PSFred,
Good description of what the expansion valve does. I would add one thing though. The reason the expansion valve is designed to hold the temperature at about 35 degrees is that if it allowed the temp to drop much further the evaporator would quickly freeze solid. As you can imagine, on even a slightly humid day you will get a great deal of condensation on the evaporator as it cools the air. If the temp of the evaporator drops to freezing the condensation freezes and you can no longer pass air through the coils. The expansion valve has a hollow tube that usually is placed right in back (downstream of the airflow) of the evaporator. As the temp drops toward freezing the expansion valve closes up a bit to let less coolant through and raise the temp slightly to prevent freezing. One item to check if you have replaced the expansion valve is to be sure the temp sensing tube is properly placed. If it is too far away from the evaporator you will likely get a freeze up and rapidly diminished cooling as less ands less air gets through the evaporator. Expansion valves are usually trouble free except when a compressor blows and debris has been scattered in the system. Then they should be replaced as a matter of course along with a good system flush.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold