The pressure sw. on your E320 is on the drier. There is a pair
of wires coming off it that have 2 slide-on connectors
about 4" from the switch. These will tell you if the a/c fan circuit is OK by jumping them together.
The purpode of the sw. is to put the aux fan on low speed when the a/c pressure gets high. [ usually in warmer temps]
If it is cool out , the high side pressure is not enough to trip the sw., thus no low fan. The resistor is how the fan gets it's low speed . [ if fan does not come on when hot ambiant, it is possible you are light on freon and not attaining high pressure to trip sw.]
The high speed fan is triggered by the engine coolant temp sensor. This is independant of the a/c system.
If the high fan comes on during a/c usage, it is because the a/c is indirectly puting a load on the engine and also heating the radiator to the point of tripping the engine coolant temp. [ this can be seen at the temp gage]
The point being that the a/c is merely causing the engine to heat a call for high fan.
The jumping of each switch in the manual is to test the individual circuits.