My a/c quit blowing cold 50 degree vent and now only blows cool 70 degree vent. So I hooked up my gauges and I'm getting 40 on the low and 250 on the high, when I rev it up to 2,000 the low goes to 10-20 and the high goes to 300-330 all the lines are hot except the one from the compressor low side to the expansion valve. Also my expansion valve use to make a hissing noise and now it doesnt. Now heres the part I dont understand when I shut off the a/c the pressures dont equalize the low goes to 90 and the high goes to 250. I think the expansion valve maybe bad or there is a restriction somewhere in the system. Also I converted the system to freeze 12 last year and it worked fine for the whole summer and up untill last week. Yes the system was properly evacuated and recharged to the proper amount and the sight glass shows the freon moving just like it should. Here is the weird thing it did the same thing last year, the expansion valve quit making noise and the a/c quit blowing cold so I replaced the drier pulled vacuum on the system and recharged and the expansion valve started making the hissing sound and the a/c worked great. So I think I may have to replace the expansin valve flush the evaporetor pull vacuum and recharge. Also does anyone elses expansion valve make a hissing sound my 85 also does it with a full charged R12 system and it works great. Any Thoughts?

 

Sounds like the expansion valve is stuck.

Usually I dont do this, but a few "SHARP" taps with a screwdriver handle, will dislodge any crud in the valve. If this fixes it, replace the valve, and flush the evaporator.

Your Compressor "SUCKS" refrigerant thru the Low side port, compresses it, sends it to the condenser to be "Condensed" or Slightly cooled, which changes the refrigerant to liquid form. When it reaches the expansion valve, the valve provides a small hole for the refrigerant to "Spray" into the evaporator. Think of a garden hose slightly opened. Only a small amount of water is allowed to pass, but it covers a much greater area, In a "FAN" pattern. The "hiss" you are hearing is the Spray passing thru the restricted faucet.

Your "Pressures" are a Dead Giveaway for a Bad EXPANSION VALVE. If the High Pressure spray were to be restricted, the low pressure port would suck all of the refrigerant out of the evaporator, leaving excessively low pressure on the low side, but the High side , which is restricted, screams away. Continued operation in this situation, can rupture a hose, or damage a compressor. Disable the compressor until you are ready to have it repaired, since likely the defroster function uses the compressor during Defogging the windows.

 

I am checking the AC on my 1995 Wiindstar before a long trip.
My Ford service manual had nice diagrams for the clutch cycle times, off and on time and pressure charts vs ambient temperatures. It give the initial set up with respect to engine speed and blower configuration. Then comes the caveat....these charts do not apply to vans with the secondary auxillary systems which mine has---drat!. That is the subject of a technical service pub I cannot get my hands on. Does anyone have a handle on how to check the dual systems for optimum performance? Do I shut down the blower on the back system etc, and then use the charts? It does make a difference if the back system blower is running, as that will stop the cluch from cycing, and it will run constantly. I live in Houston.

Regards