94 E420 AC compressor not engaging

[billb95]

A friend told me that I should try adding freon since the compressor won't engage unless there is freon in the system. So I plugged a can of 134a in, pierced the can and released the freon with the car running and the AC on. The compressor never came on, the can's weight didn't seem to change. The can did not change temperature (aren't they supposed to get cold as the gas leaves the can?).

If the system was completely empty, do I need to add more than one can to get the compressor going or does it sound like something more is wrong here?

Thanks.

Bill