Atwood 8940-III fan won't stop

I would say the Module board was damaged when the old fan motor seized and the Fan relay on the module board is sticking or burnt/pitted. Never had or heard of this type problem, but the odds are replacing the module board will fix it. Doug

the board has a time delay for turning the gas valve on/off.Also the fan delay for turning the fan off. Here is the link for troubleshooting you furnace. The first thing a all techs do is check all connections on the board also battery should be fully charged. http://bryantrv.com/docs.html

fischer wrote:
the board has a time delay for turning the gas valve on/off.Also the fan delay for turning the fan off. Here is the link for troubleshooting you furnace. The first thing a all techs do is check all connections on the board also battery should be fully charged. http://bryantrv.com/docs.html

Good suggestions, I will expand a bit:


Sounds like the OP battery (s) are low or your converter/charger is not functioning.
Fan slows = low battery! Fan may continue to blow because the controller does not have enough voltage to control the furnaces functions.

Make sure that you are plugged into shore power, check your battery monitor if you have one.

Plugged in after about 30 min your battery monitor should show that the battery voltage is high.
This is not a charged indication as some believe... It takes hours to charge a low battery!

Here is a direct link to the furnace section of the manuals provided by Bryants.

http://bryantrv.com/docs.html#furnace

Jim

The circuit broad has a 'blower power'........old timed delay relay incorporated onto board

T-stat calls for heat
Fan gets power thru circuit board
Fan comes up to speed......sail switch makes up/high temp limit switch already closed...power goes back to board and then to gas valve/ignitor
Heats blows......
T-stat set point gets satisfied....t-stat contacts open
Power to gas valve gets dropped
Fan continues to run until 'blower power' shuts down

Fan not shutting down.......
Bad contact on connector at board. Pull/clean board contact surface with pencil eraser and check that pins in connector are not bent
OR
bad 'blower power'..........NEW board


Low battery ,,,,,,,,,
Fan would not start and if it did it wouldn't come up to high enough speed to move enough air to satisfy sail switch.
Gas valve wouldn't stay open....heating wouldn't happen

Do you have the thermostat fan control set to auto?

Old-Biscuit wrote:
The circuit broad has a 'blower power'........old timed delay relay incorporated onto board

T-stat calls for heat
Fan gets power thru circuit board
Fan comes up to speed......sail switch makes up/high temp limit switch already closed...power goes back to board and then to gas valve/ignitor
Heats blows......
T-stat set point gets satisfied....t-stat contacts open
Power to gas valve gets dropped
Fan continues to run until 'blower power' shuts down

Fan not shutting down.......
Bad contact on connector at board. Pull/clean board contact surface with pencil eraser and check that pins in connector are not bent
OR
bad 'blower power'..........NEW board


Low battery ,,,,,,,,,
Fan would not start and if it did it wouldn't come up to high enough speed to move enough air to satisfy sail switch.
Gas valve wouldn't stay open....heating wouldn't happen

Nice description of function, but:

Read the first post by the OP...

"Now it runs, cycles on and heats great. When it comes to temp, flame cuts off but fan only slows a lot""

jim

Thank you for all the responses so far!

Been connected to shore power since October. No issues anywhere in rig with the 12v system. When fan straight wired to battery, runs at speed. Board and fan getting 12.39v to them. There is a discoloration at the motor connector on the board .... probably from short.

New info .... when cabin comes to temp, flame shuts off, fan slows, and LED flashing/clicking atarts. I waited 5 minutes with mo change. When I bump thermostat to higher temp, fan speds ip and flame does not come on. Clicking stops. Blows cold air until I turn off system at thermostat or outside switch. Reset outside, and it will cycle and flame/heat once. Repeat.

All I am reading suggests that there is a board control issue with fan control or flame sensing or something around that part of cycle. I will take board to dealer tomorrow and have tested.

joebedford wrote:
Do you have the thermostat fan control set to auto?

The Auto function on an RV tstat has no function on furnace mode. The only exception is the hybrid Atwood Excaliber model furnace with 2 fan speeds, but that model is usually on Hi line 5th wheels. Doug

I should have stated to check the voltage at the red motor wire when the fan goes to slow speed. The fan should ALWAYS have the 12.9 or higher voltage at the fan if it is running. If it does NOT(lower than the voltage of the RV), then the control module is the highly probable problem as the motor gets its voltage direct from the Fan relay on that board. There are 2 red wires connected on 2 spade terminals of that board. One is the supply 12 volts for the motor and the other is the direct wire to the motor after the fan relay closes. BOTH of those wires should have 12.9 or higher volts when your malfunction happens. IF they BOTH have lower than line voltage, then the control board is not the problem. If one has line voltage(supply) and the other has a lower voltage then the board is indeed defective. You CANNOT test for this problem with a Circuit board tester as the problem appears to be HEAT related, since the furnace runs normally until it tries to cycle off. Doug

TomTHaullie wrote:
Thank you for all the responses so far!

Been connected to shore power since October. No issues anywhere in rig with the 12v system. When fan straight wired to battery, runs at speed. Board and fan getting 12.39v to them. There is a discoloration at the motor connector on the board .... probably from short.

New info .... when cabin comes to temp, flame shuts off, fan slows, and LED flashing/clicking atarts. I waited 5 minutes with mo change. When I bump thermostat to higher temp, fan speds ip and flame does not come on. Clicking stops. Blows cold air until I turn off system at thermostat or outside switch. Reset outside, and it will cycle and flame/heat once. Repeat.

All I am reading suggests that there is a board control issue with fan control or flame sensing or something around that part of cycle. I will take board to dealer tomorrow and have tested.

Like I stated earlier.......blower power on board