Dometic 2

[Airbag]

I just installed a new Dometic roof air and it does something contrary to the manual. It is a 520 series with mechanical thermostat. My concern is the book says the fan will run continuous and only the compressor will cycle.

The fan shuts off when the compressor cycles. Does this sound right?

[TomC]

Some do that, and there may be a switch for automatic fan (cycling with the compressor) or continuous running fan.  Good Luck, TomC

[Airbag]

Some do that, and there may be a switch for automatic fan (cycling with the compressor) or continuous running fan.  Good Luck, TomC

I would prefer it to shut down with the compressor but it worries me because the book says it does not. There is no switch that controls that function.

I have not run it long enough to see if truly cycles yet, when you turn the temp back or warmer it shuts everthing off.

[Lin]

I have not seen one that does that.  I once wired one to work like that using an inline thermostat since I thought it would give a break from the fan noise.   Here however, since the thermostat is internal, the unit will be less responsive to the temp changes, which still may be okay.  It would be surprising if the unit functioned only like that instead of there being a switch allowing you to make that choice.  If you don't find such a switch, call tech support.

[Airbag]

Well I have gone out to the bus and ran the new AC again just to make sure it was not my imagination. and it is worse than I thought. With the fan switch in max AC mode and the temp set to full cold it works great for a short while and then shuts off and does not restart. My front thirty year old unit works great. This is AC number two that I have put in the bus with no soap. I thought by buying a new unit I could not miss. I will have to wait for the holiday to be over before I can get Dometic on the horn. I have been having the worst luck with AC lately between my house and the bus. I hold no ill will here I just thought that a new unit would have been tested and run through some kind of QA.

Maybe I should put a swamp cooler on the roof. 

[Nick Badame Refrig/ACC]

Hi Rick,

Just to eliminate some probables, lets checks some things out.

If the compressor shuts down prematurely, the first thing to check is the power source. Do you have the proper size wireing from the building

to the A/C. Long runs of power cords will eat up enough current to cause low voltage at the compressor.  Low voltage to a compressor will overheat it

and it will cut out on internal overload. Once that happens, it could be hours before it will restart. [you must check voltage while the compressor is running]

Next, if voltage is correct, check the amp draw of the compressor. It must match the amp specs on the serial plate. If overdrawing, it will also cut out on overlaod

and give you the same symptoms as above.

If the compressor is the culpret, no problem, I will get an RGA for you on tuesday and have another unit shipped to you.

I'm truely sorry for your A/C troubles. Someone is trying to teach you something about A/C's, I think... Maybe you could become a tech?

Enjoy the rest of your holiday
Nick-

[Airbag]

Hi Nick
I am running it with the 8KW gen and it is powered at the same breaker box that the front Dometic is receiving power from. It has it's own 20 amp breaker and actually has a shorter wire run than the front unit. I don't have a power pedestal at my house yet. I can check voltage easy enough at the breaker while it is running. Would that tell us anything? It is wired with 12 gauge single conductor. I just assumed since the front AC is happy this one should be also. Oh no there I go assuming again. 

I just checked voltage with both units running and I have 120V exactly at the box and all outlets in the bus. The unit ran for about 15 minutes before shutting down. The blower and compressor.

The blower runs in fan only and heat position.



Will the unit cycle off when the temp is set to it's coldest position? I would not think it should. I waited and waited for it to start again and it did not it just got hot in the back of the bus. If you recycle the fan switch it will come on and do the same drill.

Nick
I am starting to think airconditioning is like doing aircraft fabric work (voodoo) with aircraft fabric you can do everything by the book using a covering system from wood or bare metal up through fabric, stiching, surface tapes, clear, silver and color and things still go wrong. A man in the next county can spray some WD-40 and walla you have fish-eyes. A man takes a hot shower a mile away a walla you have blushing. I have had fabric jobs go wrong from a low pass by an airplane with a smoke system on, sprinkling drops of smoke oil the size of flea turds.

[Nick Badame Refrig/ACC]

Hi Rick,

It sounds like the thermostat sensing tube could be too close to the evaporator or in the wrong place.

You said the compressor and the fan cycle off..?? this could be wired differently by the factory. I would double check

your wireing at the unit. It may be possible the upper unit and the inside ceiling unit could be connected wrong?

