I've got a 2005 M6800 cab with the AC unit in the roof. I've done a few AC systems but I'm at still DIY level and I've gotten over my head...again. Cooling was poor so I checked the pressures against the service manual specs. The system was cycling but the pressures were low. I opened the roof unit to make sure everything looked okay and the evap was clean. Next I vacuumed it down, cold filled 1.1lb on the high side per the manual then finished with vapor fill on the low side to the manuals specified 2.2lb. The compressor cycled normally and the pressures were IAW the manual. Yea. I buttoned up the top unit and double checked since the gauges were still on. The pressure were high on both sides with the high side climbing over 350 before I could shut it down. To make a long story short, I've repeated this whole process several times and it works fine every time until I put the lid on the unit. All that's up there is the thermostst, a pressure switch and the expansion valve. I've played with wiring. pushed on the valve, etc and I can't duplicate the problem without putting the lid on. It happens right away and I can open the lid quickly to verify there's no icing. I'm stuck fellas - what am I missing???
M6800 Air Conditioning Mystery
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Not fully understanding what is going on but have you looked at the condenser yet? Poorly operating one will create high preasures.
Maybe I wasn't clear - I don't understand it either. The AC unit on the roof is two pieces. It's basically a tub with a lid. In the tub is the fan and the evap. With the lid off the entire system operates normally as far as I can tell. When I put the lid on the compressor engages and the pressures go up and up and up. It's really weird. The low side begins to pump down but at 35-40psi it starts climbing along with the high side. With the lid off the low side pumps down to about 25 and the high side gets to 190'ish before the compressor cycles off.
kubotafreak
Well-known member
Equipment
GRAND l6060, L3560, B6100, gr2100, tg 1860, g1800, g1900, g2160
possibly an expansion valve failing intermittently
or low airflow across the condenser, less likely low airflow across the evaporator
or low airflow across the condenser, less likely low airflow across the evaporator
Could coolant control valve for heater be allowing hot coolant to pass through it?
That's what I'm thinking right now. I'm not sure but possibly the valve is stuck open. There isn't much airflow across the condenser with the box open so maybe the refrigerant doesn't evaporate and the pressure stays down? I'm going back this morning and try experimenting with airflow.
TX Jim - the heater lines are disconnected right now.
I did an airflow test with the lid off. When there is little airflow the system operates normally. When there is good flow, the pressures go way up. I'm ordering a control valve since I can't think of any other possibilities. One question: The manual doesn't say anything about replacing the receiver when the system is opened. The online parts drawings don't even show a part number for the receiver. Do i need to worry about that?
If you evacuate for 24/48 hours, replacing the receiver drier usually isn't necessary. I evacuate for a minimum of 24 hours because I have lots of time on my side and I am not doing this for a living. I then charge with nitrogen and write down my readings and then again after 2 hours. By that time I expect that the system is stabilized, and there should be no drop in pressures. 24 hours later if I notice a drop in pressures then I know that I have a leak somewhere. I will evacuate the nitrogen and put a small charge of refrigerant in with dye and start the system to allow the compressor to spin for about a minute and shut it down. Then I start looking for traces of the dye. If I can't find any traces of dye I break out the sniffer and hope to find the leak. Most times it will be a compromised "O" ring. On my 1962 Chevrolet I have a minuscule leak that I cannot find sp every spring I add a little refrigerant to the system till the sight glass clears and call it a day. If I were to spend $2000 on an electronic leak detector, I might be able to find it, but refrigerant is inexpensive enough that I just don't care to chase it any further.
You Sir, are a patient man! I bit the bullet and ordered a new receiver dryer just to be safe but I'll remember the extended vacuum trick. My guess at the moment is that the system is sealed. I think the refrigerant was low because the control valve malfunctioned and the pressure popped the relief. I'll pressurize with nitrogen as well to be sure. Thanks!
Also, check the oil in the compressor. Most service manuals suggest draining it into a measuring cup and then replacing the oil in the compressor. An example, you drain 6 ounces of oil from the compressor, and it is suggested that you replace it with 8. The total oil in a system is based on each component and if it is being replaced. Once again, as an example, the total oil requirement is 14 ounces. 2 ounces would be for the reciever drier, 3 ounces for the evaporator, 4 ounces for the condenser, 5 ounces for the compressor. In this example if it is a cleaned and bone dry system some technicians inject each amount into the individual components, or they might put all the oil into the compressor. When adding oil to the compressor, activate the clutch with the engine off if it is belt driven and turn it about 20 full turns to distribute the oil into the lines. Then when starting the engine, engage the compressor in small burst of 10 or 15 seconds for about 10 times. When adding freon, it is usually measured in pounds, which requires a calibrated scale that the cylinder sits on. If you are using cans of refrigerant, I suggest that you have a pail of warm (150 degree) to get all the refrigerant out of the cans. Most technicians only fill the unit on the low side unless they have very sophisticated machines that are designed to pump the refrigerant in on the high side. When you are evacuating the system with a vacuum pump be sure to use fresh oil in the vacuum pump because the moisture in the system is gathered in the vacuum pump oil. My experience with air conditioning is working with mobile air conditioning system as used on autos and trucks. The principles are the same, however, as the newer the system, the more complicated they become. I am sure that the same is true with tractors with air conditioning.
Great info - thanks!
The manual has a chart that specifies the amount of oil for each component. The valve doesn't store oil but the receiver is listed as 10cc or 1/3 ounce. I admit to being lazy - was thinking I'd put an ounce in the receiver before assembly and call it a day. I hear what your saying about draining the compressor oil. Is that needed tho? My limited experience on auto ac is that the less I mess with the better chance I have.
Never really gave much thought to my evac pump before. I'll order that oil today.
You need to know how much oil is actually in the system. Too much oil and it pools in the compressor and not enough the compressor will seize up. I know of a number of people that don't subscribe to checking oil and have been lucky to have gotten away with it. If I don't know the history of the failure or the system as a whole I rather err on the side of caution, and I flush all the components. Whenever possible I only use the refrigerant that the original system called for. I never mix oils or refrigerants, for obvious reasons. Incompatibility of oils or refrigerants many times isn't known until something fails as a result. Compressors can get very expensive to replace, not to mention that there is a lot of work that should be done if the compressor has a failure. Working on air conditioning is like being a surgeon. You have to be methodical and cleanliness is an absolute. This is why I don't work on modern mobil air conditioning, since I never learned about it and I am old enough to know that it has a learning curve because of its design of components.