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2002 Diplomat PBT.  Furnace has never worked since I have owned the coach.  I have a thermostat that controls both ac units and one of them can be set for furnace.  My question is how can I check the furnace to see if it is the furnace control board or my thermostat/ac controls that are bad?  Or can I bypass those controls with some new wiring and try to turn the furnace on to see if it works?


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I would start with making sure the master switch for the furnace (remove the outside cover) is in the on position. I had to turn one off cause once it started it would not turn off (was a bad t-stat) then check your power an grounds at the furnace control board.

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OK....Fundamentals.  Here is the PRINT.  The way it works, and you MAY know this...if so, others will learn.

The 5 Button Thermostat has a Furnace Setting.  I don't KNOW which Zone....but suspect the front one.  If you have 13.5K units, there is a "Control Module" located and accessible from the inside (remove the cover) in the Plenum.  If you have a 15K (maybe upgraded?), the module is under the outside cover and you need to get on the roof.

IF, guessing, that the Furnace Option is ONLY available on Zone 1 (Front), then the module will have a row of DIP switches.  The ONLY switch that should be on is the FURNACE one.  That tells the module that when that Zone (Zone 1 is Default.... the rear one will have DIP Zone 2 turned on) is to send out a signal.

There are TWO wires attached the the Module.  They run directly to the Furnace.  There is NO Voltage on them.  They are a REMOTE DRY SWITCH.  When you use FURANCE and set the temp to a high setting and the system temp sensor says... NEED HEAT....then the SWITCH inside the Control Module is closed (like closing a LIGHT switch)...  BUT, there is NO VOLTAGE on these wires.  The wires run directly to the furnace.  When the wires are ON or the internal switch is closed....that signals the Furnace to LIGHT UP....and run.  Once the temp gets satisfied, the thermostat says OFF...and the internal switch inside the Control Module will OPEN.  NO CLOSED contacts....  NO FURNACE.

You can TEST the furnace.  FIRST....you have to have 12 VDC power coming in.  If you look on the PS7A plug....That is the pin in the upper LEFT corner.  It goes to the mating connector....and the pin in the UPPER LEFT provides 12 VDC.  Check that harness.  You have to have 12 VDC power to the Furnace.  OK....now you have power. IF no power here...follow the prints.  The power comes from the EMS system.... sort of complicated... The EMS has incoming 12 VDC to it....and then it passes it up to the other plug...

Then, the control module is pins 2 & 5 on the connectors.  If you use a paper clip on EITHER side of the connector (must be connected) and jumper pins 2 & 5, then the Furnace should start.  OK...for you information.  See the Red and White pins on the Furnace plug.....the go to the AC/Power Feed P10.   Those Red and White Pins (2 & 5) mate up with P10 AC Power Feed....they are NOW pins 1 & 2 there....and then on the C11 side....they keep going....to Pii2.  THEN.....they change colors.  Inside the Control Module....they eventually are the BLUE WIRES.  These TWO Blue Wires just turn ON and OFF.

That's it.


Turn OFF the Thermostat (switch at the bottom).  Wait a minute or two.  Then HOLD in the TOP and BOTTOM (Mode and Zone) buttons.....while holding them IN....turn on the Thermostat.  FF will appear. This resets it.  Any time you "mess" or disconnect or do anything to the Module, you need to reset it.

TIP.  NOW....STOP using the bottom switch. It is the future....leave the SWITCH ON.  Then use the MODE button and toggle through to OFF. The switch is a weak point and there are NO new replacements out there....some used or refurbished ones....and the fancy aftermarket one ain't cheap.

Good Luck....let us know.... 

2004 & Earlier HVAC Print - Furnace Knight Diplomat & others.pdf

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