early 70's Presidential blower issues

[Tom]

7600 series
Blower kicks on and off while burner (natural Gas) is lit -but not all the time. sometimes it cycles correctly.

Suggestions?

OK- found the Fan switch. Info on tag as follows:
200185
T-O-D 60T82
Fan switch
9-71
F105

[Robert]

Hi Tom,


What is complete furnace model number ?



Thanks,
Robert

[Tom]

Good morning Robert and thanks for hosting this service. Unfortunatly I just got to work and won't be able to reply w/ more info untill this afternoon. I found and taped back together the origional install guide for the furnace last night so correct info shouldn't be a prob.
Tom

[Robert]

Hi Tom,


You can get furnace model number off of the data plate/sticker on

front facing of furnace.



Thanks,
Robert

[Tom]

model #7670 (hope this is what you are asking for).
Tom

[Robert]

Hi Tom,


Can you give me the complete number, as in 7670-??? .


7670-856, 7670-859, 7670-876, etc.... .



They have different switches, so need complete model number to give correct info.



Thanks,
Robert

[Tom]

Fun, fun, fun, The setback thermostat went out sometime today which kept the furnace running-Woke up for a midnight bathroom run and noticed it wouldn't turn off. So... I just put the old manual thermostat back on.
Jus' been one of those days. I guess the furnace or the good Lord was trying to tell me to 'get 'er done'.
Anyway,the info you requested is #7670-856.
Again, thanks for your help. I work on the pulse boiler heaters at work but this thing is so different I really appreciate your help.
Looking forward to your reply,
Tom
PS- sorry this reply is so long, as has been my day.

[Robert]

Hi Tom,

Without being there myself, would first suspect the Fan Switch.

If the furnace is showing these symptoms each cycle, then jumper the fan switch and see if that keeps the blower running.


You should also check the main burner orifice and burner itself for blockages.


I would believe either a bad fan switch or heat chamber not getting hot enough and therefore cooling down to quick with blower on.


Fan Switch # 7975-3281 can be seen and ordered through PayPal

from left side of page here if needed:


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



Thanks for inquiring,
Robert

[Tom]

Fan switch- OK. I'll order one.
What are the other 2 switches (thermo discs?) one is listed on the install manual as an AUX sw . Any ideas? I guess the other is a high limit sw?
Since this is my first winter in a 35 yr old mobile that a widow had her sons maintain I am also questioning the fact that there is a possibility of the gas valve still being for propane since this park was on LP 10-15 yrs ago. How do I identify weather or not the valve was replaces or at least has the correct orifice and spring in it?
Again, thank you for your input, Robert,
Tom

[Tom]

PS- I adjusted down the pilot and the burner flame as they were both burning mostly blue but had considerable orange tips. If I can prove that the gas valve is correct, then I'll bring home a u-tube manometer from work to check natural gas pressure. Could you recommend the required pressure for such an old unit?
Thanks,
T

[Tom]

I see that the replacement fan sw is rated at approx 120 deg and the one in the unit now only 105 deg. How will this affect cycling of the unit?

[Robert]

Hi Tom,


Those are the upper and lower limits.

The Fan Switch I suggested is rated at 110* and will heat up 5* more before blower comes on, won't effect cycle.


Look at gas valve and should be a conversion stem, it will have either Nat. Gas or LP with one upright and other upside down.


Whichever is upright dictates what GV is set for.


Behind GV is burner orifice and stamped on it is orifice number, what is it ?



Nat. Gas input pressure should be 5"wc and manifold should be 3.5"wc unless otherwise stated on the data sticker for furnace.




Thanks,
Robert