Camper Air Conditioning

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Information for a Duo-Therm by Dometic AC unit that sits on top of a Camper.


The compressor kicks on and this part was sparking.

What it is

A positive temperature coefficient resistor (PTCR) is a resistor that varies its resistance with its temperature. A resistor that changes its resistance characteristics as a function of its temperature is also called a thermistor. So the PTCR is technically a thermistor.

Figure 1 shows a PTCR wired in a circuit. Whenever a thermistor increases its resistance with an increase in temperature, it is said to have a positive temperature coefficient (PTC). If the thermistor decreases its resistance with an increase in its temperature, it is said to have a negative temperature coefficient (NTC).

What it is doing on your RV


On most newer RV air conditioner units, it is very common to burn out the start capacitor and PTCR (positive temperature coefficient resistor) when using the unit in hot/humid conditions ...OR.... when you are stressing it by adding an additional load to the trailer 120VAC wiring, such as a hair dryer or microwave.

In effect, it leaves no 'head room' for the capacitor to draw from when the compressor starts and calls for the extra voltage boost while the other appliances are drawing the excess amperage, so it burns out. It might not be a bad idea to carry an extra set of capacitors in your trailer as a safety measure.

BTW, they ARE covered under your Dometic warranty and you should report your experience and demand re-imbursement from either Airstream or Dometic directly.

Where to get it/ Part Number

  • 3106732005
    • Hard Start Kit

Resetting the AC System

I posted this on the 2007 model year quality thread, because I reported the problem there, but it seems like it should go here also.

On our last night out, our Dometic air conditioner stopped working. It had been working fine in the 95-degree FL day and keeping the inside at a comfortable 74 degrees. Then about 5:00 p.m., it just would not come back on. After checking everything I could get to without getting on the roof, we switched the unit off. The voltage was on the low side, but still in the green area of our plug-in meter. We left to get some dinner, and buy a fan so we could have a tolerable night.

When we returned a few hours later, it came on again, but after cooling the inside down to the set temperature, it switched off never to come on again. Voltage was in the middle of the green area. We had a warm but comfortable night thanks to the fan we bought.

I called Airstream and they said to take it to the nearest Dometic Factory Authorized Service Center. Last Friday I towed our Airstream to Giant RV World; Melbourne, FL. The technician ran through his diagnostics and could not find anything wrong. He reset the controller, and the unit ran for several hours performing splendidly. I asked him to show me how to do the reset if it happened again. Here is the process:

  1. Switch unit off with the on/off switch on the thermostat
  2. Hold down the top (Mode) and bottom (Zone) buttons.
  3. Switch the unit on.
  4. When FF appears in the display, release the buttons.
  5. Unit should now operate normally.

I hope that I don't have to do this again. If I do and it gets to be a frequent activity, I'll be returning to the service center asking for a new controller. Everyone should take note of this procedure, as I couldn’t find it in any of the factory literature.

Thanks Lew, for posting the start capacitor and PTCR service instructions. I'm filing both of these procedures in my "Junior RV Tech Manual."

Eventually Replacing the Caps


  • Sacs, the hard start kit that you have posted a photo of will replace the start capacitor and motor starter originally used.
  • I also emailed these guys. The WSX-7 hard start kit is what I just said. A kit, and the WSX-7 label includes the cap.
    • if you purchase a supco spp6 you have to take the cap and the part labeled wsx-7 out and put it inline.