July 2015 - In 2014, when camped in Montana during a hot spell, I ran the air conditioner (Dometic Duo-Therm Cool Cat Heat Pump model 41001.511). The unit cooled great for about 30 minutes. Then I heard a loud "swish" and the unit started putting out heated air, hotter than the ambient air temperature, rather than cooled air. Somehow it had switched over to heating mode. We turned it off for about 10 minutes, then tried it again. The same thing happened, only this time it switched over to heating mode after 5 minutes of normal cooling.
After that trip, I did some research and learned all about heat pump reversing valves. These valves reverse the flow of coolant through the unit so that heat from outside is transferred inside to warm the motorhome. Since the valve is sealed and an integral part of the refrigerant system, it is cleverly switched between cooling and heating modes by the use of an electromagnet, also called a solenoid. In many RV heat pumps, the normal default mode is for it to heat. By applying power to the solenoid, the reversing valve is moved to the cooling mode. You must maintain power on the solenoid for it to stay in cooling mode. If you turn off power to the solenoid, a spring will automatically push the reversing valve back to heating mode.
It seemed that on my unit, the solenoid was not keeping the heat pump in cooling mode. Either the solenoid was loosing power after several minutes, or it was not able to keep the reversing valve moved to cooling mode. There is a relay on the control board that uses a 12 volt DC signal from the wall thermostat to turn on the 120 volt AC current to the solenoid. When you turn your wall thermostat to "Cool", it activates the relay, thus powering the solenoid and causing the reversing valve to move to cooling mode. Perhaps my relay was defective?
After gaining access to the air conditioner (details below), I unplugged the solenoid power lead from the relay and piggy-backed it onto the power lead for the compressor motor. This way I knew that the solenoid would be getting 120 volts whenever the air conditioner was turned on. I then tested the air conditioner and it there was no change. It would cool for 10 minutes and then make a swishing noise when it switched over to heating mode on its own.
Further reading told me that it was common for failed solenoids to work for a short time before cutting out. I must have a failed solenoid. I ordered a replacement solenoid from PPL Motorhomes at a cost of about $60 which included shipping. (Solenoid coil kit $42.29 plus shipping). This comes with leads of the correct length and the correct connectors pre-attached. I installed the new solenoid then tested out the unit. Success! I ran it for over an hour and it showed no sign of cutting over to heat mode.
Update 07/2016: Used the air conditioner extensively during a recent camping trip and happy to report that it still works perfectly. The replaced solenoid did the trick.
Air conditioner with white cowl cover and rear fins removed (click for larger image). The air conditioner is pretty easy to un-attach. Just remove the four screws holding down the two rear brackets (where you can see the white sealant on my unit). Also remove the four screws at the top of the unit toward the front that connect the air conditioner with the black plastic shroud.
I simply removed the old solenoid and its leads after unplugging them. Note how the old leads are run, because you want to run the new leads the same way. Then I screwed in the new solenoid and ran the new leads to the electrical component box, using plastic ties where needed. I plugged in the leads and was good to go. Reassemble the metal cover, replace the black plastic cover over the air conditioner, push the air conditioner back in place, and re-install the hold down clips and the four top screws. I used a liberal amount of sealant at the clip screws since this will be getting wet and you don't want water getting down inside the van. Don't forget to re-install the two fins that stick out the back of the unit. These help separate the cool input and warm output air at the back for better efficiency.
You may also need to replace one or more of the rubber well nuts that the stainless steel screws go into when putting the white air conditioner cowl back on. I had one come out. Roadtrek suggested I replace all of mine since my van is over 10 years old. As far as I can tell, these are M5 x 0.8 rubber well nuts.
Modifications & Info >