Dometic 15.5 A/C short cycling

[crm2525]

Hi, we have a 2019 2401RG and randomly the air conditioner compressor short cycles, meaning that it will turn off and within 5-10 seconds the compressor will come on again. Of course the dealer we purchased it from looked at it and said no problem found, and then tried to upsell me a new different brand a/c on a 3 month old trailer. I believe the thermostat to be the issue but have no real way to diagnose this, and definitely have no desire to put it back into the shop for 2 or 3 months again. Has anyone else run into this before?

[cdv3286]

I have the same issue. I recently went through and better insulated my duct work in the AC to see if it was sucking in cold air and causing the probe to think it was cooler than it was... have not had a chance to see if it works yet. I am never lucky enough to get the fix on the first shoot!

Posting to follow and bump up in the forum!

[backtrack15]

Do a google search for this issue. I expect you'll find some threads on a couple of RV forums about it. I remember reading a few threads a couple of years ago. I don't recall the solution. If, due to your research, you decide it is likely the thermostat and/or the control box within the AC, you can request these parts directly from Winnebago towables and install them yourself. I did so for another reason. They shipped them right to my house, no charge and no hassle.

Edit: oops - just noticed this is an old thread. oh well.

[imnprsd]

All these things run on the same principles, but have their own nuances.

Sound like something is turning off your compressor.

* Do you get the same "short cycling" on generator power and shore power?

* Is your thermostat? Maybe, but I doubt it. All these things do is "Call" for the AC to turn on, and if it makes the call it usually is not the thermostat.

==> You can use a multi-meter to see what voltage you are getting? I think it's + or - 12Vac. (Can anyone verify this?)

==> I saw a video by the AZ Expert who used a 12Vdc portable drill battery as your source to take the place of the Thermostat Call so you can diagnose the AC at the circuit board. (Slick, but I have never done this myself.)

* Is it your PTC-R shorting out, aka Motor Starter? Usueally, not unless you see melting of the outer plastic case. This happens when repeatedly plugging into less than 110V at the shore power pole.

Assuming you have a PTC-R (Motor Starter), because some do not come with a one and some do not, the function of this PTC-R is suppose to open after the compressor starts... in mili-seconds.

...And this is good. Otherwise a short will cause your Compressor Start Windings to remain "on" and that will burn-up your Start Windings inside the compressor... if...

* Your overload circuit protector (another PTC-R) that sits on top of your compressor's head monitors the heat off the compressor head and will open if it gets hot... shutting off your AC.

So your "short cycling" sounds like a heat problem caused by A) a burned out start winding or B) a bad PTCR or C) more likely your RUN CAPSCITOR IS BAD!
...Just a guess.

==> Always start by measuring your RUN CAPACITOR first, then investigate your START CAPACITOR & MOTOR STARTER, aka Hard Start Kit. (Again, assuming your roof top AC came with a HSK?)

Note: If you have a bad run capacitor your compressor will not run right and I suppose it can over heat too! ...Always check your Run Cap Value first, and always use the right uF value designed to run with your AC compressor.

Then you can do a little tuning with the Start circuit; I.e., some people remove the OEM-Hard Start Kit and go with a SPP6 or SPP4E... but you need to watch come You-Tube Videos before you go troubleshooting this yourself.

NOTE: The circuit diagram is for my Basement AC, which is larger than you roof top AC. So don't assume this circuit applies to your situation. I'm just including it so you can see where the PTC-R is located in the circuit.

[kitsonkt]

Did you ever find the fault or issue with your a/c?? Curious because I think mine is doing the same thing.

[imnprsd]

kitsonkt: How hot is is outside where you are trying to start your AC? And how long does your compressor run before it shuts down and then restarts?

In a car, short cycle occur primarily due to low pressure or no pressure in the AC line. However, our ACs do not come with Schrader Valves to check for this, but you can add one or buy a port-pinch as I call it on Amazon.

Heat affects the PTC-R, but more heat would in theory keep the Hard Start Capacitor "out of the circuit" ...making it hard to start your AC, because and "open" start circuit takes the start capacitor out of the circuit.

...Assuming you have a "hard start kit" in your roof top AC to start with?

If you do not have one, and you are having difficulty starting your compressor, you can always add a Hard Start Kit in parallel with your run capacitor... but use a small start capacitor. (The start capacitor value I published are for my Coleman-Mach (2-Ton) Basement AC, which includes a 88-108uF Start Capacitor, and I'm guessing the size you want is smaller than this.)

Note: Some people do away with the run capacitor and use a 3-N-1 Supco device, but I can't school you on that, because I have never gone this route.

I'm not familiar with roof top ACs, but there are YouTube video on this subject and the principles are the same. Specifically, you use a "Hard Start Kit," which includes a Start Capacitor and PTCR-Motor Starter... to help the compressor start.
After that, the PTC-R opens the circuit and just your RUN CAPACITOR is left to drive the phase shift needed to make your compressor run.

So this hard start subject may not apply here. I only mention it for a full understand of the circuit diagram I posted above.

That said, you guys are describing a shut-down problem... not a circuit breaker problem turning off. Is that right?

If this is the case, you have to check the value of your RUN CAPACITOR, before you do anything else, as this may be why your compressor is heating up... then you system shuts down... but the thermostat is still calling for your compressor to run... and you see it as a "short cycle" problem. ...And if you have low 134A then that might explain why your compressor is short cycling and not tripping a breaker.

OTHER CHECKS INCLUDE

* AC relay check/replace
* PTCR Check
* Overload PTCR bypass, but be careful of this one.
* Ohms check to your compressor windings

I hope this helps! Good luck.

[crm2525]

No, I haven't pursued it to much due to randomness of event. I still suspect the thermostat because it will also change settings by itself including switching to celsius.
I will post again in June, going to the beach in the first week. Good luck to you

[kitsonkt]

Thanks for the well informed reply. I’ll have to check the run capacitor. I’m not completely technically or mechanically challenged but I’ll have my son-in-law do the work. Lol