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AC Fan Auto option not working

I have 2 dometic ac/heatpump units, fortunately both units are running and cooling.  My issue is the auto fan (cooling) option does not work. Both the continuous fan options of low and high are working .

Have new capacitor, new control board and new thermostat.  Have also reset thermostat.

Any suggestions for what to try next to fix problem?

Susan Wright
2003 U320 38'
Greyghost
2002 Jeep Wrangler Black



Susan Wright
2003 U320 38' tag Build 6219
GreyGhost
Motorcader18153 "if I ever get it on coach"
2002 Jeep Wrangler

Re: AC Fan Auto option not working

Reply #1 –
I'm not familiar with your AC's, do you have a manual that explains what the auto function is supposed to do?  Does it switch between heating and cooling according to the temperature, or does it choose between low and high fan speed, for example? 
Jerry Whiteaker former owner 96 U270  36' #4831 Austin,TX-Owner Mods LCD TV w/front cabinet rebuild - LCD TV bedroom - Dual Central AC, either can cool coach w 30 amp - Skylights at roof AC openings - Drop ceiling for ducting of AC - Shower skylight white gelcoat/wood/epoxy frame - Air Springs/Shocks replaced - 2014 CRV - 8K Home Solar - Chevy Volt

Re: AC Fan Auto option not working

Reply #2 –
Jerry, when set on Auto, whatever temp setting you have set will come on when that temp is reached and shut down when done. 
Not sure if the switch has a mercury bulb in it or not.  That could be the issue.  At least the units are working!
Joe & Dottie Allen
Sold!  December 2023.      2000 U320; build # 5645
Our coach " Maxine"
Motorcade #  15922;  Escapee 150950; FMCA F330833; Boondockers Welcome;  Harvest Hosts;  Thousand Trails
'98 U320 from 2000-'06
USAF '62-'66

"Do not spoil what you have by desiring what you have not; remember that what you now have was once among the things you only hoped for." ―Epicurus

Re: AC Fan Auto option not working

Reply #3 –
Susan, it's not clear what you mean by "does not work".  If you have the 5 button Dometic DuoTherm Comfort Control thermostat like mine, there is a 2 minute delay before the air conditioner(s) kick on.  Does this sound like your situation?  Woody.
2001 4010 U320 build #5865 "Bluto-d-Bus" since 09/18
2006 Honda Element ESP Toad
Full timing since 2016 in Western MT
Copilot: Sitka
1975 GMC 260 Avion: sold

Re: AC Fan Auto option not working

Reply #4 –
Woody yes I have the 5 button Dometic Duotherm .....yes that is the situation(guess I should have thought of the term...kick on) but I have waited longer than 2 minutes ...have put the thermostat setting down to very cold and it will still not "kick"on with the auto fan selection....

I am leaving my home on Monday for a 4 month sojourn around the country.  Not going to worry about it too much as I do have air, but do like having the option of using the "auto fan" function so air will cycle on and off according to temperature.

"Juicesqueezer".  Yes that is how it is supposed to work but mine doesn't currently...we tried a new thermostat but that did not fix the problem either.

Will keep plugging along....Thanks for all comments

I have named my coach GreyGhost because sometimes things work and sometimes they don't.
Susan Wright
2003 U320 38' tag Build 6219
GreyGhost
Motorcader18153 "if I ever get it on coach"
2002 Jeep Wrangler

Re: AC Fan Auto option not working

Reply #5 –
Susan,

Did the A/C units run on auto before you changed out the "control board"?
Or did you change out the control board to try to fix this problem?

The reason for these questions is that the dip switches on the control board may be set wrong. 

Mike
Pamela & Mike 97 U 320

"It's not what happens to you, but how you react to it that matters."

Re: AC Fan Auto option not working

Reply #6 –
 Mike...changed the control board trying to fix the problem.....Thanks ,Susan
Susan Wright
2003 U320 38' tag Build 6219
GreyGhost
Motorcader18153 "if I ever get it on coach"
2002 Jeep Wrangler

Re: AC Fan Auto option not working

Reply #7 –
Update........went out to GreyGhost this am and did yet another thermostat reset.....the auto fan function is now working on both units......it is a good Ghost.

BTW....a technician was doing all of the aforementioned work......he gave up.....I did not have to pay for parts....so all is good...just a lot of time and aggravation

Thanks to all


Susan Wright
2003 U320 38' tag Build 6219
GreyGhost
Motorcader18153 "if I ever get it on coach"
2002 Jeep Wrangler

Re: AC Fan Auto option not working

Reply #8 –
Similar problem...

I replaced the Start capacitor.. Shorted out the screw driver when it slipped. when I checked the AC after install  I had only low on the fan speed.. I tried to fix the board and that did not work. I ordered a new board and installed it... everything but auto is working.. So its either on the board or the  thermostat.. I am going to change the AC out soon.. waiting to see if the 12v store will have a 4th of july sale..

THe board was not the original number to the AC .. they say it is the newer version so I am not sure if its the board or the Thermostat at this point so I am just going to replace it I think

Possible something was hit on the board or a switch or wire Moved or damaged on the installation of the capacitor.

Re: AC Fan Auto option not working

Reply #9 –
Quote from: Duellyduo –
Update........went out to GreyGhost this am and did yet another thermostat reset.....the auto fan function is now working on both units......it is a good Ghost.

BTW....a technician was doing all of the aforementioned work......he gave up.....I did not have to pay for parts....so all is good...just a lot of time and aggravation

Thanks to all

4 button or 5 button Thermostat? If the 5 button what is the reset procedure?

Re: AC Fan Auto option not working

Reply #10 –
Mine is 5 button

a. Turn the ON/OFF switch to .OFF. position.
b. Simultaneously depress and hold the MODE and ZONE
push-buttons while turning the ON/OFF switch to .ON..
FF should appear in the LCD display until the MODE
and ZONE push-buttons are released.

After you do the above turn the thermostat off then on again.  Hopefully this will work and when you try the auto fan option it will take a while to kick on.

Susan Wright
2003 U320 38' tag Build 6219
GreyGhost
Motorcader18153 "if I ever get it on coach"
2002 Jeep Wrangler

Re: AC Fan Auto option not working

Reply #11 –
Glad to hear you fixed it yourself, Susan!  Kudos on your perseverance and tech savvy.  Woody.
2001 4010 U320 build #5865 "Bluto-d-Bus" since 09/18
2006 Honda Element ESP Toad
Full timing since 2016 in Western MT
Copilot: Sitka
1975 GMC 260 Avion: sold