I have a 1999 Durango and trying to recharge the a/c. The system is evacuated and shows -30psi. However, I'm not sure how to jump the compressor to keep it running so it will accept new coolant. Anything I'm missing please let me know. Thanks in advance.
SA

 

I'm pretty sure you have to use a machine to recover the pag oil/left over R134a,vacuum,check for leaks,then add oil/recharge it. If you don't have any leaks, most places will recharge it for $60. I always used a machine at work, I don't know how to do it without one. Maybe you can try sites like this... http://www.id-usa.com/how_to_recharging.asp

 

If you have 30" Hg. showing on the low side gauge you can start charging without any problems. The "-30 psi " you see on the gauge is actually 30" Hg. (30 inches of mercury not -30psi) The system is in a vacuum and will "suck in the charge. After you put the charging hose on be shure to hold the can of refrigerant upsidedown (charging hose connectionat can pointing down). And whatever you do do not attempt to run the compressor without any refrigerant in it because if you do the next step will be replacing the compressor. The reason you can't start the compressor at this point is because there is no refrigerant in it, whichin turn has activated the low pressure cut-out. The low pressure cut-out is a pressure switch that breaks the electical circuit when system pressure drops below a predetermined setpoint established by the manufacturer in order to protect the compressor. So go ahead and drop in a charge, but make shure to charge the system with the correct ammount and type of refrigerant, in this case R-134a is the correct type of refrigerant and the propper charge by weight can be found on a sticker located under the hood. P.S. if you have the system in a vacuum there is no refrigerant in it because it is in a vacuum. If you really want to change the compressor oil you'll need to remove the compressor from the vehicle and literally pour it out, reinstall it, put the system back into a vacuum, and charge it with the correct amount oil and refrigerant. R-134a systems use polyl-ester for the oil and esters are hygro-scopic (they have an affinity for moisture, attract it and hold it), so if this oil is exposed to open air it will take the water vapor from the air andhold it, if it gets too much moisture in it, it will be impossible to vacuum out the moisture. So make absolutily sure to pull the system into a deep vacuum before charging.