At least on the W126 the aux fans are not switched on based on pressure, they are turned on based on temperature.... The two are very related though

You need to get a set of gauges and see what is going on. If the expansion valve is blocked up or sticking the high side pressures could spiral out of control.

In general 35*F below ambient is doing fairly good.

-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.