There are a couple of other things you can look at, the first being your low pressure switch. As I mentioned these switches are designed to disengage the compressor during a "low pressure" situation, however, like any thing else they've been known to fail, an easy way to check for this is to eliminate the switch from the circuit entirely by jumping it out. The second thing I would check for is a possible blockage, you can do this by recovering the charge and pulling a good vacuum on the system then replacing the filter/dryer and recharging. I do not know what motor you have so it is difficult to make a positive diagnosis for your particular problem but it sounds like either there is a low pressure switch malfunction or a low system charge. Give the low pressure switch jump a try and see what happens if the comp runs then you know that you must replace the switch, I've also seen temperature switches give false readings as well, unfortunately there a lot of variables here so it comes down to the process of elimination. | 
