Do you have 12V to the clutch when the pressure switch is jumpered? If so, then you have a clutch issue. Check the air gap and adjust as needed. If that fails, replace the clutch (or compressor).
More details can be found here: http://www.sanden.com/originals/imag...uide_Rev.2.pdf
1992 XJ 2" lift -- RIP
2000 XJ 3" lift -- Australian Ford Big Brakes up front, rock rails, ZJ rear discs (with WORKING parking brake), C4x4 rear bumper/tire carrier, Hella H4 headlights, IPF driving lights, OEM skid plates, OME springs/shocks, JKS LCA's, JKS Sway Bar links.