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Old 12-04-2007, 02:23 PM   #1
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Diagnose roof top a/c units

Does anyone know how to repair one of the standard roof top camper/bus a/c units. I have an old one that was working fine and then just stopped blowing cold. I am assuming it needs a recharge and the low freon? level kicked off a protective switch and won't let the compressor come on anymore. Any details available. I know they cost a fortune to take to a bus or camper mechanic. Any tips?
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Old 12-04-2007, 03:41 PM   #2
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Re: Diagnose roof top a/c units

If you know where that low pressure switch is, disconnect the wire harness going to it and jump it with a paper clip or something. If the compressor kicks on, you either have low freon or a bad switch. Do you know what kind of coolant it has in it? Is it R-12 or R-134? If it's R-12, just buy a new A/C that runs on R-134. If it runs on R-134, then you should be able to buy one of the automotive A/C servicing kits at Wal-Mart. The pressures should be the same.
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Old 12-04-2007, 04:53 PM   #3
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Re: Diagnose roof top a/c units

I'm having a/c issues too. My new bus has a supposedly new ac/heat pump. It works great in a/c mode, but won't make heat. When it's making heat the outer radiator should be cold and the inner one should get hot....but just the opposite happens. It runs in air conditioning mode even when it's supposed to be be heat mode. grrrrrrrrrrr
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Old 12-04-2007, 06:41 PM   #4
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Re: Diagnose roof top a/c units

The low pressure cut-out did its job and that's good. The problem is that you're most likely low on refrigerant. Hopefully it is an R134a or R22 system in which case it's easy enough to buy more refrigerant to bring it back up and test to make sure it's working. I would be putting dye in it when you do though. That stuff had to go somewhere and if it leaked once it will leak again. My hope for you is that it is R134a and that it is just "normal" leaking. R134a has amazingly small molecules that can sneak through the smallest of small openings. In many situations it is far more cost effective to add a pound every year rather than try and get rid of EVERY microscopic pinhole. Just make sure you use gauges so you don't overcharge.
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Old 12-04-2007, 09:24 PM   #5
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Re: Diagnose roof top a/c units

The most common place for an air conditioning system to leak, is the seals on the back of the compressor. If it's an R-134 system, I'd drain it out, disconnect the two lines on the compressor, swap those seals real quick like, reconnect the lines and service it back up. If it's R12, you might even be able to convert it to R-134. I don't know. You'd just need an accumulator drier. Anyways, if you can swap the lines pretty quick, there's no need to put a vacuum on the system.
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Old 12-05-2007, 12:57 AM   #6
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Re: Diagnose roof top a/c units

I've done a few R134a conversions now. The kits we got required that you swap all the seals and o-rings in the system. You then used ester oil which is compatible with both PAG for R134a and mineral oil for R12. It mixes with both refrigerants. We met with great success with the conversions we did, but I will agree that swapping the accumlator or receiver-drier depending on what the system has and draining the other components is the better way to do it.
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Old 12-06-2007, 01:51 PM   #7
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Re: Diagnose roof top a/c units

Sounds like the compressor is not running. Take off the cover and remove the compressor terminal cover-probably have burned off one of the wires; that's the most common point of failure. The unit should be r-22, but is a sealed system-if it's low on refrigerant then there is a hole somewhere. If the wires are good on the compressor check to see if there is voltage at the terminals. If no voltage look farther upstream. Hope this helps...
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Old 12-06-2007, 03:58 PM   #8
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Re: Diagnose roof top a/c units

I pulled three Dou-therm A/C units off a custom truck box yesterday, they were R-22. Let us know how things pan out, mine were never powered up but sat in the rain without their covers for months. So i may have issues with them to work through

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