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I left work early to play with my expansion valve. This was a good day for this since it was 100 degrees in the shade. The vent temp readings I describe below are a result of the high heat and humidity, and are not typical for this car (when it's working correctly).
I started by just watching the pressures to see where they started at 1500 rpms. In particular, I was watching the low side to try to get a fix on the correlation between low side pressure and vent temperatures. I did this with the car in the sun and both front doors wide open.
It took at least five minutes for the low side to drop to 40psi. At that pressure, the vent temperature reading was 82. In another five minutes the low side has reduced to about 35 and the vent temps were in the mid 70's. Finally, the low side settled at about 32 and the vent temp went to 72.
Next, I tried adjusting the charge. Reducing the charge so the low side went to 30 resulted in a vent temp of 70. Reducing the charge so the low side went to 28-29 resulted in a vent temp of 72. Interestingly, when the charge was reduced so that it went below 30, the low side started to move up and down between 25 and 29. This produced a steady 72 degrees.
In order to see if increasing refrigerant flow would produce more cooling under these conditions, I pulled the bulb away from the evaporator tube. Low side pressure immediately increased to 40. This cause vent temps to return to high 70's.
I then added more refrigerant in very small increments. Just a little bit produced a stable 29psi. I added a little more and got to 30, but the vent temp was still at 72. I added a little more, pressure stayed at 30 and vent temp nudged down slightly. I added a little more, low side pressure stayed at 30 and the vent temp went to 71. I again added a little more, low side pressure went to 30 and the vent temp went to 70. Not being someone to leave well enough alone, I added yet another small amount, the low side stayed at 30 and vent temp went to about 69.9.
Having found the sweet spot in the charge, I tried pulling the bulb out again to see if increased flow under these conditions would result in increased cooling. Again, low side pressure went right up, and so did vent temps.
So, it seems to me that increasing flow to increase cooling can only be effective over a very small pressure range. Otherwise, you give the evaporator more refrigerant than it can vaporize. There also seems to be a quantity range of refrigerant where adding or deleting does not noticeably effect either pressures or cooling.
posted by 24.50.5...
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