You should be able to charge it to 40psi or so in order to get the clutch to engage. Connect up your gages and hook the freon can to your low pressure side. When you start filling and the pressure exceeds the low setting, the clutch will engage. Sometimes I have to rev the engine a bit to get it started, but it's usually not a problem. If you are completely filling the system, you need to put it on a vacuum for 30 minutes or so before filling or you won't get anywhere. | 
