I just went through the enjoyable process of replacing every single component of my AC system. After completion it held vacuum and was then charged at the local shop. My compressor started cycling off and on almost immediately and now only stays on for 2-3 seconds before shutting off. I hooked up a gauge set and found the pressure around 25 when it first kicks on. It quickly drops to 20 or less and then it kicks off. The air temps are only in the 50's so am I low on refirgerant or is it normal for the pressure to be low because of the lower air temps? I dont recall other vehicles Ive owned doing this so I have a bad feeling.

 

Sure sounds like you have a low charge. This causes the compressor to cycle quickly since the low side goes into suction, triggering the low pressure cutoff switch. I'm not sure what the spec is on the XJ's AC system but typically with R134 you want to see somewhere around 30-40 psi on the low side with the engine at 1500 rpm.

You may have a leaky fitting. (That's why I like to use Nylog on the O-rings, it cuts way down on sealing problems compared to using plain refrigerant oil on them.) Years ago when I installed a new condenser in an AMC Eagle that I had at the time I forgot to put the O-ring in one of the fittings! When I finally figured out what had happened I was surprised it had held vacuum and was just a slow leak once charged.

Easiest way to check for leaks is with an electronic leak detector if you have one available. (The shop that charged the system may have one.) Otherwise, if you have dye in the system you can check for leaks with a UV light.