Sunlight entering a vehicle can account for up to 60% of the interior heat load that the system must overcome.
Sunload sensors monitor the intensity of sunlight and adjust the HVAC control to improve the comfort level of the driver and passengers on sunny days. Sunload sensors are usually located at the top of the dash and are often mounted on a removable plate, speaker grill, or defroster vent. There is often one on either side of the dash to accommodate differences in sunlight exposure.
Most sunload sensors are the photodiode type, which provides increased resistance as the light intensity increases, so the signal from the sensor drops as the sun shines brighter. The operating range is between 0 and 5 volts. As the sun load increases, the voltage decreases. The signal is sent to the unified meter and A/C amp, which provide instructions to the system to adjust blend doors and fan speed.
Bright sunlight conditions can result in the fan speed increasing, and blend doors opening to improve cooling.
NOTE: Sunload sensors do not respond to fluorescent light, which is read as darkness. When the sunload sensor is checked indoors or in the shade, an open circuit might be indicated. Always check the sunload sensor at a place where sun shines directly on it.