Forum Discussion

redmandaniel's avatar
redmandaniel
Explorer
Jun 04, 2014

Furnace blower comes on when AC compressor comes on

Okay this is a convoluted story so bear with me, I discovered some floor rot in my 2006 Coachmen Captiva 265ex (that whole nightmare could be a forum post on its own) as part of my repair I have to remove the suburban Gas furnace. I removed the furnace a couple of nights ago, I was careful to shut off the gas and remove the gas line, I also took good note of how it was wired (I took pictures) to ensure I rewired it exactly as it was. While I was doing this my AC was running, and I noticed nothing out of the ordinary, I sat the furnace aside and went to bed. The next afternoon when I returned from work I opened the camper and it was hot inside and the AC would not come on, it acted as if it had no power, no hum, no fan or nothing. I checked the breaker, it was good, I checked the fuses on the DC side, they were all good. I removed the coleman mach thermostat and checked the small glass fuse on it and it was good. I then cut the 12v neg and positive wires on the back of the thermostat to see if it was getting the requisite 12v power, it was dead, no power. Since I knew all of the DC fuses were good, I rewired the thermostat and replaced the furnace and rewired it as before. And sure enough when I turned on the fan to the AC unit it worked perfectly, so I then moved the switch to cool to engage the compressor. As soon as the compressor came on the furnace blower came on as well. And as soon as the furnace blower spun up the AC shut off completely.

Now here are my questions
1. why in the world won't the thermostat work when the furnace isn't connected?
2. why in the world does the furnace come on when I turn on the AC compressor?
I am absolutely certain I rewired the furnace correctly, remember I had pics from the previous night and I used color coded tape to be extra sure.
3. did my disconnecting of the thermostat and subsequent reconnecting cause some crazy reset or something?


Any help will be GREATLY appreciated

Thanks,

redmandaniel
Claxton, GA

6 Replies

  • redmandaniel's avatar
    redmandaniel
    Explorer
    Jun 05, 2014
    Thanks for the quick response, I did check the wiring on the TStat, and it was copacetic. So I've got a new TStat on the way, we'll see if it makes any difference,

    Thanks
  • 2oldman's avatar
    2oldman
    Explorer II
    Jun 04, 2014
    redmandaniel wrote:
    I removed the furnace a couple of nights ago, I was careful to shut off the gas and remove the gas line, I also took good note of how it was wired (I took pictures) to ensure I rewired it exactly as it was.
  • We_Cant_Wait's avatar
    We_Cant_Wait
    Explorer
    Jun 04, 2014
    You didn't mention if you took pics of the "T" stat wiring before you disconnected it, could you have rewired it wrong?
  • 2oldman's avatar
    2oldman
    Explorer II
    Jun 04, 2014
    I can only surmise there's something wrong with the thermostat.
  • Golden_HVAC's avatar
    Golden_HVAC
    Explorer
    Jun 04, 2014
    Yes it can happen. In many RV's the thermostat is connected to both the furnace and ducted A/C unit, that is how mine is.

    Double check the wiring again. That is all I can say that has changed, and needs to be back to normal before it will work correctly again.

    The furnace fan should not be on the "Green" wire from the thermostat. It should only be controlled by it's internal thermostat - that should be set for about 135F on and 110F off temperature, and the furnace fan should be coming on when "Red" to "White" is closed. That is "Red" is the +12 volt line, and "White" is the typical wire color for heating in most residential systems. Once the fan has reached it's normal running speed, then a fan safety switch can close, allowing power to the furnace gas valve and ignighter system. The fan will shut off when power is no longer applied to the "White" or "W" terminal, and the thermostat is below 110F air outlet.

    The compressor is normally on the "Y" terminal, and normally a (you guessed it) Yellow wire. Green is normally the fan motor high speed in residential systems, don't know what color your RV manufacture might have used though. There are typically 2 fan speeds on a RV A/C system, so you might have (should have or do have) two wire colors that should be bringing on the A/C fan motor.

    If the compressor is running while the fan is not, it will quickly overheat, and if the compressor overload does not stop it quickly enough it can get hot enough to melt the compressor motor's internal windings, might as well replace the unit if that happens. So never let the compressor run while you are trying to figure out this problem. You might have to disconnect the black and red wires into the compressor to stop it from running while you get this all figured out.

    To disconnect those wires, remove the rooftop assembly and disconnect the three wires coming out of the compressor, making sure to mark them all.



    Fred.

About Technical Issues

Having RV issues? Connect with others who have been in your shoes.24,287 PostsLatest Activity: Jul 22, 2025