Atwood 8520-IV not starting fan

[Old-Biscuit]

Originally Posted by BobbyT49

Hi to all,
I just ran across this thread and unfortunately, I'm having similar problems with my 8535-IV. I have power at the reset switch and the circuit breaker, but no fan or attempt to fire at all. My unit does not seem to have the TDR as in the picture by un4givn85. I also have a blue wire that connects to the control board with the red, white and yellow wires. Not sure how I could check the sail switch or high limit switch. I'm thinking that the control board may be shot. Should I check anything else before ordering one? Any thoughts would be appreciated.
Thanks, Bob

Yours uses BLW/PWR circuits on Circuit Board (replaces the TDR
function)



When T-stat Closes (CLICK--Calls for Heat) you should have 12VDC on RED wire on circuit board at BLW to Motor
*Should also have 12VDC on Blue Wire ---connector to circuit board)
Got DC and no Fan Start...then circuit board has failed

**With On/Off switch on/CB reset you should always have 12VDC on PWR to Circuit Board
No DC....blown Fuse in DC Dist Panel for furnace (or From A/C Control Module)

[BobbyT49]

Thanks Old Biscuit,
I appreciate the response. I'll check that out. I figured its either the board or the thermostat is not sending the signal to fire. Someone prior to me owning this, has the thermostat on one zone but both AC's work. Wondering if they screwed up the signals to the furnaces, as the other furnace is working fine. If I understand, if I have DC at the blue wire it should be getting the command to fire.

Thanks
Bob