%$^%$^%$^ spiders
I hate spiders, and now I find out I have been freezing my azz off for 2 weeks because of this blasted damn spider....







UPDATE:
Ok, after cleaning the bejessus out of it, it has stopped fast-cycling, but is now seeming like it cannot keep up.  For example, it is 35 outside and 60 inside, right now.  The thermostat is set at 68 and has the light indicating the system is on lit.  The furnace is running more normally now, but it doesent seem like the air is as warm as it should be and that there now seems like the pause between cycles in too long.  I know this contradicts my previous problem.  Other than vacuuming dust and changing filters I haven't done anything to this furnace.


Was changing the filters yesterday and noticed the whole thing was dusty so I got the Vacuum out and sucked uo as much of the junk as I could get to.  Now my furnace is cycling on and off rapidly.  The fire looks good, and everything seems normal but it goes "Click" and cuts off 5 seconds or so into the blower cycle.  But then a couple minutes later, it goes again.

It's a Coleman Presidential in a MH and it has been working flawlessly up to this point..

I can post pix but it is pretty simple.

HELP!  HELP!  HELP!  I need to figure out where to look and what to fix!!Pix:

1
2
3
4
5
6
7
8

Check to make sure there isnt a big hunk of dirt or whatever inside the blower wheel.
Pic 3 and 4 show limit switches. jump one at a time and try the furnace. BE CAREFUL

Flame sensor dirty?

Look to see on the board if there is an indicator light. It will usually flash a series that you can refer back to the inside cover of the unit to self diagnose. I'd check the limit switches first.

Ok, help me out, what does a limit switch look like and where would I look for it?  

dirty flame sensor or did you knock the wire off the flame sensor ?

Sounds like the plenum high limit was bumped.  Look for a small box with (usually) a sliding vertical scale, reset your plenum temperature to somewhere around 190 degrees F.  Or perhaps you need to replace it.

kind

limits are little push buttons (i see them here usually red) There are about 5 on the ones I see. Usually they will all be wired in one color wire to trace easier.

do a google search. tons of info and troubleshooting on them. I just looked

Ok I googled but I am having a hard time finding useful info.  My furnace [1972 vintage] is stone axe simple, no lights no control board [at least not like I see online].  

I posted pix of the furnace parts I am working with in the Original post.  

My thermostat is electronic so I don't seem to have a mechanical anticipator, and in any case, I never touched the t'stat.

HELP!

Why do you have the end of a thermo couple hanging out of the furnace? It looks like whoever replaced it never botheres to remove the old one.
The reason your furnace is cutting off sounds like there is a high limit switch opening up. "meaning there is excessive heat build up in the furnace .

Do you have a volt meter to read voltage or resistance across the limit switches?

One ploblem that I see with is you need to clean the blower wheel "dirty blower wheel= low air flow = excessive haet build up in the combustion chamber.
The dirty blower wheel is probably not the problem but can cause the unit to act like you described.
You need to pin point the switch that is opening up turning the unit off. Listen closely to see if single out the one opening up/clicking
Do you have a pic of the wiring diagram to post? look for a rectangular box with a round metal disk inside that has temperature markings on it "I dont think Coleman had this type back in 72 but it may. You could have turned it down too low by accident from cleaning .
A limit switch is basicaly a temp. switch that breaks the circuit when it reaches its designed max temp for that perticular switch and its location in the furnace.

did you notice any sooting or particularly orange flame? Any soot in the vent that was not there before? you need a good blue flame .
Make sure the flame sensor is lined up close to the pilot flame and has good contact with it too . You could have bumped that or moved it.
See the copper line with the gray metal end on it coming out of the furnace? That should have been puller out when the thermocouple was replaced this is the old one just laying in there and could have been bumped or pulled and got the flame sensor mis aligned.
CLEAN THE BLOWER WHEEL. You are not moving as much air as dirty as it is and causing excessive heat to build up in the combustion chamber.

Somewhere on that furnace there is a high temp limit designed to stop the burners in the event of a blower failure. It appears that one of those disc shaped switches has to be the limit.  Turn the power off and jump out the wires to those switches and see what happens. Only jump out one at a time so you know which one is defective.

