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Old 09-01-2013, 08:16 PM   #1
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A/C woe - thermostat?

Our Dometic Penguin A/C has a problem where sometimes the compressor just doesn't start, although the fan runs. When the compressor does work, the A/C works beautifully. Adjusting the temperature seems to make it start working again. Do you think this is a sign of a bad thermostat or board or something else?

Model number is 620215.321; I'm pretty sure it's original to our '95. Thanks for any suggestions. Some of the parts look expensive so I'd like to be reasonbly sure it will fix the problem.
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Old 09-01-2013, 08:45 PM   #2
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Correction/update - it seems to be about 10 degrees (F) off. So the thermostat is set for 65 degrees, and it kicked on when the air going in was at about 75 degrees (using a laser thermometer to measure).
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Old 09-05-2013, 06:52 AM   #3
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Anyone have any suggestions or thoughts on this?
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Old 09-05-2013, 06:56 AM   #4
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Are you using an extension cord? If so that may be the problem. If not, are you sure there is adequate power at the plug-in? I think one of these may me the most likely problem.
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Old 09-05-2013, 07:21 AM   #5
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My understanding— which becomes less certain with advancing age— is that the thermostat setting is a target value, not an absolute. When you set the thermostat to 75°, that does not mean that the A/C comes on when the temperature rises to 75°, or that the A/C shuts off when the temperature drops to 75°. What it means is the A/C will try to regulate the temperature so that the temperature range is centered on 75°. It will come on a certain number of degrees ABOVE 75°, and will shut off a certain number of degrees BELOW 75°.

Not sure how MANY degrees above or below, but ±10° from the setting for turning on or off seems excessive. I would expect something closer to ±3°~5° from the setting.

Having said all that, I think we've become too good at measuring temperature, and we become fixated on the numbers. Which is important for a refrigerator/freezer or when cooking meat, but not so much for an A/C. All we really need to know on the A/C thermostat is, push it up for warmer, down for colder, and fiddle with it until we're comfortable. Forget the numbers.
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Old 09-05-2013, 08:01 AM   #6
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There are two hefty capacitors in AC units, and they don't improve with age.

One is for the starting procedure and the other is used while running.

So...... replace the "start cap" and the "run cap". Not terribly difficult to do, nor expensive.
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Old 09-05-2013, 08:41 AM   #7
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Originally Posted by Protagonist View Post
~~
Having said all that, I think we've become too good at measuring temperature, and we become fixated on the numbers. Which is important for a refrigerator/freezer or when cooking meat, but not so much for an A/C. All we really need to know on the A/C thermostat is, push it up for warmer, down for colder, and fiddle with it until we're comfortable. Forget the numbers.
Heresy! What inoffensive thing could couples squabble about without numbers on the thermostat?!
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Old 09-05-2013, 09:58 AM   #8
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Thermostat problems

Thermostats do go bad but often they just need cleaning. Try blowing it out with contact cleaner like Radio Shack sells. Or in some cases you can take it apart and clean the contacts with a Q-tip and alcohol. TV remotes respond well to this kind of cleaning.

Interesting story, after buying my first Airstream (a well used '74 27') I ran into the previous owner at a rally. He asked if I had replaced the A/C yet? I said no it works just fine and he nearly had a stroke. Seems that was the main reason he had traded the trailer in on a Bvsn. The little bimetallic spring had let the contact get out of position. So I removed the t-stat cover and flipped the contact back where it belonged and all worked fine. Those old Armstrong A/C were built like a tank.

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Old 09-05-2013, 11:01 AM   #9
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Quote:
Originally Posted by Protagonist View Post
Having said all that, I think we've become too good at measuring temperature, and we become fixated on the numbers. Which is important for a refrigerator/freezer or when cooking meat, but not so much for an A/C. All we really need to know on the A/C thermostat is, push it up for warmer, down for colder, and fiddle with it until we're comfortable. Forget the numbers.
To be clear: I had it set to our usual 75-ish, and it just wasn't kicking on. The camper was extremely warm and stuffy - at least 85 degrees, with high humidity, and neither of us sleeps comfortably in those conditions. In fact, friends of ours were tent camping with us (fairly hardcore tent campers), and they gave up and found a hotel. Without the compressor running, there was no dehumidification, and it wasn't good in there at all. But once I adjusted the thermostat down to 65, the compressor started almost immediately and the camper was in great shape inside of an hour.

