Dometic AC stops cycling

Discussion in 'Towing, Vehicles, Maintenance and Repairs' started by Handeman, Jul 21, 2017.

  1. Handeman

    Handeman
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    Our Dometic AC and Single Zone LCD thermostat is set for the temperature we want. The unit will run until the inside temperature gets down to the set point. Once it gets to that point the AC does not come back on. The inside temperature can go 10 or more degrees above the set point. If we turn the thermostat off and back on, the AC will run another cycle and then not come back on again.
    We tried settings the fan to low instead of auto and it still shut off when it reached set point.
    Why doesn't it continue to cycle?
     
  2. BankShot

    BankShot
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    I am by no means an electrician but offhand my first guess would be that your thermostat might be the culprit. A sensor has probably stopped working or it could even be corrosion has formed inside that in turn is keeping the 'signal' from being detected, etc. I'm sure someone with an electrical background will jump in shortly and give you some better ideas of what may be going on. If you don't have the ability to do so yourself then I'd have a qualified electrician take a look at it and run some tests to determine if it is the thermostat. If it isn't then he can also check the signal path and locate the problem for you. Hopefully it is just the thermostat as normally that is a quick and inexpensive fix, Depending of course who the electrician is.................. :rolleyes:

    Good luck with this, BankShot.................(aka Terry)
     
  3. mdcamping

    mdcamping
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    some things to try

    cycle power completely off to tstat, check connections in back, make sure they are tight. if it comes with backup batteries replace. If it's programmable, check the program. This all said, good chance it's the Tstat that is the problem.

    Here is some other things to consider

    Did you notice a cooling on symbol/LED Light with this problem? or does the symbol/led light only show when you do the manual re-cycle, that would point to a issue with the tstat or contacts inside tstat. If symbol/led light is always on see next paragraph

    notice any recent changes with temps coming off the a/c? is running longer than usual? There may be a chance the outdoor unit is struggling/locking out and only on the manual reset does it restart.

    Hope this helps
    Mike
     
  4. Handeman

    Handeman
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    Terry & Mike,
    Thanks for the replies.

    We've been able to refine the symptoms some. We put the system on low fan and set the temperature. The fan ran and the a/c cycled all night just fine. In the day we set it to auto and the a/c and fan cycled on and off keeping the temperature fine all day. In the late afternoon it cycled off and never came back on. Last night we set it on high fan and a few hours later, with the fan still running, the a/c stopped cycling. In each case, clicking the On/Off Mode button to cycle the system through off and back to a/c mode gets it working again.

    I have not been able to check the connection yet but it does seem like that might be a problem since it is so intermittent. From the owner manual, it sounds like the thermostat connection at the a/c unit is a standard phone jack and plug. That sounds like the most likely culprit to me.

    We'll be back home this afternoon so I'll start digging into it.

    Jeff
     
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  5. mdcamping

    mdcamping
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    Though this is NO guarantee, plug in connections/contacts and especially ones exposed to outdoor elements over a long period of time will sometimes get pitted. Try cycling and cleaning the plug connections where they make better contact. I have had success with this in the past on appliances.

    Good luck!
     
  6. BankShot

    BankShot
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    Ironic this happened to you as just late yesterday I went out to the coach to so some initial checks prior to leaving on a trip and found that the kitchen ceiling fan would not on. The cover would raise and lower no problem but the fan refused to spin at any speed. We have a mobile guy that does any work I am not capable or doing so he stopped by and found that the small circuit board on the fan itself had gotten corroded over time and was the culprit. We are hoping he can locate a new one because if he can't we will will either have to do without a rotating fan in the kitchen area or buy a whole new fan unit. He said it was just that over time moisture/crap in the air had caused the corrosion. I about fell over when I found out what one of those little puppies cost to replace.........:eek: Seems that problems such as this are all just part of the "RV experience"............

    Again, good luck with your AC problem.................

    BankShot...............(aka Terry)
     
  7. CenFla Camper

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    Handeman:

    Have you figured out the cause for your AC issue yet? I am having a simliar problem with my Dometic AC unit / LCD T-stat. We went camping this weekend and the unit seems to function properly during the day in hot humid Florida weather, but then by night the system will cycle off and never come back on. I have to shut off the thermostat and turn it back on for the system to complete one more cool cycle only for the system to go back off. Friday night, I was able to run it on low after a while for the rest of the night, but no such luck Saturday night. Fortunately, I keep an emergency fan stored for such a situation. My T-stat connections appear secure and no error codes. I've seen similiar issues mentioned on the web, but no one has posted a follow-up solution.

    Thanks!
     
    #7 CenFla Camper, Aug 27, 2017
    Last edited: Aug 27, 2017
  8. DonnieO2

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    Could it be that in the high humid days they are freezing up? How long is the unit totally off? Try to leave it off for several hours before cycling it on again. If system runs ok for long period and problem occurs again it could be the AC unit might be low on freon.
     
