Hm, according to the documentation that I had read the low speed of the aux fans was triggered by the temp sensor on the receiver / dryer (and that this is actually a temp switch not a normal pressure switch) and that high speed is activated by coolant temp like you said.

I wish they had made the temp switch at the R/D dual stage so it could activate the fan's high speed too... it seems like on some hot days it could do some good. But I tell ya what those fans are LOUD on high speed... move a lot of air too! You can feel them running in the steering wheel!

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.