Coleman 7900 Series Intermittent Blower

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Coleman 7900 Series Intermittent Blower

Postby GarryW » Tue Nov 27, 2007 9:46 pm

--Thermostat seems to be fine
--New gas control valve
--Removed Spider detritus from orifice
--If the switch mounted to the blower squirrel cage is the Upper
Limit Switch...or not...I pushed the little red button back in
--Burner ignites, the blower cycles for about a minute or two then
stops...burner still lit
--cycle repeats several times until thermostat reaches desired temp

Did I read somewhere that this unit is a two-stage burner? If so, that doesn't happen.

Fireplace and space heater getting old...Thank you!!!
GarryW
 

RE: Coleman 7900 Series Intermittent Blower

Postby Robert » Wed Nov 28, 2007 5:09 am

Need complete furnace model number. May be a faulty fan switch.


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RE: Coleman 7966A856 Intermittent Blower

Postby GarryW » Wed Nov 28, 2007 7:34 pm

Model 7966A856
GarryW
 

RE: Coleman 7900 Series Intermittent Blower

Postby Robert » Thu Nov 29, 2007 12:05 am

Fan Switch # 7975-3281 on the left side here:

http://www.mobilehomerepair.com/ColemanGas.html


Click on pic for enlarged view.


Jumper it and see if it stops the short cycling, then remove jumper and see if it resumes.

If so, replace it.


It can be ordered at the link through PayPal or by calling toll free to 1-877-263-7860 between 9A/5P CST Mon thru Fri aand charging to Visa or Mastercard.


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RE: Coleman 7900 Series Intermittent Blower

Postby GarryW » Thu Nov 29, 2007 12:50 pm

Will this switch be inside the box that the system switch is mounted on?
GarryW
 

RE: Coleman 7900 Series Intermittent Blower

Postby GarryW » Thu Nov 29, 2007 10:27 pm

--Jumped switch...kept running
--Replaced switch :( problem still there
--Jumped new switch...keeps running...sigh

Should I leave jumper on?
GarryW
 

RE: Coleman 7900 Series Intermittent Blower

Postby Robert » Fri Nov 30, 2007 3:14 am

Ok, it will keep running when jumpered. Did you jumper and leave running for awhile ?


Set t-stat for how you normally do and jumper it is what I was meaning.


IF it never cycled again on and off, it was a bad fan switch. IF it was something else, it should have still cycled on and off.


Since you replaced it and it is still doing it, test across the upper limit with manual reset.


With unit running fine, it should read 0vac across the terminals.


When the unit blower cycl;es off, if it gives a reading suddenly and has to be reset again, it could have become weak also.



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Robert
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Postby GarryW » Fri Nov 30, 2007 3:48 pm

To test the old switch...jumped and left it running for a while, manually reduced thermostat setting to end call for heat...blower kept running.

Installed new switch...problem recurred...(blower stopped, burner still ignited)...cycle repeated.

Jumped new switch...left running...t-stat ends call for heat...blower runs until jumper removed.
GarryW
 

RE: Coleman 7900 Series Intermittent Blower

Postby Robert » Sat Dec 01, 2007 7:03 pm

Test Upper Limit as suggested.

During tests, t-stat should be set where you normally set it.


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Postby GarryW » Sun Dec 02, 2007 4:14 pm

Don't have V-O-M...believe I can borrow one tomorrow.(Monday)

Meanwhile, just a thought. Is it possible that the blower is short cycling because the burner flame isn't as intense as it should be?

I went back to the owner's manual...The gas valve on this unit is a "step-open" type valve. It opens to a "low-fire" level and then after a few seconds "steps open" to a "high-fire."

Since the valve is new, I would be inclined to suspect whatever switch tells the gas valve to step open as the possible problem here. What do you think? GW
GarryW
 

RE: Coleman 7900 Series Intermittent Blower

Postby Robert » Sun Dec 02, 2007 8:35 pm

First see if you got a 2 stage GV, most times when replaced it is a single stage now.


Does burner flame look weak ?


Yes, that will cause that if it's a weak flame.




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Postby GarryW » Mon Dec 03, 2007 11:12 am

I can't recall what the flame looked like before.
While re-lighting the pilot as necessary from time to time, I don't remember ever noting a difference between what would have been the two burner stages...but it does seem that the audible "whoosh" as the burner kicks in is not as loud as it used to be and the sound of the burner as it waits for the blower to kick on is not as loud as before.

It's difficult to diagnose these things anyway, and not having a meter doesn't help. The blower switch I replaced was only 9 dollars and if you can help me isolate another switch or other reasonably priced component that has a good chance of being the culprit, I'd like to take one more shot at fixing this thing myself before seeking professional help (for the furnace, not psycho-therapy for me) Thanks...GW
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RE: Coleman 7900 Series Intermittent Blower

Postby Robert » Mon Dec 03, 2007 7:06 pm

Did you jumper the upper limit with manual reset ?


Without a meter and proper equipment, not much else you can do.


Follow wiring diagram and see if the 24vac low voltage drops out anywhere when the burner goes out.



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Robert
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Postby GarryW » Tue Dec 04, 2007 10:10 pm

Jumped the upper limit switch...no help.

Y'all be sure to tune in next time for the another episode of "As the Furnace Burns."
GarryW
 

RE: Coleman 7900 Series Intermittent Blower

Postby Robert » Fri Dec 07, 2007 2:31 am

Hi,

Not much else to do without proper equipment and tools.

Could be low voltage circuit dropping out or burner not building up enough heat, but without tools, no way to know.



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