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?

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