Forum Discussion

mikemc53's avatar
mikemc53
Explorer II
Jul 19, 2021

Dometic AC Issues

OK, it's a new rig (3500 miles bought new) with a Dometic 15K BTU roof top unit. First trip was really hot and AC worked extremely well, though there was a time or two when it would seem to want to run forever and I looked for and couldn't find the sensor thinking it might be in a bad location. Anyway, everything worked out and we parked it for a couple of weeks. Now on our second trip with today only being in the mid to upper 70's the unit has been acting up. It worked fine until mid-afternoon and stayed off for quite a while. I had t set at 75 and the place was getting pretty stuffy so I kicked it down to 74 and then 73 and then 72...nothing. I shut it off (the breaker had not been tripped) at the thermostat, let it sit for a minute and when I turned it back on it started right up and the compressor came on. After it ran for a while I kicked it back up to 75 and heard the compressor shut off but the fan ran for a very long time and then it shut down. Again it would not come back on even after a long break and dialing it down to 71. Coach was getting warm. Shut off the thermostat and after a minute turned it back on and it fired up, again - compressor too.

Any thoughts?
  • dougrainer's avatar
    dougrainer
    Nomad
    Jul 20, 2021
    It is installed in the evaporater fins and accessed from inside just above the return air filter. It looks like a thin white wire with a black bulb on the end. It is 2 wires. I forget which is which. You either connect the 2 wires to bypass or leave them open to bypass. Just cut the wire in the middle of its length. If the compressor runs either way and STAYS running then your Sensor is bad. If not, then just splice the wires back together. Doug
  • mikemc53's avatar
    mikemc53
    Explorer II
    Jul 20, 2021
    Thanks gang. Where would I find the freeze sensor and what does it look like?
  • 3_tons's avatar
    3_tons
    Explorer III
    Jul 19, 2021
    dougrainer wrote:
    Freeze sensor defective/intermittant/installed wrong. Doug


    Sage advice from a knowledgable Tech….
  • mr_andyj's avatar
    mr_andyj
    Explorer
    Jul 19, 2021
    When you say, running, do you specifically mean the compressor is running or the fan. Does the fan always run to move air in the cabin and you are saying that the compressor never comes back on?

    Often the temp sensor is in the return air duct where you can see it, it looks like a copper wire with a fat end. pencil size usually. This needs to be in the air flow, not tucked away. Unlikely the issue though in your case.

    All the wall thermostat does is operate like an on/off switch. The temps control the switch. You should be able to literally pull the wall wire out and connect the two wires together and this will turn the compressor on. If you suspect a bad thermostat you can try this, but you will have to manually un-do the wires when temps are reached. Not very good for normal use, but good for a diagnosis.

    Voltage or freeze sensor of course more probably causes....
  • Hemling's avatar
    Hemling
    Explorer
    Jul 19, 2021
    Our is a Coleman unit, but does the same thing. Mine is a little over 4 years old and the first two years or so everything was fine, but then the A/C would cut out, and sometimes the thermostat itself would go blank too. Sometimes you could just reset the temp and it would kick right back on, other times the thermostat was dead. It is hard wired so not a battery issue. I pulled the unit off the wall and the wires going into the thermostat were all corroded, so I cut them back to clean wire and reinstalled. It still did the same thing. I have a new thermostat on order. I'm thinking in my case the corrosion on the wires leads to shorting connections within the thermostat itself. It has done this at numerous different sites over the past two years, so I rules out low voltage. Also, sometimes I can get it back to life by wiggling the thermostat around, tapping it, generally abusing it. . . so that's where mine is if that helps.
  • scbwr's avatar
    scbwr
    Explorer II
    Jul 19, 2021
    First thing that pops into my mind is possible low voltage at the power post. Do you use something such as a Progressive Industries EMS unit which protects your unit and makes it possible to track the voltage? On our last outing, I couldn't run both of our AC units because one leg of the 50 amp circuit was only about 104 volts, which caused my EMS unit to shut down power to the MH.

    If you are still on the trip, at least check the power post with a voltmeter. If low voltage isn't the problem, then it becomes a question of possibly the thermostat or the AC unit and I can't help with that.

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