I'm not saying it was 77 when the thermostat should have meant it was 75. I'm saying it was frustratingly hot when it should have been comfortable.

Quote:
Originally Posted by Aage View Post
There are two hefty capacitors in AC units, and they don't improve with age.

One is for the starting procedure and the other is used while running.

So...... replace the "start cap" and the "run cap". Not terribly difficult to do, nor expensive.
Quote:
Originally Posted by genearnold View Post
Are you using an extension cord? If so that may be the problem. If not, are you sure there is adequate power at the plug-in? I think one of these may me the most likely problem.
My concern with both of these theories is that when I played with the thermostat, it started working perfectly, and worked perfectly for the entire night, cycling on and off - except it was set at 65 and cycling on and off around 75 degrees. (No extension cord, 30 amp connections.) I'd think if it was a cap or a power problem, it would be trying to cycle on at the correct temperature, and failing, rather than working perfectly at the "wrong" temperature setting.

Not to say that the caps shouldn't be replaced, or that we weren't having a power issue (at several different campgrounds - it's unusual for me to have an issue at any campground; I've ordered an AC volt meter to mount on a wall, though). But I'd think a power or cap problem would cause issues with the compressor starting and running, rather than the thermostat being off.

Quote:
Originally Posted by tony6373 View Post
Thermostats do go bad but often they just need cleaning. Try blowing it out with contact cleaner like Radio Shack sells. Or in some cases you can take it apart and clean the contacts with a Q-tip and alcohol. TV remotes respond well to this kind of cleaning.
Good idea - I'll pull it apart at some point and see what it looks like. The A/C in my B190 was pretty much all mechanical, and one night the thermostat went bad and froze me out. This one has at least some electronics.

Quote:
Interesting story, after buying my first Airstream (a well used '74 27') I ran into the previous owner at a rally. He asked if I had replaced the A/C yet? I said no it works just fine and he nearly had a stroke. Seems that was the main reason he had traded the trailer in on a Bvsn. The little bimetallic spring had let the contact get out of position. So I removed the t-stat cover and flipped the contact back where it belonged and all worked fine. Those old Armstrong A/C were built like a tank.
I've had that happen with other things, like an old monitor. "It goes dim and fuzzy!" "Never had that happen."
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Old 09-05-2013, 11:59 AM   #10
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What kind of thermostat do you have?
Here's a link to putting in an inexpensive digital replacement.

Mod #100: Honeywell, Dometic and Suburban Digitial Thermostat Install | ModMyRV
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Old 09-05-2013, 02:55 PM   #11
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Quote:
Originally Posted by daveswenson View Post
What kind of thermostat do you have?
Here's a link to putting in an inexpensive digital replacement.

Mod #100: Honeywell, Dometic and Suburban Digitial Thermostat Install | ModMyRV
Mine is mounted on the A/C itself. I understand newer setups have a separate thermostat, which sounds great, but I'd have to run the wires and all, which I'd rather not get into. Based on the owner's manual, apparently the separate thermostat was an option with my A/C, but Airstream didn't do it.
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Old 09-05-2013, 05:47 PM   #12
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Originally Posted by Skater View Post
Mine is mounted on the A/C itself. I understand newer setups have a separate thermostat, which sounds great, but I'd have to run the wires and all, which I'd rather not get into. Based on the owner's manual, apparently the separate thermostat was an option with my A/C, but Airstream didn't do it.
Never mind.
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Old 09-05-2013, 06:04 PM   #13
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Quote:
Originally Posted by Skater View Post
Mine is mounted on the A/C itself. I understand newer setups have a separate thermostat, which sounds great, but I'd have to run the wires and all, which I'd rather not get into. Based on the owner's manual, apparently the separate thermostat was an option with my A/C, but Airstream didn't do it.
skater, if you have a slide switch for fhe temp setting the contacts may be worn or dirty. some electrical contact cleaner may help. if you take the control panel out, an electronics tech should be able to replace it. i had it done on my '94. think tv or ham radio repair shop.
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Old 09-05-2013, 07:43 PM   #14
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What i'm gathering is it works fine, but is 10 degrees off.
So it may pay to look at the back of the knob. You may find a screw. That would be to adjust or calabrate the temp reading to the accual temp. This is the direction I would head first. Keep it simple. Road vibrations move things. It would help here if we had a photo.
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