The Solar Sensor

The solar sensor is on the top of the dash, just behind the windscreen - here is an explanation of how it works:

In the morning or in the shade, when it's cooler, the a/c system comes on quietly and efficiently as expected when you start your car. If, however, it's been sitting in the sun, the a/c system comes on normally when the car starts but, after a few seconds, the blower system goes into ''high gear'' to cool down the cabin quickly.

Then, after the temperature has been reduced, it goes back into normal mode once again. It may not be apparent with your test methods but, as another poster has said, it's not like Jag or anyone else would install a piece that didn't have some purpose.

That sensor feeds info to the ECU that controls the cabin environment along with the thermostat and other sensors in the cabin. The "bump" detects high temps due to sunlight and tells the ECU that the sun has been shining bright into the cabin and to crank up the fan speed as soon as a cool air temp is detected in the airflow of the a/c. Once a more normal temp level is detected indicating that the super heated air has been evacuated, the system goes back into normal mode.

Jim B. writes, "It works that way on my car, anyway. I still recall the first time it happened after we only had the car for a week or so. It sat in the parking lot and was super warm. When the fan went into hypermode, it caught me by surprise as it was the first time it had done that. I thought I'd turned something on but it was doing it all by itself."