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Old 05-18-2022, 05:57 PM   #1
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Another Dometic A/C

Did some research here to no avail

Dometic AC on 2018 Voltage. One of three.
Starts and cools nicely. 85° outside and blowing 57° inside. For a while. Then it just does not cool.

I have not been up to look at it as I don't know too much about how ACs run. Just "The line from the evaporator to the compressor should be cool and probably sweating. The line from the compressor to the condenser should be hot." Don't miss the polarity of a capacitor -

It is hard getting older and not being able to fix everything anymore.

Perhaps a few hints to get me started before the climb.

Thanks.
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Old 05-18-2022, 09:09 PM   #2
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Sounds like you either have some marginal wiring connections/burned wiring or possibly a bad run cap on the compressor.

98% of AC problems are electrical. Few are refrigerant related. Since yours cools OK when it does run - yours fits into the 98% class.
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Old 05-18-2022, 09:16 PM   #3
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The OP doesn’t say it quits running, it just quits cooling. First thing I’d do when it quits cooling is remove the filter cover on the ceiling and look up at the fins inside so see if they are iced over.

When it quits cooling and you turn it off for a while, does it start cooling again and repeat the cycle?
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Old 05-19-2022, 06:46 PM   #4
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The OP doesn’t say it quits running, it just quits cooling. First thing I’d do when it quits cooling is remove the filter cover on the ceiling and look up at the fins inside so see if they are iced over.

When it quits cooling and you turn it off for a while, does it start cooling again and repeat the cycle?
Thank you. Sorry not enough details. The fan keeps running but it quits cooling. When it worked for the inspeciton and a short time after purchase it cycled on and off with thermostat setting. It would "turn off" and then "on" etc.

I will run it tomorrow and watch the fins.
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Old 05-19-2022, 06:48 PM   #5
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Sounds like you either have some marginal wiring connections/burned wiring or possibly a bad run cap on the compressor.

98% of AC problems are electrical. Few are refrigerant related. Since yours cools OK when it does run - yours fits into the 98% class.
OK. something is not making the compressor come on. Is that usually the capacitor?
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Old 05-20-2022, 02:53 AM   #6
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As said previous, generally on these units the issue is electrical connections.

The capacitor COULD be the issue. It helps phase shift incoming power in the start winding of the compressor to get the motor to turn,p.

There will likely also be either a thermistor device or what is called a hard start relay and capacitor on the compressor as well. They are not always used or needed, but their use does improve the compressor motors starting torque - something that is needed on units like this because of the short cycling that can happen due to temperature swings/loads.

The run capacitor for these units is also typically but not always, a dual capacitor - 2 capacitors in 1 can, 1 for the compressor, 1 for the fan motor. You can replace the dual with one of the same ratings, or replace them singlely with capacitors of the same value (you just have to wire the commons correctly).

Have you crawled up on the roof and looked at the wiring yet?

Replacement capacitors and hard start components can be ordered from Amazon.
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Old 05-20-2022, 08:47 AM   #7
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As said previous, generally on these units the issue is electrical connections.

The capacitor COULD be the issue. It helps phase shift incoming power in the start winding of the compressor to get the motor to turn,p.

There will likely also be either a thermistor device or what is called a hard start relay and capacitor on the compressor as well. They are not always used or needed, but their use does improve the compressor motors starting torque - something that is needed on units like this because of the short cycling that can happen due to temperature swings/loads.

The run capacitor for these units is also typically but not always, a dual capacitor - 2 capacitors in 1 can, 1 for the compressor, 1 for the fan motor. You can replace the dual with one of the same ratings, or replace them singlely with capacitors of the same value (you just have to wire the commons correctly).

Have you crawled up on the roof and looked at the wiring yet?

Replacement capacitors and hard start components can be ordered from Amazon.
Thanks.
I have not looked inside or outside yet.
We just got back from 3200mi. trip.
I am just trying to plan my summer.........with others help I have a boat guy but not an RV guy. Need suspension looked at too.
I have been a pretty handy person, keeping my machines running in the past but not so good on AC. I don't know why.
I will get to the roof and look for icing inside. I thought today but...no.
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Old 05-21-2022, 07:45 AM   #8
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Thanks.
I have not looked inside or outside yet.
We just got back from 3200mi. trip.
I am just trying to plan my summer.........with others help I have a boat guy but not an RV guy. Need suspension looked at too.
I have been a pretty handy person, keeping my machines running in the past but not so good on AC. I don't know why.
I will get to the roof and look for icing inside. I thought today but...no.
We’re here to help when you get a chance to look.

Don’t let anyone convince you to go buy a set of gauges or a can of stop leak or install a tap to check the refrigerant without the proper guidance of what your doing.
Your being a handy guy - we can walk you through the steps needed to resolve any issue you may be having with the unit.

My Dad had me pushing refrigerant back in the ‘60’s in my early teens. I’ve worked in manufacturing designing, have serviced, and have taught HVAC at a local junior college - and I still don’t know everything there is to know about HVAC.

