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Old 04-28-2012, 08:45 AM   #1
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So You Think Your DASH A/C Unit is No Good

I called and talked to an engineer today that works for the company that bought out the company that makes Monaco dash air conditioners in our motorhomes. This guy is not your local technician but is a design engineer that knows how they should work. I wanted to talk to somebody that was in the know on how our dash AC units should be. This is what he told me.

The dash AC units that we have are NOT designed to cool the whole MH or the whole front section of the MH and are only designed to blow cool air on the driver, the passenger and defrost the windshield. If you take an accurate thermometer and measure the temperature differential between the return vent on the lower part of the dash by the floor and the output air out of the dash vents the minimum difference should be 25*. Any better than that such as 30* or 35* difference then consider yourself lucky. He said if you want it to be cool in the front section or the whole MH then you will have to run the generator and roof AC units. The compressor for the dash units is the same size as used in a large car, SUV or pickup truck but the area of the MH is many times bigger so it will not do a good job cooling.

Some recommendations are to first run the generator and roof AC units until the inside cools down then the dash may work better and keep up. Also keep the door going to the back half of the MH closed and keep the dash control in MAX so the vent to bring in outside air is closed.

If your dash unit fails to maintain at least a 25* differential between the return vent and the supply vent when in MAX then consider getting it recharged with R-134.

Hope this answers some of your questions. It is not bad, it is just designed to be bad.
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Old 04-28-2012, 10:53 AM   #2
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I had read a suggestion recently (sorry, forget the author) to tie that recirculating door open permanently. Generally you don't want to warm cold outside air, or cool hot outside air anyway! That logic made great sense to me, so in preparation for our first long trip through the south and south west, I did just that. That door on ours is easily accessible under the hood/generator access door. Took just a couple of minutes to assure it would seal against any outside air intrusion.

Now, after running west through all the gulf states, then through Texas and New Mexico over the last few weeks, I'm happy to report the mod works great. Better than I thought it would actually. Because of the crazy hot temps we've been running in, we're running the generator when on the road, always. With the roof AC's running, we don't need the cab AC compressor running at all. With the cab fan running in it's bi-level heat mode, temp control full cold, the cab heater is picking up the cool air off the floor in front of the passenger's seat and blowing it on us, leaving us quite comfortable with the cool air being blown on us - even on 100 degree days - without the chassis AC compressor engaged. We're not even running the blower on high! With our experience to date, would recommend the mod to anyone capable!
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Old 04-28-2012, 11:09 AM   #3
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We had to have a new AC compressor put in our old Eagle and first time I used it was not happy. But then the motorhome had been sitting out in the sun and that was one hot sucker in there.

The next time we took it out, it had been under our motorhome cover, we even put tech shield in the top to protect the top of the motorhome from so much heat on it. It wasn't to bad inside and we went for a long drive checking things out, along with the dash fans it kept it nice and comfy, even for DD in the second seat back.

I know we will have to use the gennie and front AC at times and we will find a comfort level among them.
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Old 04-28-2012, 11:54 AM   #4
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We bought a spring loaded adjustable shower curtain rod and a heavy shower curtain that we placed between the ends of the two slides behind the driver and passenger seats. It helped a bunch in keeping the cool and heat from the dash air in the cab area. On extremely hot days, especially during the afternoon, we would have to resort to the roof air but it worked well most of the time.
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Old 04-28-2012, 12:29 PM   #5
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I'm surprised that Monaco didn't install ceiling mounted fans for the driver and passenger. Like everyone, I'm not satisfied with my dash air; so when I changed my IM Satellite from 12v to 110v (what an improvement!) I used the 12v circuit to wire a fan to blow on me when driving. Just moving air makes a tremendous difference in your comfort range. Hard to believe that they still don't install nor offer them as an option.
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Old 04-28-2012, 01:36 PM   #6
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Ahicks, your older system must be different from the newer systems. On cool with the newer systems if you turn the knob to MAX on cool then the vent door to the outside closes and it just recirculates interior air.

