I add to the above questions and comments by asking, Does the A/C light on the instrument panel blink when you get the no-cooling condition?

Pressure tables and some FSM's can be deceiving, which is why many pros don't use them. For example, at a low-side pressure of 18 PSI, the evaporator would turn into a block of ice in no time, preventing any airflow. Bear in mind that pressures are great for diagnosing a system but can't tell you ho...

Has any work ever been done to the system, especially recently? Compressor replaced? If not, I recommend you begin by recovering the refrigerant that's in it now (or having it recovered), evacuating it well, and recharging the system exactly to specs: 16 ounces net weight of R-134a, and 4.9 fluid ou...

I don't think there's anything intrinsically wrong with installing a pressure switch on the low side. My guess is that manufacturers leave it off figuring that a binary (or trinary) switch is sufficient, and/or to reduce costs.

A trinary switch monitors the high and low limits only on the high side of the system, and it also controls the fan(s).

Thanks...I didn't notice that.

With pressures like that and with the HP safety valve venting, my first thought is that it's overcharged. Did anyone add any refrigerant lately?

Also, use extreme care when working on this system. A burst hose or other component is a distinct possibility with such high pressures.

Pulled a vacuum down to 29 in hg my gauge may be reading 1 in hg off though. I added oil 2.5 oz to compressor and .5 to evaporator Vacuum looks good, although using a micron gauge like the pros do in addition to your Manifold Gauge Set would give a more precise reading. I shoot for around 500 micro...

The specs that I have for your vehicle say 8.5 ounces of PAG-46. That's what I thought. I don't remember where I saw it, but I remembered 8oz. I don't know how much he put in or how much was already in there. What are the other specs for my AC? I either can't find them or they are inconsistent from...

They did not install Duramax engines in any military Humvee's so there are no "specs" for what I am doing I regret the misunderstanding. Since we don't know how much refrigerant a system like yours takes, there is a way to "sneak up" on the approximate correct charge by starting...