What's this for
 

It's called an A/C Orifice tube

http://198.208.187.182/internet/Part...artnbr=15-5758

RE: What's this for
 

It's kind of like a valve to keep the flow consistent.

RE: What's this for
 

The orifice tube is used to help spread apart the refrigerant molecules thus cooling them down enough to be used in the evaporator core. This part is cheap and is highly suggested to be replaced any time a part is replaced on the AC system. If this part gets clogged with particles, your AC won't work correctly. If you don't have one installed at all, the AC won't cool. I just replaced the evep core, accumulator, and orifice tube on mine recently. I am not that experienced with AC systems but this is what I've recently learned.

RE: What's this for
 

It's a tube to keep your orifice cool.

RE: What's this for
 

is it easy to replace or do you gotta dig down in there to replace it? It's cheap and i'm tryin to get a few cheapy cheap things off rockauto all at once, so far it's just a tstat and the gasket, but this thing was like 2 bucks lol.

RE: What's this for
 

It's not hard at all to get to. It is at the base of the radiator where the AC line comes out of it and goes vertical. You will need a set of good small needle nose pliers or something to reach down in the tube to grab it. Try twisting the orifice tube back and forth while pulling straight up on it. The new one will fit snuggly back in place of the old one. Also, be sure to replace the rubber seals on the external Aluminum tube tube or it will almost be guaranteed to leak. These seals are cheap to buy, but since the tube more than likely hasn't been pulled apart since it was originally installed, the seals will be mis-shapen and will no longer give a good seal. Also, if you need to buy the tool for disconnecting the tubing, get the plastic assortment kit. It should be around $10 and have several sizes of tools. The diecast metal all-in-one tool didn't work for crap for me, so I had to make an additional trip for the plastic set.

RE: What's this for
 

I have a '98 Ram and my orifice tube is in a slightly different location. If you will look at your condenser, the radiator looking thing in front of your radiator, there is a high pressure line that comes out of the bottom of it and runs along the upper side of your passneger side fender and goes into the firewall, connecting to the evaporator. Basically your orifice tube creates a restriction on the high pressure side and releases the gas into the evaporator creating a low pressure gas which has the cooling effect via heat absorbtion. Anyway, you can see the orfice tube about midway down the line on the fender. You can recognize it two ways: 1) there are two small indentions where the orifice tube is, be aware that you may have to remove some insulation to see the indentions. The indentions are what hold the tube in place within the line. 2) You can feel a dramatic temperature difference in the line where the orifice tube is. As the HI temp HI pressure gas passes through the tube is cools dramaticallly. The A/C must be running of course. Also keep in mind that the system MUST be evacuated prior to disconnecting the lines as the tube is in a high pressure line. You will also need a tool to get the tube out and a five dollar fuel line/A/C disconnect tool. When you disconnect the line it will come apart at the firewall and the exit of the condenser. The orifice tube is mid way up the aluminum tube. Most A/C place have the tool and should let you borrow it. You may want to replace the O-rings as well on the connections. I would not replace it unless you have put in some stop leak of something that may have clogged it. Otherwise its not really worth fooling with.