Index Home About Blog
From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: dometic air problem
Date: Mon, 25 Jun 2001 01:31:13 -0400

kim webster wrote:

> I have  dometic air on my 1982 comfort light. For some reason the
> compresser will only work below 85 degrees farenheit. Naturally this
> makes it quite useless as an air cnditioner. I have messed with the
> thermostate with no effect. Does this need to be replaced or is there
> another part that could be causing this problem? I have an rv repair
> manual but it wasn't much help for this problem. If anyone else had
> this same problem please tell me which part to replace. thanks a
> million.

The compressor works harder and draws more power as the temperature
rises.  If the overload is tripping, it's quite possible the
overload is weak.  Easy enough to replace - just pop the cover off
the rig, then pop the terminal box cover off the compressor.  The
overload is a round device hooked in series with one of the
compressor leads.  If getting the OEM part is too much trouble,
simply write down the compressor brand, model# and serial# and visit
your friendly local HVAC supply company.  They'll be able to fix you
up with the proper Klixon general replacement part.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: EU2000i Generator + Air Conditioner = Not quite good enough
Date: Wed, 25 Jul 2001 12:53:10 -0400

Kirk Grau wrote:

> The engineering types will probably think to themselves that it is obvious
> that the amp draw goes up with outside air temperature, but you can make
> sure you are maximizing your bang for the buck by making sure the AC is
> operating at it's peak efficiency by cleaning coils/filters, etc.  This will
> minimize amp draw.  Having said that, it will probably only help your
> situation appreciably if the coils are really messed up, but it is still a
> good idea from an AC maintenance standpoint.

Ummm, us engineering types know that dirty inside coils and filters
REDUCE the current draw by reducing the load on the fan and
compressor.  In fact, blocking off some of the air flow is the way
to get by on the hottest day when the generator is otherwise barely


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: RV AC question
Date: Wed, 16 May 2007 20:27:08 -0400
Message-ID: <>

On Wed, 16 May 2007 10:48:25 -0700, wrote:

>On 16 May 2007 04:26:48 -0700, wrote:
>>Compressor will start and run for about 3-4 minutes then kick off.
>>Problem started after being in a campground with 108 volt supply.
>>Compressor bad? or Run Capacitor?
>I have a similar issue, and am interested in the answers. In my case
>everything was fine until the low voltage occurred. My kill-a-watt
>meter told me it dipped down to 99 volts at one point.
>The AC (Coleman 13500, circa 1990) will actually take my 6500 watt
>Onan to it's knees. On shore power it trips the breakers. I installed
>a run capacitor. It helped, but the end result was the same. Since the
>new AC's are $500-$600 plus install, it would be great if a repair
>would be less.

Do you have an Amp-Clamp and can you tell me what current the compressor is drawing?
did you replace an existing run capacitor or install a "kick starter" hard start
capacitor?  Have you verified the continuity of all the wiring between the compressor
and run cap, if it has one?  With the Kill-A-Watt plugged in, what total amps show?
What is the power factor?  On generator power, does the compressor overload open?  Is
the compressor actually running?

This kinda sounds like a partially failed run capacitor.   This is characterized by
high running current at very low power factor.  If the run cap fails completely, the
compressor usually won't run but if it partially fails, the result is operation but
at very high current.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: RV AC question
Date: Thu, 17 May 2007 07:31:14 -0400
Message-ID: <>

On Wed, 16 May 2007 18:09:18 -0700, altar nospam <> wrote:

>On generator power, I get cold air for a while. It starts the
>compressor with little effort. When it cycles off, then on again is
>when the generator tries like mad. Sometimes it pushes it, but the
>next time it won't.

What Alan said.  This is the key missing information.  The only other thing that
bothers me is that the stalled compressor draws current until a breaker trips.  That
makes me think the overload (AKA Klixon) might have welded contacts.  Very common
failure after many overload events.

After you verify that your unit is attempting to "short cycle", you might want to
replace the Klixon as a matter of course.  It's a black cylindrical object mounted
under the compressor's terminal box and in contact with the can.  It's usually held
in with a simple spring clip.  A 5 minute job to change once you get the cover off.

The easiest way to test for short cycling is to turn the thermostat as cold as it
will go to take it out of the picture, turn the unit on and let it run for awhile.
Then turn the control back to "fan" and wait a few minutes (3-5).  Turn the switch
back to AC.  Repeat this cycle several times to make sure.  If the compressor starts
OK each time then you've zoomed in on the thermostat causing short cycling.

