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Billy
Billy, HVAC Technician
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Hi, I have a Nordyne LP force air 86,000 BTU furnace with

Resolved Question:

Hi,
I have a Nordyne LP force air 86,000 BTU furnace with a Robertshaw gas valve 7C6-C7F-512. sometimes the furnace don't not start after pre ignition. The troubleshooting code from the control module is,

"Three flashes-the control is in lockout because a failed ignition
attempt, a gas valve error, or a false flame sensed during the warm
up period. If false flame has been sensed, the control will return to
normal operation, and repeat ignition sequence when the false flame
is not present."

Because this is intermit problem I can't pin point the problem. Could the gas valve be faulty, or could there be something else to look at before replacement of the gas valve?
Submitted: 5 years ago.
Category: HVAC
Expert:  Billy replied 5 years ago.
The pressure switch can also cause this code by remaining closed. When the switch stays closed and the furnace goes to start the board already has an input that the switch is closed. I would check the pressure switch and make sure it is moving freely
Customer: replied 5 years ago.
In the user manual my model M1BC086ABW does not call out a pressure switch only in M1M and M1G models.
Expert:  Billy replied 5 years ago.

I know, and that creates some confusion as to what input makes the valve think flame is present. I mention this because I have found that the pressure switch can make this code appear. The only things that can make the board think it s already running are -

 

sigal from flame sensor (never the case)

circuit board receiver for flame sense

pressure switch stuck in closed position.

 

I will pull the circuit board info for this model.

Expert:  Billy replied 5 years ago.
Do you have 24 coming out of pressure switch?
Customer: replied 5 years ago.

The wire diagram i have only have control module, gas valve and igniter. outside of that the aux. limit, fan switch and hi-limit are channel through blower relay or the thermostat.

Because it's intermittent it hard to pin point. during start up i can hear relay clicking from the control module. first click sends 24 v then drops off, again after about 10 seconds a second click 24 volt are back. this is were the ignition starts or it shuts down and I get error, 3 blinks from the control module.

Expert:  Billy replied 5 years ago.

To decipher just keep the meter on the 24v supplied to the module....if it drops, it is an input from a safet or pressure switch. If there is no drop TO the control but 24v drops OUT of the control then the control is bad. Then with the meter, if power drops TO the control, you can measure the 24 from the limits and then from pressure switch to see what one is dropping.

 

I can be more specific if you can give me the model number from the control module.

Customer: replied 5 years ago.
Hot Suface Ignition, UT 1018 Series(NNN) NNN-NNNN
Expert:  Billy replied 5 years ago.
Great , what is hooked to PSI and PSO? should be pressure switch in and pressure switch out
Customer: replied 5 years ago.

The diagram don't call out PSI and PSO.

Expert:  Billy replied 5 years ago.
On your control, isnt there a PSI and PSO terminals?
Customer: replied 5 years ago.
Ok i see what your are talking about. I will check this out when i get home. Thanks for pointing me in this direction. this will give me a new start on finding the problem. i will give a update later.
Expert:  Billy replied 5 years ago.

I think the issue i the control has been changed and is not reflected in the diagram. See here -

 

http://www.hired-hand.com/Download.aspx?ID=331&Type=doc

 

It breaks down the inputs from the limit and pressure switch. If psi and psi do not break open on shutdown will cause this...I will leave this open for you.

Customer: replied 5 years ago.
I think I found th problem. There is switch inside the motor assy. after doing a ohm check it was working intermediately. I bent the prong lead and i working fine. I want to replace it. but now having any luck finding the part number and the motor as well. The motor is a Frasco 7121-4921 Type U21. The switch is 1TM2814W. any help?
Expert:  Billy replied 5 years ago.

Excellent find!...that is what was breaking the PSI and PSO, but instead of a true "pressure switch" it is the centrifugal switch on the draft motor. If it is at all possible to repair the centrifugal switch, it will save you a lot of money.

 

The motor assembly is most likely fine and it is just the switch that mounts on the end that is bad. I will see if I can find any info on them. I admire your persistance.

Customer: replied 5 years ago.
I got it back together and seems to be working with starting a lot quicker. here is the switch.graphic
Expert:  Billy replied 5 years ago.
That would be great if you can keep that one running . The switch and the motor were discon and replaced with different motors and switches.
Customer: replied 5 years ago.
Ok we will let it run to see if it's works over the next few days (or more). Thanks for you help being persistent with me on finding the switch. Well i learned a lot about furnaces in the last few days. Have a Merry Christmas. Again thanks for your help!
Expert:  Billy replied 5 years ago.

you are very welcome....you probably know the unit pretty good now!

 

Happy Holidays!

Billy, HVAC Technician
Category: HVAC
Satisfied Customers: 20264
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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