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2004 Monaco Diplomat. No fuel (LP) to furnaces or cooktop.


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Started diagnostics by testing LP switch. Have 12 v DC at off position and 13v DC in on position. LP tank is 2/3 full and bleeder valve has LP when opened. Furnaces run and I can hear the lighting module clicking. I reset the T stat and it turns the units on by increasing the temp setting. The previous owner had the front air conditioner replaced. I have yet to check the DIP switch positioning. The furnace breaker is functioning in the bedroom cabinet. The 15 amp fuse is intact. The air conditioners do not produce cold air when turned on. With the written documentation found so far I'm hoping that my challenge lies in the DIP switch on the roof AC unit. 

Am I missing anything else? Any help would be appreciated.

Larry

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Sequence of operation of the furnace is:

1. Thermostat sends 12 volt control signal to the furnace control board.

2. Furnace control board sends power to the blower relay which, after several seconds delay (*1), powers the blower motor.

3. Air flow from the blower operates the sail switch allowing 12 volt sense current to pass through the firebox overheat switch and on to the furnace control board that air is moving AND firebox is not overheated.

4. Furnace control board starts an internal "purge timer".

5. After uninterrupted purge time, furnace control board simultaneously powers the furnace gas valve (loud CLICK), starts igniter pulses (tick, tick, tick...) and starts an internal "ignition failed" timer.

6. Burner ignites (fairly loud "WHOOSH" and sound of gas burning) and the flame is detected by conducting a (very small) electric current to "ground". (*2).

7. If flame detection does not occur before "ignition failed" timer expires, power is cut to the gas valve (slightly less loud CLICK than in step 5) and program drops back to step 4 above. 

8. Normal heating continues until a.) thermostat turns off control signal OR b.) Sail switch opens indicating loss of air flow OR c.) fire box overheat switch opens indicating overheat condition OR d.) flame detector indicates loss of flame.

9. Loss of control signal, sail switch, firebox overheat switch, or flame detect will cause the furnace control board to immediately shut off the gas valve.

10. If the shutdown was caused by loss of any of the safety switches (sail, overheat, flame loss) the blower will continue to run and the furnace control board will keep cycling through the startup steps. If the shutdown was caused by the control signal going off (thermostat stops asking for heat) then the blower will turn off after a delay of several seconds.

(*1) The blower relay is a thermal relay that takes a significant amount of time to turn the blower "on" and "off".

(*2) Flame detection is tricky and relies on sensing the small amount of current produced by ionized gas (the flame) shorting the flame rod to ground. To accomplish this, the flame rod has to be in constant contact with the flame rod. The control board produces a high voltage (several hundred volts) that is applied through the ignition coil. (The flame rod also serves as the "spark plug" for the ignition.) One thing I skipped in the above narrative is that the flame rod is sampled during the purge time to make sure that it's not shorted out. If current flows through the flame rod BEFORE the gas valve is turned on that constitutes a fault condition that will prevent the gas from being turned on. I have seen situations where the flame touches the rod for only a short period of time and then deflects away causing a shutdown. If this happens, the furnace will go through a long delayed heating cycle where it goes through the safety steps and lights, only to shutdown and go through the cycle again and again. I have also seen a burner rust out and the resulting flame miss the rod completely.

You mentioned the dip switches in the forward A/C unit. These really have nothing to do with furnace operation. The furnace is controlled by a 12 volt control signal that goes directly from the thermostat to the furnace control board. The only effect mis-setting the dip switches would have would be in screwing up operation of the A/C or heat pump (if so equipped). 

 

 

I just noticed you said "no gas to furnace or cooktop".  So that leaves the furnace out of the discussion altogether. 

Gas from the tank to the coach goes through a two stage regulator and a solenoid valve that's controlled by the propane detector/alarm and by the shutoff electric switch near the tank fill port. Look for a stuck regulator or stuck solenoid valve. I have had both of these items fail over the years. I also have had to replace the furnace solenoid because it wouldn't open if dry camping and the battery wasn't fully charged. (That one had me waking up freezing several times with the furnace dutifully recirculating cold air!)

 

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There are 2 blue wires coming from the furnace is used to turn on the furnace.  A contact closure or shorting the 2 wires together will turn on the furnace.  Purchase a mechanical thermostat and install it at a place where you can extend the 2 wires to the thermostat.  A Honeywell model RTH111B would work perfectly.  The instructions in the box will tell you where to hook up the 2 blue wires.  

Then you will not have to worry about Dip switches, and programing to use the furnace.   Chuck B 2004 Windsor

I would recommend a trip to your local propane dealer to repair any propane issues in your coach.  Chuck B 2004 Windsor

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