I have a 92 Crown Vic with 160,000 miles. A can of refrigerant was attched with the car running, doors shut, and a/c on high, but the clutch on the compressor would not work. The temperature out side was somewhat cold, in the middle 40s. Also, there was some tape covering a small vent on top on the dash. I was thinking this might be some sensor for the climate control. Could either of these be a factor, or what else could be the problem, assuming the compressor is fine?