knowledge(which could be flawed) and research are pointing me to a bad control board?? Can anyone confirm this?? Or are there some more tests I can do to isolate the control board as the problem
Thanks!

here is a pic of the wiring diagram:
 
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Old 11-17-09, 09:21 AM
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Well the first thing I would suggest: Take a continuity test of the "fuse link".
There are 4 items that will cause this senario with that furnace:
1-bad fuse link
2-open limit switch
3-bad transformer
4-bad board

When taking a continuity test of any component be sure to disconnect at least one from it.
 
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Old 11-17-09, 10:15 AM
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Normally, when you push in the fan compartment door switch, you will hear a "chunk" noise which is the heating fan relay closing.

This furnace design is a bit odd, the heating fan relay has to be closed to turn the fan OFF. When the relay is open, the fan turns on.

The fact that you are getting heat proves that the low voltage circuit is working properly, and the heating fan relay should be closing when the door switch is closed.

I would therefore agree with you that the circuit board is defective and needs to be replaced.
 
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Old 11-17-09, 03:22 PM
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While you are at it, if you haven't had the furnace serviced in the past year or two, you should take the burners out and brush them off with a wire brush to remove any rust and debris.

Also, remove the brass fitting on the bottom of the pilot burner to expose the pilot orifice and remove and clean the pilot orifice.
 
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Old 11-17-09, 04:42 PM
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I can't see on that board, anything that specifically controls the fan. Maybe a fan-limit or thermodisc control is used, in some other location, and it is stuck closed. In fact, that diagram is so lacking it does not show how or through any temp sensor, relay, etc., that power gets to lo (red wire)speed or gets shut off, specifically.

You also have an antique enough furnace that maybe you have some limit control(even a flame roll-out) that has the 2-terminal thermodisc, with the little red reset pin inbetween, and got tripped, causing it to run.
 
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Old 11-17-09, 06:08 PM
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Originally Posted by SeattlePioneer
While you are at it, if you haven't had the furnace serviced in the past year or two, you should take the burners out and brush them off with a wire brush to remove any rust and debris.

Also, remove the brass fitting on the bottom of the pilot burner to expose the pilot orifice and remove and clean the pilot orifice.

OOOPs on my part. guess I jumped to conclusions without rereading post. Indeed if you have heat then it isn't three of the items I posted. Sticking HFR on the board as you have idenified. For some reason I was thinking: "no heat continuos fan"
 

Last edited by mbk3; 11-17-09 at 06:36 PM.
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Old 11-17-09, 08:22 PM
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Have continuity between terminals on limit switch.
By tracing wires I cant see anything else that would control the blower besides the fan limit switch or the HFR.
So I am pretty sure its the HFR which is attached to the board so... new board.
Thanks for the help
 
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Old 11-17-09, 08:36 PM
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Originally Posted by ecman51`
I can't see on that board, anything that specifically controls the fan.

The fan is controlled by the heating fan relay 2A on the circuit diagram.

The normally closed contact is used to turn on the fan motor, as shown in the 120 VAC portion of the circuit diagram.

That means that when the 2A relay is not energized, the normally closed relay contact causes the fan motor to run.

When the 2A relay IS energized, the normally closed contact opens, and the fan shuts off.


The 2A relay coil is controlled from the Gas3 test position on the circuit board. Gas3 is energized with 24 VAC when the pilot switch heats up, energizing the main burner gas valve and the timer circuit. The diode just before the timer circuit converts the 24 VAC to 24 VDC, which then operates the timer circuit.

When the timer cooks off, it biases the transistor to the right of the timer circuit. That opens the circuit for the Heater Fan Relay 2A coil, which opens the 2A relay contact, which turns the furnace fan on.


Fun, eh?
 
 

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