One of the air conditioners in my fifth wheel is acting up and I can’t afford an rv tech right now so I’m handling it on my own. I have a pretty decent electrical background so I’m not going to kill myself touching something I shouldn’t so don’t worry. The fan and compressor switch off and on independently of each other. They’ll both turn on then one will turn off while the other is still running and vise versa then it’ll turn back on.. Sometimes the fan will kick off but the compressor is still on and humming. They’ll only stay on or off for a couple minutes at a time. The fan will also come on when I turn the breaker on with the thermostat unplugged. I’ve already tested the capacitors and replaced one which wasn’t testing in spec but the problem persisted. I’m hoping one of y’all can point me in the right direction of what to check or replace next. Thank you. Also it is a Duo-Therm. Model #59136.331

You don't give any info on the year of the unit, the type of thermostat, or if this is a new problem so it is hard to give you much help. I will say that some of the newer digital thermostats will do more or less what you are describing so I don't know if you have a problem or not.

I think we'll need to know the Thermostat and perhaps "control box" model numbers as well as the rooftop unit itself.Is it also a Heat Pump?Personally, I'm not good at this, only mentioning what a tech or experienced DIY would need to know to try and help. Our friend, member and tech Chris Bryant has a lot of manuals on his site. Here's one on Duo-Therm troubleshooting but he has many more.Clearly, it's OK, and common, to run Fan without Compressor. But the opposite is almost never OK. Many RV A/C units start both the Fan and the Compressor at once, together. That is the utmost amperage an A/C can draw, so sometimes the manufacturer starts Fan first so its starting amp surge is over before cranking up the compressor. It would be OK to start compressor first, too, but fan has to be running within seconds to keep system pressures under control. If your fan and compressor are randomly switching on and off, that's a problem. If compressor's running without fan, that's a problem. All I can offer in closing is this - Some wall thermostats have batteries, even though the A/C has control voltage to it. I know that in home A/C, bad batteries can cause the A/C to cycle On/Off.

I also have read what j-d said. To minimize maximum current draw, some AC units stage the starting and stopping of the fan and compressor.

Sorry, didn’t realize any of that was necessary. I don’t know the exact year on the unit, it isn’t listed on the panel with the serial and model number. The trailer is 2003 though so that can give some sort of idea. The thermostat is a Digital Dometic comfort control which I’d say it’s safe to assume it isn’t original to the trailer but has been there the whole time I’ve owned it.. That being said, this is a new problem. The ac has functioned flawlessly for the last 5 years. I think this started the end of last summer when a buddy was borrowing my trailer.And yes it is also a heat pump. I’ve never used or known how to use it though lol

Heat pump? Probably at the end of its useful life. Heat pumps just dont have a great track record. I would do a few cursory checks, but plan to replace it.

Air conditioner problems | Good Sam Community

11 Replies

wa8yxm
Explorer III
Jul 01, 2018
THe fan is normally controlled by a relay that is in turn controlled by the Thermostat. I will however tell you of a problem I had not T-Stat related.

The Compressor may turn off due to freeing up of the condenser or due to "I'm cold enough" signal from the T-Stat..

ANd both of them the motor can overheat and shut down.

The problem I had.. Brand new motor home 13 years ago..

Both AC and Compressor would stop or start together (normal the compressor had a slight delay for reasons already mentioned) sometimes one would work, sometimes the other, both or neither.

Turns out I have a few screws loose (Don't say it) like the ones in the electrical distribution (Breaker box) panel which clamp the wires.. one took 3 1/2 turns before I was happy. Many were loose
dougrainer
Nomad
Jul 01, 2018
robatthelake wrote:
Have you tried a Hard Reset?
Google has the instructions!

Resetting a Dometic CCC system does nothing for operational problems like this. Doug
robatthelake
Explorer
Jul 01, 2018
Have you tried a Hard Reset?
Google has the instructions!
Born2hunt
Explorer
Jun 30, 2018
Thanks for the info so far, I’m going to look into what Chris Bryan’s has to say.
The model/ part number on the control box is 3109226.005 and it is a duo therm item as well.

No I haven’t done any work on the unit besides swapping out one of the capacitors.
dougrainer
Nomad
Jun 29, 2018
Low coolant will not affect Fan or compressor short cycling. What is odd is you state you had the Tstat disconnected and the unit came on. THAT points to a upper control board, but that would be extremely rare. You have what is known as the 5 button CCC tstat system. A compressor staying ON, when it should cycle OFF, points to a sticking compressor relay on that upper board. If that is the case you must replace the upper board. Have you had any work done on the AC system? Replace the other AC? Capacitors also will NOT cause this type problem. Doug
SageCrispin
Explorer
Jun 29, 2018
Sounds like low gas to me. Do you see any oil around any of the lines? Use a spray bottle full of soapy water and spray it on all the connections or rub spots on the tubing. If you get bubbles anywhere, you'll need a tech. The good news is that the compressor still runs.
donn0128
Explorer II
Jun 29, 2018
Heat pump? Probably at the end of its useful life. Heat pumps just dont have a great track record. I would do a few cursory checks, but plan to replace it.
Born2hunt
Explorer
Jun 29, 2018
Sorry, didn’t realize any of that was necessary. I don’t know the exact year on the unit, it isn’t listed on the panel with the serial and model number. The trailer is 2003 though so that can give some sort of idea. The thermostat is a Digital Dometic comfort control which I’d say it’s safe to assume it isn’t original to the trailer but has been there the whole time I’ve owned it.. That being said, this is a new problem. The ac has functioned flawlessly for the last 5 years. I think this started the end of last summer when a buddy was borrowing my trailer.
And yes it is also a heat pump. I’ve never used or known how to use it though lol
maillemaker
Explorer
Jun 29, 2018
I also have read what j-d said. To minimize maximum current draw, some AC units stage the starting and stopping of the fan and compressor.
j-d
Explorer II
Jun 29, 2018
I think we'll need to know the Thermostat and perhaps "control box" model numbers as well as the rooftop unit itself.

Is it also a Heat Pump?

Personally, I'm not good at this, only mentioning what a tech or experienced DIY would need to know to try and help.

Our friend, member and tech Chris Bryant has a lot of manuals on his site. Here's one on Duo-Therm troubleshooting but he has many more.

Clearly, it's OK, and common, to run Fan without Compressor. But the opposite is almost never OK. Many RV A/C units start both the Fan and the Compressor at once, together. That is the utmost amperage an A/C can draw, so sometimes the manufacturer starts Fan first so its starting amp surge is over before cranking up the compressor. It would be OK to start compressor first, too, but fan has to be running within seconds to keep system pressures under control.

If your fan and compressor are randomly switching on and off, that's a problem. If compressor's running without fan, that's a problem.

All I can offer in closing is this - Some wall thermostats have batteries, even though the A/C has control voltage to it. I know that in home A/C, bad batteries can cause the A/C to cycle On/Off.