Help me understand the technical of my truck's A/C

[mooch91]

(Sorry, may have this in the wrong sub-forum).

Have a 2007 F350 6.0 with about 4000 miles on it. I've got a pretty good understanding on A/C systems but don't have full knowledge of the Ford system.

Have used the A/C rarely, but had it on last week on an approximately 80 deg sunny day on a 1 hour trip on the highway. On normal mode, the cab was uncomfortable. On max, I would say cab was cooler, but not cold. Definitely something not working right - no question about it. When I got to my destination, compressor was cycling on and off approximately every 10 seconds. Ran the truck again today with A/C on and compressor was doing the same.

Hooked up some gauges to watch with the assistance of a friend who is a home/industrial A/C tech. Static pressure was about 50 psi (at 60 deg F ambient temp today, so probably not the best day to test). Low-side pressure would build to 40-50 psi, compressor would kick on and low-side pressure would drop to about 20-30 psi, at which point compressor would turn off. On the high side, when compressor would kick on, the pressure would slowly start to raise above 50 but would only get to 90-100 psi before the compressor would turn off.

Checked the whole system for leaks with a leak detector and found nothing.

Thought maybe refirgerant was a bit low and was causing a low pressure cutoff to engage.

Added a shot (suspect few oz.) of 134A and the cycling stopped, compressor stayed on. Low-side pressure around 30 psi and high side in the 180 - 190 psi range. Cab got real cool (vents were showing 30 - 35 deg F). Stayed this way for about five minutes, then started cycling again.

Shut it down and pressures equalized at about 52 psi, a couple of psi above the 50 we started with.

Checked again for leaks with a leak detector and found nothing.

Stopped there figuring we'd check again after I could do some research on the Ford system and we got a warm day. We weren't sure if there were temperature cut-off switches or other controls that were working against us with such a cool ambient day.

My preliminary guess is that the truck was a tad low on refrigerant based on the way it behaved when it got a few oz. There's nothing that leads me to believe the compressor isn't working when it's on (even before, it managed a little bit of cooling) or that there is obstruction in the system (high side pressure was not excessively high).

Anything else I should be checking?

If it was in fact a tad low, what would cause this? Being a closed system, and one that's only two years old, I would have expected to find a leak, but nothing evident.

I did find TSB 08-13-5 which talks about dessicant issues clogging the orifice tube and heard about one that describes an underfill of refrigerant from the factory (but could not find the text to see if it applies to my 07). Anyone have the text of the latter?

Thanks in advance!

[CCdually6.0]

I agree that it sounds as though the system is low of freon. I would start from scratch by having the system hooked to a recovery station. Pull it down, test for leaks and refill with the proper amount. Most of the newer recovery stations will report what was recovered. That should tell you how low the system was. If the system passes a vacuum test but still shows signs of leaking under load, you will probably find an o-ring is leaking under pressure.