1998 Isuzu Rodeo AC not engaging

Guys Had a ? about AC I got this Isuzu Rodeo and AC Doesn't work. When ever I jumper it from relay AC compressor kicks in, try changing relays but no luck. What else should I check? What's the relay connected to does anybody know. Also guy from work told me to check filter from AC not sure what he was talking about.

http://i30.photobucket.com/albums/c337/Gizmo420/AC1.jpg

What is not mentioned in there is the high pressure and low pressure switches. High pressure shouldnt be a problem since it hasnt run. If there is low/no refrigerant then the low pressure switch will open and keep the clutch from engaging.

Thanks Dave I will try checking system for low freon will need to jumper compressor to check pressure. Thanks again!