Thermostat - Atwood Furnace

[wheels]

I have a 1997 Fleetwood Mallard TT. It has an Atwood gas furnace. When the electricty is plugged in and the room temperature is below 40 something degrees the fan will run all the time. There is a little off/on switch on the thermostat and I have that in the off position and it still runs. I have pulled the fuse to disconnect the furnace for now. Would like to have the power plugged in when we are working on it or loading it. Is this the way it should work, or would a new thermostat correct this situation?

[RV Wizard]

You need to diagnose to see if in fact it is the t'stat, control wiring short or a bad time delay relay. Start by disconnecting the furnace from the t'stat and re-power the unit. Let us know what you find and if it is still an issue we need to know the model of furnace you have.

[Dave78Chief]

Might be in lockout mode. If the burner did not light or relight, the control borad times out after 3 tries and goes into lockout mode. Many older furnaces did not disconnect the fan motor when in lockout (would run run battery dead). If, the fan is running as you say, turn the thermostat setting down until it turns the fan off (or if there is an off switch on thermostat turn it OFF). Wait 30 seconds then turn it back on and see if the furnace lights. If it does, it was in lockout. Now, wait to see if it will cycle off and on properly. If not, then there is a problem. For reference, normal operation during start up there is a short period the fan will run before ignition to purge the burner chamber of residual gas. The fan will again run for a short time after the burner shuts off to allow the burner assembly to cool down.

Dave

[wheels]

RV Wizard & Dave78Chief-

I did what RV Wizard said since I heard from him 1st. I took the thermostat off - put the fuse in - the fan did not run. I put the thermostat back on, put the fuse in and the fan ran. The thermostat was in the off position (on the little switch on top of it). I had the thermostat set so that it was as low as it would go. Then I took the thermostat off again, put the fuse in and the fan did not run. I'm looking in the Users instructions and it's and ECALIBAR 8500-III Series. Not sure if this is the model number - MPD 36236. Effective 9/95. Thanks to both RV Wizard & Dave78Chief! ~Wheels

[Dave78Chief]

1) I took the thermostat off - put the fuse in - the fan did not run.
2) I put the thermostat back on, put the fuse in and the fan ran. The thermostat was in the off position (on the little switch on top of it). I had the thermostat set so that it was as low as it would go.
3) Then I took the thermostat off again, put the fuse in and the fan did not run.

Sounds to me as if the thermostat is stuck in the ON position. If the Thermostat wiring had a short in it then it would have turned the fan on when you did steps 1) and 3) above. If the time delay relay was sticking then it would have also ran at steps 1) and 3).

Refer to wiring diagram on bottom right (8500-III) of page 10 http://bryantrv.com/docs2/docs/hflamefurn04.pdf

The fuse your pulling supplies 12VDC to Terminal Board pin 1 (TB-1).
That is feed to the internal CB (page 23) and then back to TB-3.
The thermostat (page 22) simply turns on and feeds the 12VDC back to TB-4 and on to the Time Delay relay (blue wire; page 24). Also provides signal to sail switch.
After about 20 seconds, Time delay relay energizes allowing 12VDC from CB (red wire) to go to fan motor.
Gas valve/ignitor circuit is also energized but is not germane to this discussion.
When thermostat says temp is achieved, it opens removing 12VDC from time delay relay.
Time delay relay stays energized for a minute or so to allow unit cool down.
Time delay relay de-energizes removing 12VDC from fan (fan stops)

With the thermostat never being satisfied, the only thing that will shut the unit down is the high temperature cutoff switch though I would think your coach would be real hot at that point.

Dave

[wheels]

Thanks RV Wizard & Dave -

You were both right that the switch on the thermostat was not working correctly. I have the problem corrected and ran the system through it's course. It appears to be working perfectly fine now. Thanks Dave, for the detailed explanation - it helped me understand the system better. This is one incredibly awesome site! ~Wheels

[nuckels11]

We can't get our Atwood 8900-III Series furnace to work. We checked the fuses - ok; did a reset and nothing. The fan doesn't even come on. We've checked the manual and believe we have followed all the directions there. Does anyone have any ideas or experience with this furnace that they would like to share. Thanks.

[Dave78Chief]

Bob,
Your 8900-III is different in that blower motor operation is controlled by the ignitor board. Additionally, there is an LED on the control board that can flash an error code.
Installation manual: http://www.atwoodmobile.com/manuals/...2011.27.06.pdf
Wiring diagram and error code table on page 5.

1) Verify the ON/OFF behind the door and the CB are both ON.
2) Set Thermostat on and check is to see if an error code is present on the control board. If there is, refer to error code table on page 5.
3) If no codes then see if 12VDC is getting to the unit. Comes in on TB-1 and goes to CB/switch. Should be voltage on either side of CB/switch (black meter lead grounded).
4) If voltage is there, then problem could be dirty connectors on control board.
5) If that is OK then either:
A) Control board is bad (possible maybe a fuse on the board)
B) Fan is bad and may have taken board with (hopefully not). You can disconnect fan lead from board and jumper 12VDC to the fan red lead to test it.