Before you remove the thermostat, look at the sensing bulb and see if it looks like
it is positioned properly.  That is, as the factory did it, with factory
ties/clamps/whatevers in place.  If the bulb is out of place then that could cause
short cycling.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: RV AC question
Date: Fri, 18 May 2007 06:54:05 -0400
Message-ID: <>

On Thu, 17 May 2007 21:16:38 -0700, altar nospam <> wrote:

>I performed the test you mentioned yesterday. The good AC cycled in a
>little over two minutes. The bad one cycled in 18 seconds. Repeated
>test, same results. So.... simple things first, I pulled the covers on
>both (inside the coach) and examined the tubes coming from the
>The good AC had the tube hanging in a loop in the air. The bad AC had
>the tube, for it's entire length of six inches, smack up against the
>steel framework of the coach.
>I pulled out the tube away from the steel, so it hung just like the
>other one, and reassembled.
>Retested the bad AC, now the cycles were just over two minutes. Had to
>quit, it was getting cool enough outside to preclude further tests.
>It couldn't be that simple a fix, could it???

Probably almost that simple.  You still gotta change out that Klixon or you WILL end
up smoking the compressor eventually.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: RV AC question
Date: Fri, 18 May 2007 19:47:15 -0400
Message-ID: <>

On Fri, 18 May 2007 09:57:05 -0700, wrote:

>On Fri, 18 May 2007 06:54:05 -0400, Neon John <> wrote:

>>Probably almost that simple.  You still gotta change out that Klixon or you WILL end
>>up smoking the compressor eventually.
>I happened upon a piece you wrote back in 2000 regarding the Klixon. A
>great detailed explanation of where to find it, and what it looks
>like. I will check the part number on it and get an exact duplicate. I
>will check continuity first. If I do not get continuity, should I
>still replace it?

You'll have continuity.  With the contacts welded, you'll have continuity even when
you shouldn't :-)  With the exception of the odd occasion where a weak breaker is
involved, an AC with a stalled compressor drawing current long enough to trip the
breaker is prima facia evidence of a welded Klixon.  The thing is cheap - I'd not
expect to pay more than $20 full boat retail - so no reason to try to diagnose the
old one.  Just change it for a known good one.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: AC tested, new/old question. Neon?
Date: Thu, 24 May 2007 00:40:15 -0400
Message-ID: <>

On Wed, 23 May 2007 21:28:26 -0700, altar nospam <> wrote:

>Since I last wrote, I actually located one. Of all places, in the
>Camping World$ "hard to find parts" section. While they don't show a
>picture of it, they specify it for my model of AC. So I will buy it.
>Your explanation makes sense to me. Now that I have finally located
>one, it's a piece of cake. Provided of course, that CW is accurate.
>Thanks to everyone. It's been a hoot figuring this one out.

Good deal.

You can sorta test the old one by holding a cigarette lighter under the metal disc
that you can see on the underside.  It should fairly quickly pop and open the
circuit.  With that much heat it may even be strong enough to un-weld the contacts.
They'll weld back again so don't try to re-use it.

If you take the thing apart (small nut on a stud between the spade lugs, sealed with
a dab of glyptol), you'll see that it consists of a round disc of bimetal material
with a contact welded to one edge.  Current flows through the disc and heats it and
in addition, the disc is near enough the compressor to sense its heat.  When it gets
to the thermal setpoint, the disc pops into a second shape, moving the contacts apart
and opening the circuit.  A very clever design.

Every time I take one of these things apart I marvel at how small the contacts are.
As if they were designed to weld after a while.

Glad you found the correct part.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: AC tested, new/old question. Neon?
Date: Thu, 24 May 2007 08:33:26 -0400
Message-ID: <>

On Wed, 23 May 2007 22:05:48 -0700, altar nospam <> wrote:

>Thanks for all your help! Couldn't have done it without you.
>When the new one gets here, I'll definitely try the test on the old
>one, then take it apart. I trust that a new one will not arbitrarily
>weld itself closed. OTOH, this one probably came with the coach, so
>it's to be expected with age.

Like Alan said, it's probably been short cycling from day one and you just didn't
notice it.  The Klixon probably has thousands of operations on it.  Since you've
fixed the short cycling problem, the new Klixon should last forever.  Do try to be
attentive to new episodes of short cycling, especially when on the generator.  Once
you start listening, short cycling will be obvious from the generator heavily loading
for about 5 seconds and then recovering.  If it short cycles in the future and the
bulb hasn't moved then it'll probably be time for a new thermostat.

