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Old 07-09-2020, 07:47 PM   #1
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Saga of the Coleman-Mach Basement A/C

Over the years, Iíve read a number of IRV2 posts about problems with the Coleman Mach basement ACís and wondered when I would get my turn. Itís my turnÖ

Last week my sonís AC went on his house so I told him that he and his family could move into our 2007 Winnebago Tour 40TD to avoid the heat. He pulled the RV out of the barn, plugged it in to 50 amps and turned on the basement AC. Three hours later, no real change in temperature.

Started to examine the unit. I could hear both compressors kick on, I saw the amps pick up to confirm the compressors were on, both blowers working, but no cold air. Saw the one coolant line going to the coil was covered in ice which is a sign of low Freon. However, these units do not have a recharge port like cars.

Ugh. So I started doing my research and found there were some replacement basement units available. However, my unit is a 6538-871 which has a TOP return vent. The replacement units Ė 46515-811 Ė have a BACK return vent.
Also found the unit uses R-22 coolant, not R-134a which is used in cars.

Called the dealership where I bought the motorhome new Ė they donít work on ANY basement ACís anymoreÖ

My current plan is to remove the unit so I can get the top off and see if I can get a residential AC repair guy to add some taps and charge it with R-22. I donít really have any other options. My only other choice is to replace a vent with a roof AC but that would not allow much cool air to get to the back bedroom.

One other issue with the basement AC is that itís positioned behind the rear tires. So after 12 years and about 120,000 miles, all that rain, road salt, parking it in the FL Keys 15 feet from salt water, etc. have created a rusty mess.

With these units, you have to remove the plenum which is screwed on with self-tapping Torx head screws that are covered in rust and then loosen four big bolts to lower the unit so you can slide it out. As youíre working underneath the unit, rust flakes rain down on you as your impact wrench loosens the bolts. You have about 3 inches to get your ratchet with a T-20 head on to remove the plenum screws if thereís any head even left.

Today I was able to loosen the plenum and get the bolts down enough to pull the AC unit out about 2 inches.

Tomorrow I have to drill out the rivets that hold the access door to the hinges. There may be self-tapping screws on the other side of the hinge but theyíre hidden behind some trim. Seems like an easier option to put new rivets in when (and if) I finish. Then I can slide the unit onto a yet to be built platform, take the top cover off and give easy access to the still-to-be-determined AC repair guy. Looking at the electrical connections, I should be able to slide it out without disconnecting.

Since the frame that holds the AC is so rusted, Iíll also be cleaning that up, replacing the bolts, painting everything with POR-15 paint, and checking any other components that may need fixing. Some of the sheet metal for the unit is rusted so Iíll be replacing that too.

While I have everything apart, I will also determine if I can change the interior return to a side return vs. a top return. If thatís easier, I might be able to fit a replacement unit in.

Iíll give an update on tomorrowís progress and take some pics.
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Old 07-09-2020, 08:02 PM   #2
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Wow, what a mess!!! We have a basement unit too. Fingers crossed we donít run into your troubles. Weíll keep watching your adventures. Good luck!!!!
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Old 07-10-2020, 10:33 AM   #3
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Morning update.

Was able to open access door far enough so donít have to remove door.

Not enough play in the wires so had to disconnect. Very easy - thermostat wire plugs into board and two 110 electric lines.

Built a platform about 2 feet high and ready to slide out the unit this afternoon.

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Old 07-10-2020, 02:34 PM   #4
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When ours failed on our Horizon, I was talking to a residential HVAC guy about repair and when I mentioned ďmotorhomeĒ he started shaking his head. It took a little convincing to even have his guy look at it.

The tech that showed up didn't have any trouble figuring out how to get it repaired. They weren't interested in finding a replacement compressor so I ordered it online and had it within 3 days.

He installed it, brazed in a couple of ports, did the vacuum thing and recharged the system. All was good after that.

Ours had long enough electrical wires that I could run it while still hooked up to the MH.