Wannabe, I have built in hidden fans above the windshield that I can turn to help defrost but they are not like the good old fans I had in my previous MH.
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Old 04-28-2012, 03:33 PM   #7
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Ahicks, your older system must be different from the newer systems. On cool with the newer systems if you turn the knob to MAX on cool then the vent door to the outside closes and it just recirculates interior air.
Mike, mine works that way as well.

Originally (prior to the mod), the only way to get recirculating air on mine is as you say yours works, with control set to MAX. The only way you can get to MAX is with the cab AC compressor on though?

With the "outside air door locked in the closed position" mod, I found a bonus feature, discovered quite by accident! You can have recirculating air in use when on the heat side of the controls (temp control set to cold) - without the cab AC compressor running - to take better advantage of the cool air floating around the floor of your coach. Controls set this way have proven very effective even in the very warm (100+) temps we've been traveling in lately. So effective, we generally run the fan on the lower speeds to keep "I'm freezing" comments from the passenger side minimized.

On ours, this was previously unheard of with the cab AC on and the fan blowing luke warm air at best when the temps get like this.

In fairness I do run the roof AC controls down to 75 degrees to prevent the potential for that unit to kick off.

BTW, my cab AC is very functional, seems to work well with no known issues, and remains that way even after this mod. It just can't/won't pull in outside air any more, a feature I now find questionable on a rig this size. I intend to use it as I normally would do when traveling in more moderate temps.
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Old 04-28-2012, 05:56 PM   #8
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Recharging may not be the answer. There are plenty of other issues that can cause the unit not to cool properly. To a layman, the operation of the simple refrigeration cycle is pretty complicated, but to a properly trained technician, it is pretty simple and by getting some temperatures and pressures, he can easily figure out the best course of action to get you cooling again.

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Old 04-28-2012, 07:56 PM   #9
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Ken, I totally understand a refrigeration system. not sure I can be considered just a layman. For you I will change it from "recharged" to "serviced" but if the system is cooling but not cooling enough indicates nine time out of ten it just needs recharging. If it is blowing non cooled air then yes it may be a another fault but again could just need recharging. But what I am trying to do is to just tell people what the dash AC unit is for and what it is capable of doing or not doing and not get off into the details of servicing the system.
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Old 04-28-2012, 09:13 PM   #10
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The dash air, like you said, is not meant to cool the whole coach. But I have seen too many times, people, including technicians immediately want to charge more refrigerant into the system based solely on low suction pressure or not cooling enough. You have other things that can cause the same basic symptoms.

I have been designing, servicing, presenting service and operator training and selling industrial refrigeration systems and dabbling in smaller systems and A/C. Over 40 years in the field, with an engineering degree (mechanical specializing in applied heat transfer and thermodynamics) I have learned check beyond the obvious before proceeding.

I just want to caution folks, that low refrigerant may not the the solution, but that is ALWAYS the first word out of a lot of the "shade tree" crew.

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Old 04-29-2012, 07:54 AM   #11
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Quote:
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Wannabe, I have built in hidden fans above the windshield that I can turn to help defrost but they are not like the good old fans I had in my previous MH.
Mike,
I have the defrost fans also, but they don't help cool me like the fan blowing on me. I'm looking for a portable horizontal fan to place on the dash for DW. There is not a good mounting surface on the passenger side to mount a fan like mine on the driver's side.
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Old 04-29-2012, 08:51 AM   #12
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Sometimes I swear those defrost fans blow hot air. I have never had to use them to defrost my windshield. I turned them on to circulate the air and got hotter.
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Old 04-29-2012, 11:12 AM   #13
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On most systems, the compressor is on when in the defrost mode. The intent is to get dry air to the windshield. This can have a negative affect on your MPG.

I'm a smoker and whenever I light up either in my MH or trucks, I turn the control to outside air to help exhaust the smoke.



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Old 04-29-2012, 11:13 AM   #14
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Mike, if in the defrost mode, with the temp control to hot, it will blow hot air, even though the compressor is on. Same with in the Max mode.



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