Furnace won't turn on

[SailorJohn70]

I have a Coleman Mack HVAC system with a rooftop two stage (fan speed) Coleman AC, an Atwood Furnace, and a wall mounted Coleman Mack thermostat that controls both.
Furnace model # 8531-IV-DCLP-L/D
These are the original equipment so they are 22+ years old. We've owned the trailer for abt 16 years give or take (one previous owner - we bought from the original dealership). This HVAC system has worked well for us. It has not needed any work until we started fulltiming (in one location) about 16 months ago. about 14 months ago I changed the foam AC air filter to a disposable type paper filter like you commonly find on home HVAC systems. I've done minor maintenance like cleaning and draining the condensation resivoiuor, and replacing mouse chewed wires. The only major repair was last month when I replaced the AC fan motor. I recently (yesterday) changed the old analog Coleman Mack T-stat to a new digital Coleman Mack T-stat (I did this to try and fix my issue but it made no change).

Issue:
My furnace won't turn on (through the T-stat).
I've taken the furnace to the local RV shop and they thoroughly cleaned and checked the furnace and had it working on the bench.
When the furnace is installed in the TT and I connect the blue T-stat wires to each other at the furnace it will turn on, ignite, and blow hot air.
When the blue T-stat wires are connected to the T-stat, the AC and fan work as they should (so I know the wires have good connection and I'm getting DC voltage/signal from the T-stat to and through the furnace).
However, when I try to turn the furnace on at the T-stat I get nothing. (With the old analog T-stat I would get a click from the T-stat when the temp selector slide/switch was moved past the ambient temp.) Changing from the old analog to the new digital did not make any difference in how any of the systems reacted.

I am at a loss and don't know where to look next. I would really appreciate some with more knowledge/expertise to help me out here.

 

[Henry J Fate]

Best guess is the continuity of the wires to the thermostat that fire up the heat. New thermostat same problem and furnace works fine when jumped or bench tested.

I think I would probably run an auxillary set of wires for the furnace to the thermostat to see if then it werks. That could offer enough of a clue to solve the problem.

The other thing to remember is that your house 12 volt system is the control voltage for the furnace and the ac. Make sure everything is connected clean and grounded properly with proper 12 volts. If you are plugged into shore power, your 12 volt power should be in the 13.5 volt range. Unplugged you will be on battery power and should have a minimum of 12.5 volts.

 

[Gary RV_Wizard]

You've already verified that both the thermostat & the furnace have 12v power. If the furnace makes no attempt to light & run, then the problem is the signal from the thermostat (the blue wire) is not reaching it. The blue wire may go thru the HVAC controller (likely if it's a heat pump or a two-zone system with multiple roof units) rather than direct to the furnace. A quick check is to jumper a wire from the thermostat to the furnace blue-wire terminal and see if it starts up that way. If yes, look at the AC wiring you did and see if something got disconnected there.

 

[Kirk]

I recently (yesterday) changed the old analog Coleman Mack T-stat to a new digital Coleman Mack T-stat (I did this to try and fix my issue but it made no change).

So the problem existed before you changed the thermostat? I would put the original one back at least until you figure out the problem. Since the thermostat also controls the air conditioner, it probably does send it's signal to a control board in the air conditioner that then determines whether the furnace or the air conditioner is supposed to get the signal, and sends it the proper place. With that age of RV, I am going to guess that your original thermostat looks like this one.