Because they no longer use freon.
You add freon to the low side fitting,top front,on the ac compressor
You call a licensed contractor and have them check your system. If they determine that it needs freon, they can add the right punt.
Add freon
R134a
R12-if it has ac of course.
R134a
Freon is added through the ac low pressure port. The port is located on the aluminum pipe on the ac line.
With the Freon and gauge connected to the low side ac port, start the engine. Turn ac on high, and add Freon till the gauge shows full.
Freon is added in the low pressure port of the ac system. It is on an aluminum pipe on the passengers side, close to the ac drier.
Connect a freon gauge and tank to the low side ac port of the 97 DeVille. With the engine running and the ac on high, open the freon valve. Shut the valve when the ac gauge reads full.
You add refrigerant through the low side service port.