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  9. CenFla Camper

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    Donnie. Thanks for the feedback. Your comments are consistent with the feedback I got earlier tonight from a retired AC technician. He also indicated the possibility of low freon combined with being on constantly all day in a high humidity environment. Plus there was rain that added to the moisture content in the air.

    I am going to have the unit checked out since it still has some warranty.
     
  10. DonnieO2

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    Please post your findings as it might help others with the same problems.
    Thanks
     
  11. Handeman

    Handeman
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    Your problem seems very similar to ours. We have the problem, but not in high humidity.
    Fan set on Hi and runs all the time and the AC will cycle and keep temp.
    Fan set on Lo and runs all the time and the AC will cycle once or for 24+ hours, but at some point the fan turns off (which it shouldn't) and then the system never comes back on until you cycle the system.
    Fan set on Auto and fan runs when AC runs. This may work one cycle or for 24+ hours. But at some point it stops coming on and the RV gets hot.

    It's a brand new unit and cools very well when running so based on my experience with home heat pumps, I don't believe the Freon is low. I think the problem is in the thermostat or the circuit board in the AC unit.

    BTW, when set to furnace (heat), it runs the furnace just fine and maintains the temp at setpoint.

    I have not found a solution yet but will let you know when I do.
     
  12. mdcamping

    mdcamping
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    Based on what you said this does not sound like a leak.

    Usually you will get a cooling "on" indicator when space temp is above setpoint. Try tapping the tstat when it's hanging up. if you hear the compressor kick in it will prove the tstat is NG...about 10 to 20% of the time this will work on electronics.

    Being a heat pump, might also be a sensor/thermistor/board thinking it's back in the heating mode

    Good luck

    Mike
     
  13. CenFla Camper

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    Thanks Mike. I'm going camping again for the Labor Day weekend and will check the thermostat again. In the meantime, I scheduled an appointment for next week to get it looked at.

    My trailer is a 2015 model. Last year, I first had the circuit board replaced and then the whole unit because it stopped working completely. On the new unit, I would prefer that the tstat is the issue or a sensor rather than a leak.

    Mike
     
  14. CenFla Camper

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    On Friday night when the system went off, I turned off the system completely and then turned it back on the low setting instead of auto. The system ran the rest of the night. However, on Saturday night, the unit would not stay running on the low setting. We'll see what we experience this weekend. I have my counter top fan to assist just in case.

    Mike
     
  15. Handeman

    Handeman
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    The tstat show the cooling light all the time when it's in cooling mode so that's no help, but I haven't tried tapping on it. I'm not hopeful that that would work. It does work if we shut it off and wait until the On/Off led goes off and then turn it back onto cooling mode. We just don't know if it will work for one cycle or 20.
     
  16. mdcamping

    mdcamping
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    unless the tstat contacts are hanging up it's beginning to sound like the problem is in the heat pump.

    Mike
     
  17. BankShot

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    Just a question before I shoot my mouth of here but when the AC stops when you are hooked up to an AC pedestal, does the rest of the 110 stuff in the RV go off also? If so, then the problem just might be with the "transfer switch". As I am not electrician I am not qualified to say for certain this could be it but if I'm not mistaken that switch or regulator as some call it, has something to do with switching the power over from AC to the generator and vice versa. Again, just an "unedumacated" guess but perhaps worth a check...... :rolleyes:

    Hope you find the problem and get it corrected as I know all too well how things like that can be very irritating....................

    All the best, BankShot..............(aka Terry)
     
  18. Handeman

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    The tstat is solid state so there are no contacts. There are three wires from the AC to the tstat. I have not confirmed, but I suspect they are +DC power, -DC power (return), and serial comms port between the tstat and the circuit board in the AC unit since that also has to be connected to and control the furnace for the tstat to operate that too. That would still leave the problem in the tstat if it isn't sending the right serial messages to the AC control board.
     
  19. Handeman

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    Thanks for the guess, but that really isn't the problem. This is a travel trailer and there is no generator. The AC only works when plugged into "shore power". None of the other AC items are affected. The microwave stays on, as does the TV and all the other AC only appliances.
    Unlike the larger class A units, the travel trailer is much simpler in how it works since there is no generator and only one house battery for DC use until you plug in the shore power. The problem is definitely between the tstat and the AC unit. I just haven't been able to figure out which one is the problem. I would just replace the tstat, but those run $100 and that's a little steep for "maybe" its the problem. I would like to get to a more definite explanation before spending the money on a replacement that might be the problem.
     
  20. mdcamping

    mdcamping
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    If you hadn't tried this all ready, the only thing I could suggest is if the comm port is a plug in style would be to find where it lands on the opposite end, shut off all power and remove and reconnect the plugs. and while your there check all other connections, especially anything low voltage.

    Mike
     

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