How to Troubleshoot my Chevy C3500 AC System

Hi all,

I have a 2000 Chevy C3500.  I am having probelms with my AC.  I noticed the problem awhile back.  My compressor would not engage.  The AC system worked fine last summer and now that it is getting hot again I can not get it to come on.  I have already tested / done the following;
Jumpered relay to engage compressor - it does come on when I do this.
Add refrigerant to the system thinking it was the low pressure safety switch, this did not do anything and now I am afraid I have too much refrigerant and I may slug the compressor.  I have commercial / residential grade gauges but they do not fit on the auto style connections.  I need to get adaptors for this.
Checked the power across the relay coil - 12vdc and replaced the realy
checked the power at the cylinder (what I believe is the low pressure switch) 12vdc
checked the power at the compressor (connection on top) nothing unless I jumper the relay.
I tried to find a wiring diagram but I have not had any luck on the internet.

Any help you can offer will be great
Thanks