[s-cars] Recharging the a/c system-94 UrS4

[Tom Green]

To start you need to access the climate control diagnostics either directly on the climate control head or using VCDS to check for any fault codes and correct any problems there before attacking the refrigeration problem.  With the air distribution working correctly, you can turn to mastering the OBD channel 52 display for information on the a/c system operating status.  It will not only tell you if the compressor is operating, but why not if it is not operating.

https://12v.org/urs/Climate%20Control%20Channels%20&%20Codes.pdf

You probably already know if the a/c compressor clutch engages when you select a desired cooling temperature on the climate control head.  If so, don’t keep the information secret-share it.  If it does, that means there is still sufficient refrigerant in the system to create enough pressure to activate the low pressure switch on the line to the a/c evaporator coil in the passenger side plenum.  

If you have a monkey wrench to throw into this discussion, like a RHD or South African car, say so now before we waste any more time.

That low pressure switch is the key to any DIY work.  It is easily found by the electrical wires leading to this grey plastic part screwed on the aluminum tubing fitting.  You can remove the switch anytime.   A schrader valve underneath should prevent any refrigerant from escaping.  If it does leak, tighten the switch back in place. If the schrader does its job, this fitting will screw on in place of the pressure switch.  It has a quick connect fitting that the 134a top-kits fit.  You can then use a stiff piece of wire to short the contacts on the electric cap for the low pressure switch so the compressor will run when you start the engine and select a/c. This will allow the refrigerant from the container to flow into the fitting, flashing into a gas in the low pressure.  These kits have a gauge on the cylinder and instructions for determining when the system has enough charge.  

 https://www.amazon.com/Mastercool-MSC82272-Straight-Charging-Adapter/dp/B00S043QJK/ref=sr_1_36?hvadid=78546425032851&hvbmt=bb&hvdev=c&hvqmt=b&keywords=r134a+quick+connect&qid=1562120996&s=gateway&sr=8-36

This method does nothing to remove the moisture that has entered the system wherever it is leaking.  The moisture combines with the PAG 46 refrigerant to form a gray sludge that clogs the expansion tube and renders the desiccant in the filter/dryer useless for more filtering.  Both of these parts need to be changed in a leaking system, and the system checked for leaks, probably meaning replacing all the o-rings at connections and evacuating the system with a vacuum pump.  Gauge sets and vacuum pumps are available for no cost loan at most local auto parts suppliers.  I recommend you don’t use these since they will be dirty and probably used for R12, resulting in more clean up than use time.  Current auto a/c shops use much more efficient vacuum pumps to better evacuate the system in a lot less time.

You can do a lot to assemble the correct replacement parts and do the troubleshooting, but the actual refrigerant replacement should be done by a professional shop with a modern service machine.  They also use an electronic hydrocarbon detector to find any refrigerant leaks.  The money spent having one of these shops do the work will keep you from having to do this job again next year. You need to do all the work on the climate control system though.  You don’t want to spend your money financing a never-ending look into this system they know nothing about, don’t have any part, and can’t fix it in the end.  This applies to any Audi dealership service department as well.  If you can’t learn to fix it, get rid of this vehicle.