Any idea why a coleman furnace&a/c combo from a 78 model mobile home won't shut off and which part it is that needs replacing? Switching from cool to heat on the heat/cool switch on the unit itself works but it doesn't shut off. The fan and compressor and everything keeps running. The mercury switch on the thermostat appears to work like it should when manually operated but still doesn't shut it off. One suggestion I heard was the fan relay could be bad. My mother lives in a small town and she controls it with the breaker. It has done this before but messing with the thermostat seemed to fix it last summer. Thanks in advance.
Thanks Juleous. I didn't explain well but the heat/cool switch will shut the a/c off and then switch it to run the furnace non-stop if allowed to do so. I'm not familiar with what the contactor looks like and where it is at, inside the furnace&a/c or inside the condensing unit outside? Is that like the points in the voltage regulator on my old 50 chevy where I would rap on the cover with a screwdriver to unstick them? Any voltmeter or ammeter test you would suggest?
Sorry I didn't understand I guess what your problem is. Is the fan in the furnace the part that runs all the time, or is it the condensing unit outside. And yes the contactor is like the points on your old car and they are located in the condensing unit outside under the cover panel.
I went over tonight with a new thermostat and that didn't fix it. The old one was ok. Switch it to heat and the heat ran all the time. Switched it back and the ac ran all the time. Yes fan blower and condensing unit ran together except on heat and then only the blower ran. Checked by the compressor and bumped the contactor points with a screwdriver and it shut off. Sprayed the part that slides with wd40 and that has fixed it, so far anyway. I'm not sure why switching to heat would shut off the condensing unit even with the points still stuck. Thanks for your help.
The t-stat may be miswired. Make sure that the cooling wire,,(yel or blue) is on the "Y" terminal and Heating wire is on the "W" terminal. If you have the cooling wire on "o" terminal it will run all the time. If heat wire is on "b" the same thing. I will check back tomarrow to see if you have solved the problem....