The expansion valve controls the flow of liquid freon into the evaporator based on the temperature of the gas return side. They fail in two ways (actually three, I guess) -- they plug, or the actuator liquid leaks out and the valve is either wide open or completely closed, depending on the design. Usually wide open.

If plugged, you need to check the high pressure side for the presence of gray crud -- if so, you have a dead compressor and a large headache -- the crud will quickly ruin a new compressor, sometimes even with a filter in the return line. Flusing will NOT clear it out of a Benz parallel flow evaporator.

On the W124, the valve is fairly easy to remove, but you must vent the AC, so unless it's recovered (R12 or R134a), you may not be able to do this yourself.

They only cost about $30, I'd just get a new one rather than try to clear the old one.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!