Good Luck.
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Old 07-10-2020, 02:44 PM   #5
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I have owned two Winnebago Journey MH's with the basement air and absolutely loved it. No AC roaring over my head. One simple 14 x 20 house-style filter to maintain. Cleaner roof line with fewer "holes" in it to potentially leak some day. No condensation dripping off the roof. One control. Lots of advantages.

I don't have those MH's anymore and never had to deal with the "repair" side of that kind of AC that is now becoming obsolete and even harder to take care of if a problem develops. Too bad, because I'd go back to that kind of AC in a heartbeat over roof airs.
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Old 07-10-2020, 03:05 PM   #6
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Afternoon update.

Itís out. Slid it onto the platform with my son and then lifted it on some saw horses.

Support frame was so rusted in one corner that it was not providing any support. Lower rear cabinet also rusted.

Put some feelers out to AC guys about recharging.

Iíll work on rust remediation this weekend!Click image for larger version

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Old 07-10-2020, 04:25 PM   #7
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I LOVE it. You are obviously a very capable guy.

I have been repairing a similar SCS basement units for sometime. You are totally right on in that a standard RV tech knows nothing about repairing these units. If itís not replacing a rooftop then itís our of their expertise level.

And itís difficult to get a home AC tech to look at it once they hear RV. The trick is exactly what you did. Take it out, take off the covers, and let the AC tech see that is technology that they are very familiar with.

Couple of things about replacing the compressor. Insist that the tech install a filter drier on the suction side, and insist the Nitrogen purge is used during the brazing. If you donít do this, then you will be taking the unit back out to replace the thermal expansion valve soon.
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Old 07-10-2020, 09:06 PM   #8
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Quote:
Originally Posted by RichardE View Post
Couple of things about replacing the compressor. Insist that the tech install a filter drier on the suction side, and insist the Nitrogen purge is used during the brazing. If you donít do this, then you will be taking the unit back out to replace the thermal expansion valve soon.
Thanks for the tips. I have a home AC guy coming next week. He has a larger AC company and I coached his son in soccer and baseball 20 years ago so it's always helpful to be involved in the community so you can call in markers years later!
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Old 07-11-2020, 07:47 PM   #9
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The guts of the unit

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Old 07-12-2020, 09:23 AM   #10
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Good luck, just one of the problems I've had working on those, they don't have a filter/drier in the refrigerant ckts. you can easily change. Instead, some of the colemans had the filter in the last loop of the condenser tube in the form of a fiber insert in side the tube. I changed one of the compressors and on charging it discovered no refrigerant flow. The filter in the condenser tube blocked almost all flow causing icing of the evaporator coil even when over charged. I desoldered the condenser discharge tube and found it. I finally got it out, and it worked good then. It took about a week to figure it out.
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Old 07-14-2020, 09:21 AM   #11
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I have a AC guy and a welding/sheet metal guy lined up. In the meantime, Iím getting rid of the surface rust and painting it with POR-15 primer.Click image for larger version

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Old 07-14-2020, 09:37 AM   #12
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Just FYI

the new units with side return are shorter than your old unit by 4''

this allows a duct transition to be fabricated to go from top to side return fairly easily.

this attached PDF shows what I am talking about item #2 is what you need

You would be money ahead to replace and retrofit
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File Type: pdf Merdian AC duct.pdf (105.0 KB, 106 views)
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Old 07-14-2020, 09:59 AM   #13
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The job is not that difficult to do, it just takes time.Get rid of the Phillips screws. Be sure to change the duct screws to ones that can be driven with a socket and use never seize on threads
Also replace pillow block bearings with sealed ones.

My unit has cold air return on the side with a canvas material going up to the cold air return on the top. I also cleaned & painted frame. while out.
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Old 07-14-2020, 08:56 PM   #14
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Quote:
Originally Posted by Wild Card View Post
Just FYI

the new units with side return are shorter than your old unit by 4''

this allows a duct transition to be fabricated to go from top to side return fairly easily.

this attached PDF shows what I am talking about item #2 is what you need

You would be money ahead to replace and retrofit
Thanks for the info. I see where I save 4 inches on height but the replacement unit is also 2 inches deeper. With the heat shield, I don't know if I have that room but I'll have to check.
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