After reading up on ROLLGUY's custom harnesses for running the aux fan under the control of the compressor clutch, rather than the temp switch at the receiver drier, I contacted the company that sold me most of my AC rebuild parts. I'm going to be running a new auxiliary fan in addition to a Sanden and PF condenser. I asked their advice on performing this modification, and they suggested running the aux fan off of an additional pressure switch at the receiver drier, so that it would engage only when the pressure is high. Initially they mentioned a w126 pressure switch, but that doesn't look like it will fit the w123 receiver drier's additional port without drilling & tapping it out. Another option would be to replace the stock binary pressure switch (which cuts out the compressor at low pressures) with a trinary switch, which would then engage the fan at high pressure and the compressor at low pressure. My queston is, does anyone know the switching pressure for the stock binary switch? I thought I might be able to find a trinary switch that would work at the correct low pressure cutoff, and have a high pressure switch to engage the fan.

This is not really strictly necessary, the original temperature switch is probably fine, as is ROLLGUY's modification. I just think that I'd like to improve things as much as possible since I'm going to be replacing the entire system at this point.

Edit: Also, does anyone know the temperature that the original temperature switch is supposed to activate at? I'm still running the one that was in the car when I bought it, but I've never seen it activate. Would like to bench test it to see if it actually works.

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild