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Old 07-08-2022, 11:59 AM   #1
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Dometic Duo-Therm Roof Units

OK Guys, I’m not crazy. Problem started with rear unit on my 2005 National Seabreeze. Now both units are doing this. With thermostats in the off position, compressor try’s to start up for about 20 seconds. Then remain off for 1 1/2 minutes, then try’s to start compressor again. Both front and back units are doing this.
HELP
Tried replacing one thermostat, no change.
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Old 07-10-2022, 07:18 AM   #2
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Location: Eastern outskirts of Dallas, TX
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Your problem isn’t the thermostats.

Turn power off to the units at the breaker.
Climb up on the roof with the following -
1/4” socket driver
5/16” socket driver
Slotted screwdriver
Phillips screwdriver

Go up on the roof, remove the shroud from the first unit.
On one side or the other there will be an warning sticker on the electrical enclosure. Access the enclosure by removing 1 or 2 screws.
Inspect all wiring in the enclosure for burned or discolored wires or connections. Look for failed components of which there will be at least 2 - a combo capacitor for the compressor and fan motor, and a PTC or similar start device for the compressor. The PTC’s have been known to fail by literally coming apart into pieces.

Repair/replace any compromised wiring within the enclosure. It common for the crimp connectors to fail inside the enclosure due to the high current pulled by the compressors.

If all looks good there, close the enclosure back up. Move to the compressor and pop the electrical cover off the compressor. Again, check the wiring connections for burning. If all looks good there, reinstall the cover.

Repeat the same for the 2nd unit.

Let us know what you find.

IF you don’t outwardly see anything wrong - then write down the make/model of the units, find the parts listing for them, and order a new PTC start device for each unit.

A compressor that acts as you are describing has an electrical issue related to what I described above. The compressor tries to start and hums as you describe until it kicks out on thermal overheat. When the thermal overheat sensor cools down, it closes and the compressor tires to start again, and the sequence starts all over.

FWIW - REGARDLESS OF SOME ANSWERS YOU MAY GET, THIS IS NOT A REFRIGERANT PROBLEM! YOU DO NOT NEED GAUGES (beside the fact that unless previously Tampere with, there is no refrigerant port access to the refrigeration system to hook gauges to anyway - and if it has been tampered with and ports added - you potentially have a whole new slew of issues to contend with).
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Old 07-11-2022, 04:02 AM   #3
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SeaBreeze - I realize you PM’d me but for the good of the thread and others who read it I thought I would reply here.
I did fail to catch that you said the thermostats were turned off when the compressors tried to start.

It sounds like you’ve got several things going on.

First of all, when the thermostats are turned off, the units shouldn’t be doing anything. Second, if the compressor is being told to run, it should run and not do what you say it is doing - and that is trying to start but kicking out on thermal overload.

It sounds to me like you have 2 different problems - 1, a thermostat and/or control interface wiring problem, and 2, a compressor running problem.

For the thermostat wiring issue - have you or a PO made changes to the thermostats or has it been left untouched from the date of original coach manufacture? Are the rooftop units original to the coach?

For the 2nd problem, you need to follow through the outlined procedure.

I would attack the thermostat issue first, and the compressor issue 2nd subject to what you stumble across in your investigation.
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