Forum Discussion

JohnnyG73's avatar
JohnnyG73
Explorer
Jul 03, 2014

Is my thermostat faulty?

I recently picked up a 27’ 1994 Starcraft. The trailer had sat for a couple of years unused. I towed it and plugged it in…everything worked great. I then moved the trailer about 10 miles to where I stored my older camper so I could transfer everything to the new one and get the old one ready to sell. Next I will get new tires and check the axles.

As soon as I plugged the trailer in the furnace fan came on. I checked the thermostat and it was in the OFF position. The thermostat has something printed on it about a time delay so I waited ½ an hour and the furnace fan was still running (I don‘t know what time delay means?). I could hear it clicking every so often, trying to ignite I assume but the tanks were closed. The temp was also turned all the way down.

I then switched the thermostat to cool to see if the furnace fan would shut off and the AC would kick on. The furnace fan did not shut off and the AC made a light buzzing/humming noise but never kicked on or blew any air. When I checked out the camper prior to purchasing, both the AC and furnace worked great. I have no idea what could have happened on the gentle 10 mile trip. I checked the breakers…OK. I lightly tapped on the thermostat and nothing changed. I unplugged the camper and waited. Upon plugging it back in I got the same results. I cannot get the furnace fan to shut off or the AC to kick in and blow air.

I’m no expert but I am leaning towards a faulty thermostat. Considering that both the furnace and AC worked when I first checked the camper out, I doubt that both of those units could have gone bad on the short trip. Is there any reason I can't replace the thermostat with a regular house style one that controls both heating and cooling?

Can anyone confirm my thoughts or offer any ideas? Thanks, John
  • tenbear's avatar
    tenbear
    Explorer
    Jul 17, 2014
    2oldman wrote:
    JohnnyG73 wrote:
    Is there any reason I can't replace the thermostat with a regular house style one that controls both heating and cooling?
    No problem. Lots of us have done it.

    Make sure the house style thermostat is not a 24v thermostat. The one I replaced my old thermostat with uses 2 AA batteries. Many home thermostats use 24v from the furnace.
  • Boband4's avatar
    Boband4
    Explorer
    Jul 17, 2014
    On the airconditioner part - I was doing summer start up and had the same problem The ac would hum but not blow air. I found the blower shaft had a little corrosion on it. I could manually spin it. I put a drop of oil on the bearing and it worked great until the next spring when I did it again. I found I could see and spin the fan manually from the roof with the cover off, or by looking up from the inside. It is a Dometic unit.
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Jul 17, 2014
    You can replace that Coleman Mach t-stat with a Hunter t-stat.
    Hunter t-stat that uses an internal battery for the t-stat.

    HERE are the direction/article.

    Furnace fan continuously running is typically a problem with t-stat, shorted wires from t-stat to furnace or a faulty timed delay relay on furnace.

    Furnace (and t-stat for furnace & A/C) needs 12V DC to operate.
    Your converter is supplying the 12V DC power.
    When t-stat contacts close....furnace fan runs 15-20 seconds to purge burner chamber then spark occurs. Main flame lights. When t-stat contacts open (temp set point reached) main flame goes out and fan runs until timed delay relay opens. It is a separate component that has DC power from DC dist panel (fused) and then goes directly to fan motor.
    It is a normally open Relay. When activated closes, sending power to the Blower Motor. When deactivated allows Blower to run for a period of time to cool the Combustion Chamber.
  • JohnnyG73's avatar
    JohnnyG73
    Explorer
    Jul 17, 2014
    I had a little time over last weekend. I cleaned out the entire trailer and found all of it's original documentation. The thermostat says Coleman Mach right on it and the manual for the AC unit also says Coleman Mach. Much of the weekend was spent cleaning out the old Aristocrat trailer and getting it ready to sell. I had previously always had the vintage trailer plugged into an outlet in the garage that it was parked next to. The shore cable never quite reached the outlet so I used an adapter and an outdoor extension cord.

    My new trailer is now parked in the same location with the same situation...an adapter with an extension cord. I was unable to remove any wires from the back of the thermostat because they go under a circuit board without access. I would have to break it apart or cut the wires, so I left it for now.

    Like I mentioned before, the furnace fan would not shut off as soon as the shore cable was plugged in. I switched the thermostat to heat and cranked the temp up to max. The gas was open. The furnace would click and but not ignite.

    When I switched the thermostat to cool the furnace fan would continue to run. I turned the temp all the way down. I could hear the AC unit make a low buzzing noise but it would never kick on or blow air.

    I had to physically remove the hot wire from the furnace fan motor to get it to shut off.

    The AC unit on my newer trailer is larger and draws more than one in my old trailer. I am suspecting that the "draw" is too much for the extension cord that I am using and could be why it's not kicking in and only buzzing.

    It will be a couple weeks before I get another chance to work on the problem. I plan to pick up an actual RV extension cable (heavy gauge) and see if this can remedy the problem.
  • JohnnyG73's avatar
    JohnnyG73
    Explorer
    Jul 10, 2014
    I have a marine battery I will throw on it. Yes, the white is ground. I made the mistake of hooking one up backwards on my old TT and it took me a few minutes to figure out why the lights all worked but the exhaust fan ran in reverse lol
  • opnspaces's avatar
    opnspaces
    Navigator II
    Jul 10, 2014
    Most of the replacement home thermostats are for 24 volt systems. I have one of those and it works fine on 12 volts.

    I would pull the thermostat off the wall and disconnect the B or BW wire (probably red) and see if the fan stops running.

    If you wan to see if the 12v is working you can always run a set of jumper cables from your truck battery to the trailer battery cables. Remember though that on trailers usually the white wire is ground and black or red is positive. Verify the ceiling lights work on 12 volts and then see if the heater sill come on.
  • enblethen's avatar
    enblethen
    Nomad
    Jul 10, 2014
    Most digital thermostats run off an internal battery. Yes, you would have to change the battery maybe once or twice a year.
    Digital thermostats can be programed for what type of furnace. Gas or electric furnaces operate on 24 volts and will work just fine on 12 volts.
  • westend's avatar
    westend
    Explorer
    Jul 10, 2014
    House thermostat uses nominal 24VDC, there is a transformer in your house furnace that does the conversion. Some residential thermostats work with 12 VDC. Do a search on the Forum for "thermostat" or "digital thermostat" and you should get a lot of results.
  • JohnnyG73's avatar
    JohnnyG73
    Explorer
    Jul 10, 2014
    2oldman wrote:
    JohnnyG73 wrote:
    Is there any reason I can't replace the thermostat with a regular house style one that controls both heating and cooling?
    No problem. Lots of us have done it.


    I have been away from my trailer but will have a chance to work on it this weekend. I have had some time to think about the problem though and I'm leaning towards a 12v issue. I considered changing the thermostat and installing one from a home store.

    House thermostats use 110v only. The trailer has to be a 12v thermostat or else how could I use the furnace while boon-docking. It has to operate off the converter or battery. All the 12v lights are working properly to the converter is working. If the thermostat is faulty it would need to be replaced with an RV style. Am I correct in my line of thinking?
  • JohnnyG73's avatar
    JohnnyG73
    Explorer
    Jul 03, 2014
    enblethen - not yet, I'm out of time for today. I'll post here after I get a chance to try that.

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