Thanks to both of you. I was operating under the belief that the low speed fans would turn on immediately after I started the engine and called for cold air, presuming my a/c system was properly charged.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

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The low fan comes on only when the high side presure is in need of additional airflow due to high thermal load condition. Like sitting at idle on a warm day..if the ambient temp is low ,you do not have a high thermal load and the aux fan is not called for and will not come on at all. Both the Low ac fan and the High coolant temp fan only come on when either the coolant temps or ac pressures are being TAXED by high termal conditions..that is why they call them Aux Fans..They are not needed or called for under NORMAL conditions . But one usually sees a low fan for ac b/c the ac system is usually only being used when such a condition exist [ Hot day].
I have done ac work in the NE and aux fan activation is much less b/c of the climate than I see here in Fla. It is the ambient temp that is the main factor in determining if the system needs additional airflow to keep the condenser pressures in check. Again, the existance of a high thermal load condition... no load/no fan.
Just remember that if one does have a hot day with the ac on and has no aux fan activation, to a Tech, that is a tip off/ early warning sign of a low refrigerant condition. But some further testing is required to make a true diagnosis..
[ Guages to compare sw. cut-in specs to actual high side pressures]

I noted that the low fans did not come on until the high side got to 250 psi, but they did not turn off until the high side fell back down to 150 psi. Is this a dual switch that allows the fans to bracket the high pressure within such a pressure range, or is the measurement taken from the low side?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Hi Guys
It is interesting to hear what happens to make the aux fan run.
But I have a question that is related. That is when you push the A/C button on the PBC does the aux fan start to run ( I have a 88 190E ) and according to the wire dia only to thinge control the aux fan and that is the HP swt and the PBC.
THanks
Chuck