I own a 98 Navi, and the previous owner swapped the motor for a lower milage one (67k). I added some r134a to it this week from Advance Auto, but it still blows warm air. While adding the 134, my compressor clutch would engage/disengage repeatedly. as its doing this, the pressure meter on the can goes from 20psi to 0 psi, and back and forth. Is the compressor gone bad, or do I need to keep adding 134 to the system until it stops this? I don't want to destroy anything, so I chose to stop adding anything until I found out from somebody that has some insight on this.
Thanks for any help you guys can give...

 

Go read the HVAC FQs, link is in my signature below.

 

You're still low on refrigerant. However, there could be a leak and you could just be wasting money by adding another can. You could take the chance and add the can, or you'll have to bring it to a shop to be tested.

 

I had this happen to me six months after I bought my EB Expy. It ended up being the AC condenser. It cost about 500 to repair but has worked great ever since.

Usually when the compressor kicks on/off like that it's because pressure builds up enough to shut off the compressor but then drops too fast and the compressor kicks back on. Odds are you have a leak somewhere in your system.
The best bet is to have a trained mechanic work on it as the AC system requires special tools and expertise.