But you already have one basic concept down - the big line at the compressor should be cool (possibly sweating), and the little line from the compressor will be HOT (it can be so hot as to cause burns!). The service tube at the compressor (if it’s there) doesn’t matter from a touchy-feely perspective.
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Old 05-21-2022, 08:28 AM   #9
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I would bet on the icing up or the capacitor. The capacitors are not hard to change just turn off power and open up the box so you can see them. It may be an easy thing to see (my last time I had very obvious burns showing). Just go slow and take pictures so we can see what you see.
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Old 05-21-2022, 08:48 AM   #10
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We’re here to help when you get a chance to look.

Your being a handy guy - we can walk you through the steps needed to resolve any issue you may be having with the unit.
Quote:
Originally Posted by bddadles View Post
I would bet on the icing up or the capacitor. The capacitors are not hard to change just turn off power and open up the box so you can see them. It may be an easy thing to see (my last time I had very obvious burns showing). Just go slow and take pictures so we can see what you see.
Thank you so much.
Confidence builders you are. Will be away for most of next week.
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Old 05-21-2022, 08:53 AM   #11
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Question.
Can I just pull the filter to see about ice or do I have to take off the inside shroud? Visible screws? Snap tabs to break?
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Old 05-21-2022, 10:28 AM   #12
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Get the owner manual for your unit to follow the instructions on how to remove and clean/wash/replace the filter. Doing so generally is a tool-less operation.

Usually you can see the entering air side of the evaporator with the filter out of the way. I’ve tried attaching a picture of our unit with the grille and filter removed. The face of the evaporator is back in the dark recesses behind that electrical box.

IF the evaporator has a blanket of dust on it, you may not be able to see the ice formation unless the unit has been running g for several hours.
Little whisks of dust are no big deal. But if you do decide you want to clean it without climbing on the roof and tearing it all apart - then make sure the unit has been sitting idle for 24 hours or so - otherwise the dirt/dust will be muddy and even harder to remove.
A fin comb is also a nice tool to have. They come in a kit because not all fin tube coils have the same fin spacing.
Hopefully yours is t so dirty that you need to tear the unit down to “power wash” with a water hose and maybe some coil cleaner. If you have to go that far, be careful of what may drip down inside of your coach. Put a big tray or pan underneath to catch what spills over as best you can.
Always good to have someone inside the coach to warn you of too much drippage.
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Old 05-22-2022, 01:50 PM   #13
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Quote:
Originally Posted by MrMark52 View Post
Get the owner manual for your unit to follow the instructions on how to remove and clean/wash/replace the filter. Doing so generally is a tool-less operation.

Usually you can see the entering air side of the evaporator with the filter out of the way. I’ve tried attaching a picture of our unit with the grille and filter removed. The face of the evaporator is back in the dark recesses behind that electrical box.

IF the evaporator has a blanket of dust on it, you may not be able to see the ice formation unless the unit has been running g for several hours.
Little whisks of dust are no big deal. But if you do decide you want to clean it without climbing on the roof and tearing it all apart - then make sure the unit has been sitting idle for 24 hours or so - otherwise the dirt/dust will be muddy and even harder to remove.
A fin comb is also a nice tool to have. They come in a kit because not all fin tube coils have the same fin spacing.
Hopefully yours is t so dirty that you need to tear the unit down to “power wash” with a water hose and maybe some coil cleaner. If you have to go that far, be careful of what may drip down inside of your coach. Put a big tray or pan underneath to catch what spills over as best you can.
Always good to have someone inside the coach to warn you of too much drippage.
AAHHHH. The filter part I can do and have done. They were all dirty (three A/Cs). I will look in there and also get model/SN to find manual.
Thank you, thank you.
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Old 05-22-2022, 02:40 PM   #14
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OK.
I got to the unit after packing for next week.
I think when I was out using this RV I did not know how it worked. I did not realize it has a fan speed inclucing "auto". So unless the auto is chosen the fan will never turn off. This has a fan only setting and that threw me I guess.

I also think the two times I had come in to check the temps it must have coincidently been at a point just before the compressor came back on... because...
I started it set to 75°. Outside air temp was 92°. Inside ceiling temp was 100°. Floor temp was 86°.
At 15-30 sec. I heard the fan slow a bit and another hum = compressor on.
At 5min air was 52°
At 10 min air was 47°
If I held the IR thermometer in the air flow close to the fan, I could get a reading as low as 22°.
So, I was laying there contemplating how nice it would be to just have a nap when I could tell a difference in the air temp. Ahaa!
Nope.
From the pic you can see there is no view of any coils or anything except the blower and the control box. SOOO...
I turned the thermostat to 77° and the air felt about the same. Wall, floor and ceiling temps had been coming down.
After a few minutes I reset the thermostat to 75° again. Then I noticed the hum and compressor again and the air temps began to drop again.





I laid around for about 35 min. total to realize I might be nearing the time I should sell my RV, aircraft, boat and search for a room to be put in so I can be watched.

Or I am too impatient and did not run it long enough to "freeze up/malfunction".
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