Mark, The expansion valve meters the refrigerant from the high pressure side (condenser) to the low pressure side (evaporator). When the Freon goes from a high pressure state to a low pressure state, it expands or vaporizes, and gets cold and absorbs heat from the surrounding area. When the exp valve gets clogged or freezes up (from moisture in the system) it doesn't work. Also, there may be an incompatibility problem with Freon134a in some systems, When a compressor fails, metal particles tend to get caught in the exp valve, and throughout the system. By replacing the valve you have a higher rate of success with the compressor changeout. Understand that if your compressor failed, it probably sent contaminants throughout the system. You must flush all this junk out in order to be successful. I have successfully flushed several systems with plain old mineral spirits that you can buy for $3.00/gal at Home Depot. Note this is just for Freon 12, not 134a, it uses a different oil type (ester) and may not be compatible. You must remove the compressor, receiver/dryer and expansion valve out of the system before you flush through. I use heater hoses clamped onto the lines with a funnel. Pour the mineral spirits into the funnel, blow compressed air through and catch it with a bucket on the other end. repeat until it blows clean. You will be surprised what comes out! After you are satisfied the line is clean, blow pop it (finger on end and quickly release after pressure applied) to get as much mineral spirits out as you can. If you reuse the exp valve, make sure it is clean, reassemble everything, but keep in mind you must re-add oil to the system, this can be done by pouring the measured amount into the system before you connect the hoses, or suck it in the low side after you pull a vacuum. Pull a good vacuum, make sure it holds, and charge. Bob J (Went to tech school for refrigeration in the '70s) _____ From: Chrysler300@xxxxxxxxxxxxxxx [mailto:Chrysler300@xxxxxxxxxxxxxxx] On Behalf Of MRS954@xxxxxxx Sent: Monday, February 26, 2007 5:18 AM To: Chrysler300@xxxxxxxxxxxxxxx Subject: [Chrysler300] AC Compressor Hi Group, What exactly is the function of the expansion valve in the AC system, and why is it recommended to be changed when I replace my AC compressor? Thanks. Regards, Mark Souders 300H Mohrsville, PA __________________________________________________________ Check out the new AOL. Most comprehensive set of free safety and security tools, free access to millions of high-quality videos from across the web, free AOL Mail and more. [Non-text portions of this message have been removed] [Non-text portions of this message have been removed] ------------------------ Yahoo! Groups Sponsor --------------------~--> Yahoo! Groups gets a make over. See the new email design. http://us.click.yahoo.com/hOt0.A/lOaOAA/yQLSAA/8LmulB/TM --------------------------------------------------------------------~-> To send a message to this group, send an email to: Chrysler300@xxxxxxxxxxxxxxx For list server instructions, go to http://www.chrysler300club.com/yahoolist/inst.htm For archives go to http://www.forwardlook.net/300-archive/ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Chrysler300/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/Chrysler300/join (Yahoo! ID required) <*> To change settings via email: mailto:Chrysler300-digest@xxxxxxxxxxxxxxx mailto:Chrysler300-fullfeatured@xxxxxxxxxxxxxxx <*> To unsubscribe from this group, send an email to: Chrysler300-unsubscribe@xxxxxxxxxxxxxxx <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/