Let's clear up how this works. There is a pressure switch in the receiver/drier that activates the low speed fan relay. It comes on at 300PSI, which is usually above the operating pressure of the a/c system. But you can hit that on a hot humid day. No freon, no pressure, no fan. If your fan comes on at low speed at any other time, there is a problem. If it never comes on with a/c operating, then check the fan resistor on the right inner fender.
High speed is activated by high temperature, usually at 105, sometimes higher. This should almost never happen. Depending on year and engine, it's either activated by a thermal sensor which feeds the climate control system, or directly by a thermoswitch located on the engine water outlet. The high speed relay is sometimes mounted in an auxilliary fuse box just forward of the main fuse center. It's always an internally fused relay. If it's a black relay, the fuse can't be replaced and the relay will need to be discarded if it blows. Replace it with the later style orange relay which has an ATC fuse on top.
The belt driven fan has a clutch which can either be electromagnetic or viscous. Either way, it activates around 95c.
And that's about all there is to 190 fans.