Not sure at this point
Nick-

[Airbag]

Nick
I have pulled the distribution air box down and checked the wiring. It is idiot proof between the roof and the template just one mastered six pin connector and it is installed correctly. There is no chance of the compressor short cycling due to cold air induction into the return air. The bellows is installed with no leakage points visible. The power supply is installed correctly. The mounting hardware was torqued per the installation manual. The mounting surface on the roof is free of rivet heads to cause any leakage. We are just slightly off centerline keeping it well with in the angular tolerance. I have copied this page out of the manual and it shows the highlighting my mechanics did while doing an initial reading of the manual to keep them focused on the applicable sections that are model specific.

I have copied off the internet a Dometic service manual that talks about a freeze control sensor but I do not think it applies to this unit the publication number of that manual is 3108892.013 10/00. It is almost ten years old but it is all I could find. It talks about the installation of a freeze sensor. The installation manual received with this AC says nothing about any sensor installations at all. Please note the sensor illustrated on the page below out of our installation manual. Does it stay where it is because we have not moved it and it is just as shown.

[Airbag]

I found this on Fixya. The man has exactly the same problem I have with a brand new Dometic roof AC. Here is what he says.

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Posted by booregard on Jun 19, 2008
My problem is just the opposite. You turn it on and it runs for about 4 or 5 minutes and then shuts off completely. Will not start back up on the AC side. Will work on the fan side. I'm puzzled. Brand new unit, even went back and had the company replace the first one. Same problem. Any suggestions
                           
                                                              end qoute

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Is it possible the blue and grey wires on the thermostat are backwards? There are no pin identifier markings on the thermostat itself I checked. If you look at the wire diagram and assume that it is looking at the back of the thermostat then they are backwards or reversed from what the diagram calls for. I am afraid to make any changes not knowing what affect it might have.

[Lin]

I recently bought a large window AC unit at a yard sale for my garage.  Although it was new and in the box, it turned out that it was over ten years old.  Since it was old and a 2 ton unit, I decided that I would test it on the ground before installing it.  It worked and is now installed doing well.  This is of no use to you, Airbag, but I think that the lesson to be gained here is that it is a good idea to test any new equipment before installation if possible.  Installing this unit has made it your problem instead of being able to drop it back in the box and send it home.

[Airbag]

I may have fixed it. Could have been a loose wire on the back of the fan switch, It ran for 15 min straight. I will update you guys tomorrow when I have more time, have to go to the in laws now.

[Airbag]

I recently bought a large window AC unit at a yard sale for my garage.  Although it was new and in the box, it turned out that it was over ten years old.  Since it was old and a 2 ton unit, I decided that I would test it on the ground before installing it.  It worked and is now installed doing well.  This is of no use to you, Airbag, but I think that the lesson to be gained here is that it is a good idea to test any new equipment before installation if possible.  Installing this unit has made it your problem instead of being able to drop it back in the box and send it home.

The AC was installed in my shop by two federally licensed mechanics that hold airframe and powerplant and Inspection Authorisation certificates. This requires two years of classroom time eight hours a day Just for the A&P.

Required areas of study in the airframe curriculum include inspection, structures—wood, sheet metal, composite—and fasteners, covering, finishes, welding, assembly and rigging, hydraulics, pneumatics, cabin atmosphere control systems, instrument systems, communication and navigation systems, fuel systems, electrical systems, position and warning systems, ice and rain control systems, and fire protection systems.[10]

Required areas of study in the powerplant curriculum include inspection, reciprocating and turbine engine theory and repair, instrument systems, fire protection systems, electrical systems, lubrication systems, ignition and starting systems, fuel metering systems, fuel systems, induction and airflow systems, cooling systems, exhaust and reverser systems, propellers, unducted fans, and auxiliary power units.[11]

The AC is covered if installed by an approved installer. They would have a hard time disputing their qualifications. My guys did the installation like it was the space shuttle.

[Lin]

I have no doubt that your AC was correctly installed.  I merely mean that once it is installed, there is a tendency to try to fix it in place rather than remove it.  Very often this works out.  However, it is easy to put time into it only to find out that equipment is defective and must be pulled out.  Therefore, if it is possible to test something first, it is worth doing it.  The incident I mention with my AC was probably the first time I have done this because it was so heavy that I need others to lift it into place.

[Airbag]

I have no doubt that your AC was correctly installed.  I merely mean that once it is installed, there is a tendency to try to fix it in place rather than remove it.  Very often this works out.  However, it is easy to put time into it only to find out that equipment is defective and must be pulled out.  Therefore, if it is possible to test something first, it is worth doing it.  The incident I mention with my AC was probably the first time I have done this because it was so heavy that I need others to lift it into place.

I agree I just assumed it would run with no problems. Oooops there I go assuming again  You know what assume means right? It makes an (@$# out of u & me). Sorry old airplane expression.