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Old 09-04-2019, 07:23 PM   #1
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Dometic A/C Capacitors ?

Owners manual says its an 579 Brisk Air or 590 or 595 Quick Cool, on our '02 Montana 5th wheel, a Duo-Therm air conditioner melted the start capacitor and the top of the run capacitor.
If I turn on and spin fan by hand it runs.
I just replaced them BOTH, but the start cap began to smoke.

Found there is another Start Cap with a built in resistor.

2 questions!

Is THAT the start capacitor I need?

Would/COULD there be a compressor problem?

Thanx guys!

Trapper John
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Old 09-05-2019, 07:42 AM   #2
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Quote:
Originally Posted by 41magnum View Post
Owners manual says its an 579 Brisk Air or 590 or 595 Quick Cool, on our '02 Montana 5th wheel, a Duo-Therm air conditioner melted the start capacitor and the top of the run capacitor.
If I turn on and spin fan by hand it runs.
I just replaced them BOTH, but the start cap began to smoke.

Found there is another Start Cap with a built in resistor.

2 questions!

Is THAT the start capacitor I need?

Would/COULD there be a compressor problem?

Thanx guys!

Trapper John

Basically - there should be a cap for the compressor, and a cap for the fan. On my Duo-Therm, they are combined into a 3 terminal can (compressor terminal, fan motor terminal, and a common).
If you have a start cap, it is for the compressor and it would be a separate capacitor. It should also have a resistor across the top (to bleed off cap voltage to prevent arc welding the start relay contacts together). If you have a start cap, you will have a start relay.
Start caps are only for compressors, not the fans (not of this size at least).


So, knowing the above, please clarify what capacitor melted down, and if it was a combined capacitor that melted down, which terminal melted down?
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Old 09-05-2019, 04:50 PM   #3
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The side of the large gray cap melted out AND the top of the black cap melted as you can see in the photo
it is my understanding that the black is the Start Cap and the gray is Run Cap.
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Old 09-05-2019, 05:07 PM   #4
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If it was a new start cap and it failed probably a bad start relay. The start capacitor as stated above would be for the compressor not the fan. If you had to spin the fan to start it that is a symptom of a bad run capacitor. If both were bad you probably had some bad voltage and it probably welded the points shut on the relay and it needs to also be replaced. The start relay should drop out the start winding once the compressor starts. Sounds like it is not.
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Old 09-05-2019, 05:11 PM   #5
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The side of the large gray cap melted out AND the top of the black cap melted as you can see in the photo
it is my understanding that the black is the Start Cap and the gray is Run Cap.
Yes that is correct. Yeah looking at those pictures I would replace the Start relay and check the terminals on the compressor. Those are pretty melted down.
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Old 09-06-2019, 08:15 AM   #6
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The side of the large gray cap melted out AND the top of the black cap melted as you can see in the photo
it is my understanding that the black is the Start Cap and the gray is Run Cap.
Great job with the pictures! They truly tell a thousand words.


The cap on the left looks to be a dual purpose cap - hard to tell as I can't clearly see if the terminals are clearly isolated from each other.
The cap on the right is a start cap. Start caps will always have a (current) relay as a part of their circuit as they are connected with the compressor.


It's hard to tell from the picture, but it looks to me like the side of the can for the run cap has a hole in it. Is that true or an illusion?


For the start cap to look like that, I would suspect that either a) the relay didn't drop out of the circuit like it is supposed to, or b) the relay contacts arced together (possibly because there was no resistor, typically located on the top of the start cap). I guess a c) would be that one of the wires from the cap shorted to ground.


Going back to your comment "I can turn the fan by hand and it will run" - I'm sure it will, but the motor will probably also get hot and eventually burn up. It needs the run capacitor to run.


In effect, your dealing with 2 circuits here. A compressor circuit and a fan circuit. The fan and compressor share the can capacitor, and only the compressor uses the start capacitor. And then there is that start relay that I mentioned above.


So, what to do? Well, if you have an ohmmeter, you need to access the terminal box on the side of the compressor. Note the three terminals, C(ommon), R(un), and S(tart). You need to measure the resistance from C to R and C to S (you can measure R to S which should be the additive total of the values read previous). Use the Rx1 scale for these measurements.
Then, switch to the highest Rx scale and measure from each terminal to the case of the compressor or a good chassis ground. You shouldn't get any kind of a reading.
Make all readings after disconnecting the wires from the compressor terminals. Take a picture of the wires as they are connected so you can be sure to properly reconnect (hooking up wrong can possibly ruin the compressor).


I could go into how the circuit works in greater detail, but that would darn near be a book.


