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Definitely low on charge. Whether you have a leak or just didn’t charge it sufficiently the first time is unknowable. I would suggest using a dye containing refill can and get the pressure to 32-35 low side with engine at 1200 rpm doors open blower on high. Then, when you get settled, get the UV light out and find your leak.


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A CCOT air conditioning system (like we have) should never form ice. That indicates the suction pressure is at a Pressure-Temperature point below freezing, which the low-pressure switch should prevent. Unlike HVAC systems, the compressor speed won't stay constant in an MVAC system. That makes setting charge by pressure challenging. Add to that the variable cooling across the condenser (wind speed) and you get into lots of fun. So, you sit there at idle and the system stays above the low-pressure limit and all is well. Then, you get on the road, towing nonetheless, and see your low side pressure plummet leading to the rapid cycling.

Ford did issue a TSB at some point raising the recommended charge by 4 ounces. Then - the new microchannel parallel flow condenser that out now decreases it significantly. Which condenser did you install? Was it the MOTORCRAFT YJ594? Did you put a red orifice tube in? You also mentioned leaving the vacuum overnight. Do you have a micron gauge or did you eyeball it on a dial gauge? I wonder if you truly had a full vacuum when you charged it.
 

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I think you are on the right track. One thing people fail to realize, especially on these small (relative) systems is the refrigerant loss in the high-pressure hose can easily be a couple of ounces. This, combined with the recognition by Ford that their charge value was already 4 ounces low can lead to issues. In the future, when you hook up gauges on an R134a system, unscrew the port depressor on the high side (effectively closing the high side port) and open your low and high side knobs on your gauge set with the yellow hose still attached to your closed refrigerant can. This will allow the suction side of the system to pull the liquid out of the high side hose. It's a bit hard to describe this exactly without knowing what gauge set you have. I have the Fieldpiece SMAN 480, which has 4 knobs. That behaves differently than an old analog 2 knob set as I can isolate my charge port and suction port. A little bit rambling, but the basic point is every time you put gauges on it, you are extracting enough liquid refrigerant to fill the red hose and manifold if you don't do the purge. And - I'm sure you know about purging the air out of the refrigerant lines when you attach them. Don't want to be injecting air into the system each time as well.
 
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