[johnmyster]

At highway speeds your fans never turn on. Your thermostat opens up and coolant flows. Airflow is sufficient to cool the motor. Only if it continued to get hotter would the fans kick in - IE, stop and go where airflow is low.

The fans on your car run with the A/C, regardless of temperature. The extra airflow is cooling your motor to a lower point, until the thermostat cuts off cooling flow enough to maintain this new, lower temperature. You've observed a 5 degree swing. Seems about right.

You're seeing the swing because thermostats open over a RANGE. At 188 it only flows a little bit of water. However, it's enough to keep you cool with the fans running because the radiator is more efficient at rejecting heat with the fans going. Fans off, you're running at 193. The thermostat flows more water at 193, which helps your radiator to reject the heat with less airflow. Both are equillibrium cases.

Some cars have this "speed" logic. Later GMs do. The logic goes like this. Fan(s) run anytime the temperature gets above the set point(s). They also run if the A/C is activated. However with continuous speeds over 45 or 55 MPH, this A/C activation is disabled. Likely, your ECM is unaware of vehicle speed, and thus, this logic was not implemented on your generation of ECM/PCM/Body Control communications.

I'm installing electric fans on my truck and will have a manual switch allowing me to have fans with A/C around town, and to turn off that requirement on the highway. However, even with the fans decoupled from the A/C operation, they'd still kick on if the temperature got too hot. Worst case scenario, I notice the A/C isn't cold. Reach down, hit the switch, fans go. If you do put in a disable switch, be sure to do it in a manner such that the computer can still cut them on if things get too hot.