Evaporator temperature sensor adjustment
From Knowledgebase
This technique is cited for Type 85 cars, but may work for others.
Well, this is definitely one for the archives. Are you a 4000, Coupe GT or Urq owner who is frustrated by your A/C system? Does it take forever to get cold? Or never gets REALLY cold despite being in perfect working order? Well, there's the answer! Adjust your A/C temperature sensor! My CGT was converted to R134a. I have no idea how well it worked with R12 as the AC was dead when I got the car, but with R134a on a hot sunny day, it took at least 10 minutes for vent temps to get down to even 60*F, sometime longer. It would bottom out around 50*F after about 1/2 hour, and if you stopped the engine for more than a minute or two it would take for ever to get cold again. The system was in perfect order with proper pressures. I had just figured that this was the price for admission to the R134a crowd. Boy was I wrong, a quick adjustment of the AC temp sensor and it works like it should have all along. Now, on a bright sunny day, vent temps hit 56*F within 3 minutes, 50*F within 4, and it bottoms out at 40*F in city driving and 38*F on the highway in less than 10 minutes! So I figured I'd better pass this along with instructions (care of Todd Miller on AudiWorld):
The temp sensor is just to the right of the evaporator. So, remove the glove box to get to it. Remove the 14mm nut that holds it to the bracket and then CAREFULLY move the temp sensor and the capillary tube forward till you can see the top of it.
There is a small Phillips head screw on the top, this is the adjustment screw. Mark that screw and the body of the sensor for where they are at as a reference. Now turn it counter clock-wise to lower the temperature at which it kicks out the compressor. Use 1/4 turn increments and go for a drive after each one to make sure the vent temps don't go too low (causing evaporator to freeze. Use a calibrated thermometer in your air vents to accurately check the temperature.
Some people on Audiworld have reported that their sensor only needed 1/4-1/2 turn, but Dave's Coupe GT needed a full 2 turns, so proceed carefully, and make sure you measure the temperature of the evaporator accurately- vent temperature could vary greatly from the evaporator temperature.
Compressor damage
If you run your evaporator too cold, you will freeze the evaporator, particularly in high humidity. When this happens, the thermal expansion valve won't be able to reduce the refrigerant flow enough to ensure that all the refrigerant is evaporated within the evaporator. Liquid refrigerant will leave the evaporator and be sucked into the compressor. Liquids do not compress; when the compressor tries to compress the liquid refrigerant, it makes a "punching" sound. As such, pulling liquid refrigerant into your compressor is known as "slugging your compressor" and can cause serious damage. If you're trying this adjustment and hear the slugging noise, you should immediately turn your A/C off in order to disengage the compressor clutch. As is probably also obvious, you should back your temperature setting off a little and try again. You do not ever want a dash vent temperature lower than approximately 40-45F.
Content courtesy Dave, Chris Thorp, and Mark Chang
