89 300e keeps blowing the speed sensor out of A/C compressor
 

I have an 89 300e that I switched from R12 to R134 about 2 years ago. It ran great for a year then it blew the speed sensor out of the compressor. So I replaced the speed sensor this winter but didn't recharge the system until a week ago.
The compressor might have been damaged because the clutch will only engage for a few seconds at a time. I put in one can of freon thinking that in was low and that was the reason for the clutch not engaging.
I didn't want to overcharge the system so I left it alone until I could get some professional advice. Yesterday about 10 minutes after I had gotten home I heard this loud bang from inside the house. I ran outside to see what had happened and the speed sensor had once again blown right out of the back of the a/c compressor, along with all the oil and freon in the system.

can anyone tell me why the system is doing this?