I have a 1993 volvo 850 glt, the problem is when I turn on the A/C the compressor was just turning on and off simultaneously. I dont know what's the problem.

 

Could be the clutch. Do a search for adding some shims for a quick test/repair.

 

If the freon is low, it will do this to safe guard the compression. Try adding some freon to see if it is still giving you issues.

 

First thing would be to have your Freon level checked. An Ac shop can do this for you or an auto parts business can sell you an inexpensive freon pressure test/ thermometer kit and you can check the pressure itsself. You can also add more freon if needed. Note: In adding freo, you should keep the engine on fast idle and the can of freon warm, such as keeping it in warm water. The high idle speed increases the vacuum on the compressor inlet line, and the warm water increases the pressure in the can of freon, making it easier to put the freon in. A simple way to determine if the compressor clutch needs shimming is to set your AC on lowest temperature and< using a broom stick and a light( be careful of fan blade) apply a little pressure to the clutch face. If the compressor stays engaged after you remove the stick and cooling takes place, your clutck needs shimming; however, clutchs that need shimming don't often cycle off and on constantly. Your compressor inlet( larger line to compressor) should feel cool if your compressor is engaged. If not, you likely need freon. Note: There is a simple way to shim the compressor without having to remove it. Working from the bottom side of engine, one can use a screwdriver to spread the clutch plate. Crazy glue a 30/1000 inch thick shim under each tab. This puts the clutch plate only a few thousands of an inch -say 11- from the drive plate and allows magnetic force to pull the clutch in and engage the compressor. A 1 hour job at most, Done mine and it has worked fine for over a year.