Glad to have been of service :-)


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Roof air prob
Date: Fri, 01 Jun 2007 02:10:33 -0400
Message-ID: <>

The thread changed subject lines a couple of times. These are the ones I posted to:

AC tested, new/old question. Neon?
RV AC question
Installed Overload relay in AC

The second is probably the longest.

On Thu, 31 May 2007 21:12:56 -0700, "Jim Corey" <> wrote:

>> On 120 volts does it trip a breaker? How long does it take? Dim lights?
>> Make any funny noises?  Cause the blower to slow?  Can you hear the
>> overload klixon click on and off?
>Yes, house breaker (on 110V circuit) tripped, after several attempts of
>compresser to start.  Breaker on rig did not trip.  Klixon did click on and
>off.  Blower did slow.>

That narrows it pretty much.  Either the compressor has failed or the start/run aid
has failed.  I'm using the generic term "start/run aid" because I don't know whether
you have a start capacitor, a run capacitor or a start relay.  All of those would be
located near the compressor, in any event.

If the compressor is locked up, sometimes it can be freed by running the compressor
backwards.  How that's done depends on the start/run aid.  With a start or run cap,
one simply reverses the "start" and "run" connections and optionally applies 240
volts FOR A FEW SECONDS.  I have maybe 30-40% success doing that.  Before I give up I
try to jog the compressor back and forth several times.  I have a standard
refrigeration diagnostic tool called an "Annie" that lets me do that by simply
flipping a switch on the panel.  Lacking the instrument, you can do it manually by
reversing the leads several times.  Many times a piece of crap left over from
manufacturing will get stuck in the gearotor oil pump that most compressors have and
that locks the shaft.  Sometimes it can be backed out.

240 volts is used because the motor doesn't generate much torque when the windings
are reversed.  This has to be done QUICK - 1-2 seconds max - to avoid vaporizing the
start winding.  I usually jog the compressor several times on 120vac and then as the
last ditch step before getting out the brazing torch to R&R the compressor, try 240.

At this point the compressor is almost surely toast.  The concern about not smoking
the winding is not to have an arc form inside the can which breaks down the
refrigerant, makes acids and results in an expensive cleanup job. If the compressor
hasn't smoked then it can be R&R'd with little more work than unbrazing the old one,
brazing in the new one, evacuating the system and charging..

Do you have an amp clamp?  If so you can do a 2 minute test once you have the cover
off.  There will be three wires going into the compressor.  Clamp each lead.  One
should read a lot (common lead), the next a little less (run winding) and the last a
whole lot less but not zero (start/aux winding.)  If you only see current on the
common and run leads then the cap is probably open (probably exploded too :-)  if you
see about equal current on two leads and twice that on the common then the cap is
probably shorted. If you see a LOT of current on the common lead and little to none
on the others then the motor has a grounded coil and is toast.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Roof air prob
Date: Fri, 01 Jun 2007 09:13:37 -0400
Message-ID: <>

On Fri, 01 Jun 2007 08:48:52 -0400, Matt Colie <> wrote:

>By the by.... Was it you that suggested in the last AC thread to check
>the location of the temp switch bulb?  I never would have though to look
>for that one.

I think that Alan and I were writing our posts at the same time. I think he got his
up first but I hadn't seen it when I wrote mine.

A mislocated bulb is a common problem with window units.  Some units are designed to
have the bulb stuck in the evaporator to serve as freeze-up protection.  Others have
'em in free air.  Still others have them attached to the evaporator but partially

Way back when, Whirlpool (or maybe it was Fedders, can't recall) made a big honking
deal about their blue triangular shaped bulb mounting block attached to the
evaporator.  The way they hyped it I thought there must be something mysterious and
probably electronic inside.  No, just a block of rubber.  It served as a delay
element between the evaporator and the bulb.  It also held the bulb a bit away from
the evaporator but close enough that if ice formed it would touch the bulb and shut
off the compressor.  A bit of clever mechanical and thermal engineering but not
nearly the revolutionary product their ad department made it out to be.

At another level, the books and schools tell you to look at the thermostat
positioning first thing when short cycling is experienced.  With a central unit, it
may be that an inopportune placement of a vent has cold air blowing on the 'stat or
perhaps an inopportune location of furniture.  Both training and experience condition
one to look for a thermal cause of short cycling before anything else.


Index Home About Blog