And remember to turn the power off when jumping out the wires.

I will take the blower out and clean the heck out of it.
The flame is nice and blue.
I noticed the extra thermocouple but never saw a place for it and everything was working ok, so I left it.  I don't think I hit the flame sensor, it is up behind the gas regulator and I never messed with that.




OK, I think I need more info here.  If the power is off, what is jumping them going to prove?  Or am I mis-understanding?

The more I look and listen and read it sounds like a flame sensor to me. That's where I would start off. A flame sensor would be a sensor inside the combustion chamber that would be in direct contact with the flame. try locating it and taking some fine sandpaper to it to get some corrosion off. See what happens. If thats not it than check to see that you didn't loosen up something and partially block the exhaust vent.


ETA:
Than again the more I look at the pics it's hard to tell if it even has a flame sensor.

ETA:
Whoops, just read in the post above me that you do have one. Start there.

Yep.  Dirty blower for sure.  Could also be the other thermocouple (thermopile).  These send a signal of only 0.75 volt to the gas valve.  it could be shorting out.  It the pilot stays litl, then the problem is in the temp limit switch.

TAKE THE BLOWER OUT to clean.  Wear a dust mask.  Clean with a bristle brush and clean it well.

It may be cycling on the high temp limit. Jumping out the limit will allow the flame to stay on and not cycle.  Also you might check the thermostat connections in the furnace, you might have knocked one loose.

You have to turn the power back on after jumping out the limit, I was just reminding you to turn the power off so you wouldn't get zapped.

That's not a thermocouple hanging out there un-connected. You can see the thermocouple threaded into the gas valve. What you see is a bleed fitting from the regulator that somehow was disconnected... and it has nothing to do with his problem. It's cycling on the high limit, The limit is one of the 2 controls pictured with 2 wires connected to it... in either the middle of pic #3 or the bottom of pic #4, usually the limits have a number written on the like L200-2 or L180-4 which would mean it opens the circuit on temperature rise of 200 and the -2 means it has a 20 degree differential... or L180-4 means opens on rise (off) of 180 degrees with a (-4) 40 degree differential (on @ 140). A fan control is similarly marked but usually with a F120-2 or F150-4 which makes a circuit on rise like on at 140 with a 20 degree differential of 20 or in other words on at 120 and off at 100 for example. jumper out the one with the "L" designation and I'll bet it stops cycling. Don't leave it that way obviously as it is a safety device. They usually cost less than $10 and can be had easily at most HVAC suppliers

so is it running?

Sounds like you have a loose wire connection. check and see if all connections are tight.

Step 1: turn on furnace
Step 2: open up door
Step 3: stick your head inside and investigate problem

Yikes, that is cold...

bump for update

I was kidding.

No, I mean it was cold, as in my furnace makee no heat...

It's 30 outside and my furnace is barely holding 60.   Icalled a HVAC guy and he told me $50 to clean the "spider out of my orifice"  Does this sound right?

Spidermite's do get inside the little gas tubes that are in front of your ribbon/inshot burners. I've cleaned out a many here in kentucky. Alot of wannabe arm chair HVAC eddie futches in here. Any you guys licensed? I know I am.

okay kill that, I actually read what you crossed out. It could be multiple things (cracked heat exchanger, Dirty Coil, Blocked Return, Shut off Supplies, Etc.) get a temp rise.

Bad advice. Never jump one. They are safety switches what are you trying to do get someone killed. If you have a volt meter check the ohm's it should be closed (showing something other then O.L)

I don't think he meant permenantly, only to test...

I cleaned the be-jesus out of this thing, so I doubt there is a glob of dirt...

Does the flame look "normal"?  

If the air flow isn't enough, heat transfer can be limited.  You probably don't have a manometer handy...those blower wheels get awefully thick with dust and then no flow.  You did take it out, right?  I filled up a baby ShopVac cleaning one out that was only 12 years old.

The motor might not be healthy, the only check is if you can measure and compare windings, you probably don't have a legible schematic to follow.

bump for solution...