W124 has a thermistor just in behind the evaporator that senses the
temp. Will cycle the compressor to prevent freeze-up.
However, if the expansion valve is bad, it will seriously overcool,
and then the computer will not turn the compressor back on until the
evaporator thaws out, which takes a while with no airflow.
The temperature of the return line to the compressor should be 35 to
38F, whatever pressure that is on an R134a system.
You can test the expansion valve with some very cold ice (dry ice
works best, but it's hard to find and dangerous. Use a cube straight
from the freezer). With the system up and going, put the ice on the
bellows of the expansion valve (the do-dad on top). This should close
the expansion valve and drop the pressure in the return line to near
vaccum (which is why dry ice works better, it's way to cold -- ice
won't go below 32F and you need it colder). If it doesn't drop, the
expansion valve is stuck open and it will seriously over-cool the
evaporator.
Peter
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