Dave

[nuckels11]

Dave78Chief, I checked the circuit board and got the furnace to work one time. Then I cleaned the contacts but it wouldn't start again. There are no error codes anywhere. I am not sure what your mean by TB-1? A faint sound can be heard from inside the coach, but there are no error codes on the panel there either. We really appreciate your help and if there are any more words of wisdom you can provide that would be great. If it is a bad CB do you have any idea of a replacement cost?

[Dave78Chief]

The DC wiring diagram is located at the bottom of page 5 of the manual (linked to above)



As shown in the diagram, power from the your coach DC fuse panel is fed to Terminal Board pin 1 (TB-1). Thats the +12V DC Supply red wire.
The documentation says there is a LED mounted on the ignition board (item 13 in parts drawing). It should flash on an error.
The error table (page 5) says the LED will:

DIAGNOSTIC CHART - FAULT LED INDICATION
Internal Circuit Board Failure.............. Steady on, no flashing
Limit switch/Airflow problems............. 1 flash with 3-second pause
Flame Sense Fault........................... 2 flashes with 3-second pause

Ignition Lockout Fault...................... 3 flashes with 3-second pause
I will add........... No lights

1) Internal Circuit Board Failure - Obvious meaning

2) Limit switch/Airflow problems - Command from Thermostat to turn ON recieved however:
__A) Sail switch is not being turned ON. Fan blows air across switch vane causing the switch to turn ON. Either the fan motor is not running, the switch is bad, or there is an air flow blockage (mouse nest, wasp nest, mud dauber nest, etc) blocking air flow through the unit. Fan could be bad or ignitor board bad or fuse (if there is one) on ignitor board blown, or wiring problem.
__B) Limit Switch - defective (opens when furnance is to hot [High temp safety])

3) Flame Sense Fault - Ignitor board does not sense flame - High tension lead is dual purpose. Provides spark voltage to ignite gas then is used to monitor flame condition.
__A) Dirty, misadjusted or worn/broken electrode.
__B) Bad/dirty ignitor lead connections
__C) Defective Ignitor Board

4) Ignition Lockout Fault - 3 failed attempts to light furnace.
__A) Low 12VDC voltage
__B) LP gas tank empty

5) No lights - No Fan operation
__A) No power to furnace from coach DC fuse panel
__B) ON/OFF switch or circuit breaker OFF
__C) Dirty/corroded electrical connection
__D) Defective Ignitor Board



Sequence of Operation:
1) 12VDC from coach DC fuse panel is comes into TB-1 (red wire) and is routed to Switch/CB then:
__A) to ignitor board (red wire) to PWR terminal at top of board
__B) Thermostat (blue wire) via TB-2 (+Thermo)

2) Thermostat is turned on routing 12VDC back (Thermo) to TB-5 and then to (other blue wire):
__A) Sail Switch
__B) Ignitor Board

3) The signal from the Thermostat to the ignitor board tells it to:
__A) Turn on the fan
__B) Check for signal from Sail Switch/Limit switch path. If no signal recieved then shut down and flash Limit switch/Airflow error code.

4) Ignitor board sees Limit switch signal so it waits a few more seconds (purge chamber of residule gas) then commands gas valve open and high voltage ignition spark.

5) Wait 5 seconds then monitor high tension lead for flame sense. No flame sense then set Flame Sense Fault error code then,

6) Go back to Step 3) and retry 2 more times to ignite furnace. If no joy, set Ignition Lockout Fault error code.

To reset Ignition Lockout Fault, turn thermostat OFF for 30 seconds then back ON and furnace will try again.


Make sure gas is ON at tank (good flame at oven top burner? {purge piping system of air by having good flame at stove top; applies to furnace and water heater})

Battery voltage above 12VDC - Fan runs to slow at lower voltage resulting in sail switch not coming ON.

I hope that answers your questions,
Dave

[nuckels11]

Dave, These are the things I have done to get the furnace to lite; 12VDC @power pin OK,good continuity and contacts cleaned on sail switch, circuit board check by a tech ok, cleaned all contacts on circuit board, LP gas ok, fuse ok,on/off switch ok,Faint click heard 7 seconds after thermostat is turned on, Still the circuit board doesn't throw any error codes and the LED did come on when the tech checked the circuit board. fan works when jumped. What am I missing? Thanks for all the info.

[Dave78Chief]

Fan not turning ON (fan works if jumpered) - Only thing that can be the problem is:
1) 12VDC "PWR" lead from circuit breaker to control board problem (red wire)
2) Theromostat not providing 12VDC to Control board (blue wire from TB-5)
3) Control board not providing 12VDC to the fan
4) Wiring between control board and fan problem.

You will have to make voltage checks to determine what is not there.

Dave

[nuckels11]

Dave,
I want to thank you and everyone that responded on my dilemma on the furnace. Me and my buddy got it fixed yesterday. We took every measurment every where we could think of and finally he was taking measurments on the wires on the back side of the on/off switch and one of the blue wires just came unattached which was a thermostat wire. I had jiggled and pulled those wires several times to see if I had a bad connection to no success. It had just been hanging on by a thread. Thanks again to all.
Bob

[Rockcop]

I have a different problem. I get an ignition lockout fault. Furnace lights and runs for from 5 sec to a minute. Sometimes it works normally. Checked wires for corrosion, tight connection, etc and found no problems. LP tank is full and voltage is good. Any suggestions?