Question re Gas Furnace Pressure Switch

Hi All:
I'm having trouble with my 20+ year-old Sears 867.762340 gas furnace.
About once a month, it will fail to ignite the burners. It just sits there with the vent blower running.
If I short out the contacts on the pressure switch, the burners start right up and everything works normally.
I've dismantled the relevant parts. Checked the gas vent from top and bottom; cleaned the rubber sensor tubes and checked them for leaks; cleaned the insides of the metal fittings to which they attach; checked the electrical connections to the pressure switch; and checked the pressure switch itself (it responds immediately to very light sucking and blowing on its input ports). Everything was squeaky clean as far as I could tell, and I didn't find anything wrong with any of the components.
After re-assembly, the old behavior is back. Furnace fails to start about once a month.
The only thing I can think of is that the switch contacts inside the pressure switch are worn or corroded, so they don't always close the circuit.
Before I sink $70 into a new pressure switch, does anyone have any ideas about what might else be causing this problem? And are the pressure switches known to wear out?
Your help greatly appreciated.
Pete
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Oh, the ol 867.******. That one was a real sum biatch to work on.

Just plan on taking a day off once a month and go away.

Then leave the short in place. It sounds like you've already figured it out.

but did you check in between?

So was this a thorough cleaning with soap and water with an air drying or did you just give it a once around the horn?

Eww, that could be a problem. THe insides of the fittings have an exacting tolerance. You could have really screwed something up there.

Did they look like spade terminals or bath tub play toys?

Oh my! Do you write for Hustler in your spare time?

Go out of town that day of the month

Well, some are good thinkers......others are not. Where would you be?

Perssure switches never never never ever wear out. I would start replacing parts one at a time from the cheapest part and gradually work up to the the more expensive parts. This is a sure way of getting it repaired right.

No problem. Dont thank me. I do this as a public service. You can ask me anytime. I would replace your thermostat first though. After all, its almost always the thermostat. Bubba
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Wondering if thats a differential switch with two tubes ?

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On Fri, 4 Jan 2008 23:49:10 -0600, CRE8WAVES wrote:

Yes it is. Does that lead to a conclusion?
Pete
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Pete posted for all of us...

Could... are they fallopian tubes?
--
Tekkie Don\'t bother to thank me, I do this as a public service.

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This is your problem....replace it and your problems will be solved.
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windy, weather ???????

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On Sat, 05 Jan 2008 08:56:59 -0600, The Freon Cowboy wrote:

Interesting suggestion! It was quite windy when it failed yesterday, and normally we don't get much.
However, the winds were gusty and intermittent. Wouldn't you expect the furnace to start as soon as the pressure in the vent was OK? What happens is that the furnace sits with the vent blower running, and it would stay that way forever if I didn't do something.
Thanks
Pete
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Pete wrote:

condensation in the air tube.. Take it/them off and blow the itty bitty moisture out and things will be fine.. Dew point cause that. No real cure. Only happens in certain circumstances. Be sure there are no traps in the line.(low points)
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Don Ocean wrote:

Sometimes I find tiny spiders have made a mess in the tube. [This is usually when it sits for the summer, and the first firing doesn't work.]
--
Zyp



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