When does the aux fan come on? 300E
 

Is the aux fan in front of the condenser supposed to be on all the time when the a/c is engaged? On my Peugeot is does but my 1988 300E M103 it cycles on and off. Normal? Thanks.

The low speed comes on with A/C high side pressure, so if the fan running lowers the pressure enough it will cycle off until the pressure builds again.

Mine come on when the high side hits 250, then goes off when it drops back down to approx 150-175. Mine also come on when the engine temp reaches somewhere around 105C.

Correct. Low speed aux fan is switched on by the a/c system, high speed by engine heat. Two different relays in the relay/fuse box, low speed relay has a resistor to reduce the fan's current and slow it down.

Thanks guys.

Or connects to a big resistor [1989 300E], mounted near the receiver/dryer behind the driver's side headlight [juice to one side is high speed, the other is low speed]

Cool but not Cold
 

Right, mine has the big resistor.

Just curious but wouldn't the a/c cool better if the aux fan was always on when the a/c was engaged? My other European cars work this way.

This 300E has been converted to R134 and checked by a shop, evacuated, pulled a vacuum, and IR checked with dye for leaks last week. The A/C is not as cold as my other R134 retrofitted cars. On an 80 degree day I could only measure ~50F at the vent.

Air Flow and Condenser efficiancy.
 

The Condenser on the R12(From The Factory) equipped W124 Does Not Have
the Capacity to Properly cool when Retrofitted to R134.

Two possible solutions:
Reconvert back to R12...Properly!
Replace the Condenser with a Parallel Flow Condenser (More Capacity)

Well...that is probably one of my issues. Just had my w124 engine replaced and found out today they converted it to R134. It wasn't blowing cool today so I went and charged it myself. However, it would only take so much charge and I couldn't get it into the optimal "blue zone" on the guage. It does blow very cold though. Not as ice cold as it was before but cold enough. Could this have anything to do with my auxillary fans not cooling properly and engine temps running higher on the newly installed engine?