Dometic AC doing Weird things

Question

It was suggested I post here for suggestions on this. I have narrowed the symptoms down a little more since noticing this issue. It has to do with how cold I set the Thermostat. Anything colder than 66 or so, it will trip out, shutting down completely requiring a recycle of the thermostat and or the breaker. The manual states that if the evap coil is freezing, and the thermistor (freeze sensor) located on the evaporator coil signals too cold to the board, 32 degrees I assume, the compressor will shut off, and the fan will go to high to defrost the coil. Anything above that 66, it is fine. Mine doesn't do that if that's what is happening. I have noticed the true temp is off by 5-8 degrees, the actual temp being lower than the thermostat says. Instead of cycling out, the compressor shutting down and then the fan, it just makes a sharp snap like a relay opening, and completely shuts down. Strange. I strongly think a new board may be in the future...:?
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My original post to start with;
Hey all, new here but we've been a fulltimer for over 4 years. I discovered this thread through Google and my current Dometic Rooftop is setup with this type of setup, the newer digital stat and control box and board on a 15K straight cool Brisk Air, pretty new I believe. We purchased the trailer used about six months ago.

I wonder if I'm developing a board problem. At times I run the fan on low continuous at night as the compressor quicks in and out as needed, or even with the propane heat running. Several days ago, twice during the night, the fan went off. After getting the "I'm hot" nudge, I cycled the stat all the way through off and back to fan low constant cool mode and it fired right up. Again later that night, same thing.

Last night, (the other night) I couldn't get anything to start, fan or cool. So at 11pm I'm pulling the control box from the ac.. I see no problem and resort to turning off AC breaker, waiting and flipping back on. Unit starts right up. So recently, I'm running the propane heat, haha surprise lol, but before the propane heat kicks in, I hear the relays in the box, I still have it open, click about a 8-12 times as the furnace kicks on. The furnace lights and runs fine, cycles fine, but I hear the relays continue to click for about another 6 times, even after the furnace is cycled in.

Does this sound like it's developing an issue? Thanks.

Correction, its the 595 series of rooftop. Thanks for any suggestions.
Eta to show the newer controls there are there now, so it has been upgraded since the trailer was manufactured.
Dometic 3313189.000 Single Zone LCD Thermostat & Control Kit
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370843604297&fromMakeTrack=true&ssPageName=VIP:watchlink:top:en

j-d · Answer

I don't have one of these and have not worked on one, but let me offer:

What Manual do you have? Our friend Chris Bryant offers manuals on his website.

Pretty thermostat, does it require batteries? If so, are they good?

Does the manual offer testing for the defrost sensor you mentioned?

2002sprinter · Answer

Thanks j-d. I found that site this weekend, and saved a few docs from there.

It seems like a nice system, with the AC board controlling function of when the heater works also. There is a way to test the resistance of the freeze thermistor, but it's a pain, as the resistance changes according to the temperature it is. My thoughts are since this AC was upgraded to 15K btu, from the factory 13,5 btu?, that the evaporator coil may be trying to freeze due to less than idea volume of air flowing through it? Low air flow will tend to freeze evap coils. I did home and commercial AC repair for years.

I'm wondering if it would be worth adding another ac duct vent to help with the flow? I know there must be alot of air flow resistance, because when I open the slide louver on the AC cover, it roars. But all the vents flows good through out the trailer. Four, of the what look like four inch, round vents.

You know, the more I think about this, that might be the issue. One way to test is open the cover louver wide open, run the stat down to 55 or so, and see if it shuts off, like it has been from time to time.

2002 Sprinter 260-RBS, 28 footer.

Eta I just found this, nicehttp://bryantrv.com/docs2/docs/singlezonediag.pdfDuoTherm Single Zone LCD Diagnostic Mode , that lets you step through individual functions, fan, compressor, heater etc, as well as checking the reported evap coil temperature. That will tell me what the thermistor is reporting for temps. Thanks for the doc

dougrainer · Answer

The ORIGINAL single Zone tstat's had a HEAT problem. IF the inside temp of the RV was over 100 degrees, the tstat would not operate at all. I have never heard of your problem, but the odds are since you complain of inaccurate temp settings, you would be better off replacing the wall tstat. The CORRECT new updated tstat will have a yellow dot sticker on the back of the tstat. Dometic did not change the P/N, they just had the yellow dots installed to lket dealers know which are the current fixed tstats. If yours does NOT have that yellow dot, then you need to replace the tstat. Dometic "may" warranty it if out of warranty. Doug

2002sprinter · Answer

Thanks Doug, that might be part of the problem. I haven't had a chance to check the other issue, air flow. Today, the stat is a solid 7 degrees off, reading high, mounted on inside partition wall. Set on 77, the inside temp is 70. I have a chance to buy the whole new control system for $115 shipped. I'm thinking of doing that, to make sure I have a new spare in case the board quits this summer. That would give me a new stat and thermistor as well, to install.

2002sprinter · Answer

Well it's 2;30am this morning. Ac shutdown again. I immediately went into diagnostics and the thermistor said it was 31 degrees on the coil. Readable at the thermostat in diagnostics. I visually checked and didn't see any ice. So I pulled it from the coil, wrapped it in black tape to insulate it and am trying it again now. So far so good. So at this point, I need a thermostat and a thermistor lol. Yep, definitely picking up another full control setup. I'm trying to be meticulous, to help out anyone else that runs across this issue. Strange as it is. :@

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