Let me know what you find and I'll give you more direction. I'll also try to find a picture of what the start relay looks like.


I just found the circuit for the 579 and 1 other. Rather than a current relay to drop the start cap out of the circuit, they use a PTCR (Positive Temperature Coefficient Resistor) in series with the start cap. This circuit doesn't need a resistor across the top of the start cap.

In your original post you said you replaced both the start and run cap, and then the start cap started smoking. Did you get your words confused as to what started smoking?
PTCR's only fail open, so there is no way power can be continuously applied across the start cap (which is what would cause it to smoke). Is it possible it was the PTCR that was smoking (which could happen if you have a shorted compressor).
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Old 09-06-2019, 08:36 AM   #7
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Mr Mark. From your extensive knowledge it seems your'e likely an a/c tech like myself. Is that true?
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Old 09-06-2019, 10:12 AM   #8
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43-56 MFD Round Start Capacitor (220/250V)
were the numbers on lil blackie, since they are only $7 ea I ordered 2, cuz i'll replace the 1 that smoked.
I will look for that relay, and HOPE it has #'s on it for ordering replacement....UNLESS either of you 2 awesome a/c whiz's know the part #.
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Old 09-06-2019, 10:14 AM   #9
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and YES Mark, the run cap had a hole in it.
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Old 09-06-2019, 10:23 AM   #10
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is this them? https://www.supplyhouse.com/sh/contr...earchText=ptcr
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Old 09-06-2019, 10:57 AM   #11
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or would 1 of these be the correct relay?

https://www.amazon.com/s?k=dometic+h...l_8kyva18khs_b
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Old 09-06-2019, 01:22 PM   #12
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or would 1 of these be the correct relay?

https://www.amazon.com/s?k=dometic+h...l_8kyva18khs_b
Go for the supco spp6 it does the relay and start cap all in one. very easy to install
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Old 09-07-2019, 04:35 AM   #13
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Mr Mark. From your extensive knowledge it seems your'e likely an a/c tech like myself. Is that true?

Yep Dom, I've done that My Dad started me pushing freon when I was a wee lad (5th/6th grade) back in the '60's.


Later after college, I started worked for a company designing electronic air cleaners and dust collectors while doing A/C work on the side. Then went to work for a company designing commercial roof-top equipment utilizing heat pipe heat exchangers, gas fired furnaces, steam/cold and hot water coils, DX coils.
I also started teaching HVAC classes at a local Jr college - starting out teaching basic electricity for HVAC, then became one of two instructors who taught "System Service and Analysis", a class that all students earning a degree had to pass.
Over the years I've worked on a number of high profile projects around the country where air cleaning or HVAC had peculiar requirments.
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Old 09-07-2019, 05:09 AM   #14
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and YES Mark, the run cap had a hole in it.

With a hole in the run cap, I would get an ohm meter and measure the compressor windings before I did anything else (process described previously). You need to be certain of it's viability first.
This is of foremost importance before you start throwing parts at it.



You should see a C to R resistance of around 3-4 times that of the C to S resistance, and infinite resistance from any of those terminals to chassis ground or the steel shell of the compressor.
A word of warning, always, always, always replace the terminal cover on the compressor after insuring all wires are properly connected and separated. I've seen the plugs blow out, and it isn't pretty when it does.



Then and only then, assuming resistances check out OK, proceed with the following - and as previously described -



The Run cap that has the hole in it is for both the fan and the compressor. The black plastic cap is solely for the compressor, and is the Start cap. For our discussions, you need to be clear on these.
And I did follow up on my previous comments after looking at the wiring diagram for these units - they don't use a relay, they use PTCR. The Supco device mentioned previously has that included as a part of the assembly as does the one your showing that has burned up.


Then, after wiring everything up, to include any sheet metal panels, power it up and see if it will run. Then stay with it for awhile to make sure you don't let any more smoke out, or that the compressor (assuming that as it run's it's internal character takes a turn for the worse that didn't evidence itself in the above resistance tests) that it doesn't do the same thing or similar again.


Bottom line - I'm very suspect of your compressor at this point. I don't know that it was the original cause of the burned capacitors (could have been a lightening strike), but if the unit was running or attempts made for it to run with burned wires as pictured, it could have toasted the compressor.
Again, the PTC portion of the start cap assembly does not fail shorted. The cap portion can short, but they usually just vent to relieve pressure, sometimes bulging ever so slightly before the case cracks.
The hole in the side of the run cap scares me as well. In all my years I've not seen that type of failure without there having been some outside source causing it.

But to have burned like that as seen in the pictures tells me something more catastrophic (again, lightening strike?) may have occurred here.


Let me know what you find.
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