A/C Problems? Clogged Orifice Tube?

[Scott Mockry <scottmo@teleport.com>]

With the rash of hot weather in certain parts of the US, (unfortunately not
here in Oregon) I thought I would add one item to the Check List when you
have your Audi A/C system serviced.

The Audi A/C system is derived or was designed by Harrison of General
Motors fame, and uses the simple Orifice Tube/Screen or a basic restrictor
tube to meter liquid freon into the evaporator. This was used instead of
the more conventional expansion valve or other POA (Pilot Operated
Absolute) valves that regulate evaporator pressure.

The Audi and GM system is called CCOT (Cycling Clutch Orifice Tube) as this
system operates simply by just spraying liquid freon into the evaporator
tubes, where it expands, turns into a gas, absorbs heat etc. The A/C
compressor is cycled on and off as necessary by the thermostat that
measures the evaporator fin temperature to avoid freezing up the evaporator
coils.

One often overlooked item during A/C service is replacing this simple
Orifice Tube/screen assembly as the screen portion of this tube can get
clogged up with crud, especially if the compressor has failed and metal
gets propelled towards the evaporator. I ran across this recently on a 1989
200TQ that has had "3" A/C compressors installed previously, but was not
cooling despite being full of freon. The Orifice Tube was clogged up
completely with fine metal particles.

On the 1986-88 5000S/T/Q and the 1989-91 100/200T/Q vehicles, the Orifice
Tube is located at the inlet pipe on the evaporator assembly, underneath
the plastic cowling tray in the engine compartment. If the A/C system is
discharged, you can loosen and remove the high pressure line connection at
the evaporator and with a pair of needle nose pliers, pull out the Orifice
Tube/Screen and inspect it for crud.

The 88-91 80 and 90 Audis also used this Orifice Tube, which apparently is
accessible by removing a tray underneath the battery, to gain access to the
evaporator inlet pipe.

The same Orifice Tube was used by GM, so a replacement Orifice tube for a
mid 80's Cadillac will work fine.

If your compressor has failed, and shrapnel or other metal has made it way
from the compressor to the Orifice Tube, you may need to remove the
condenser, muffler and high side lines to replace or flush them out, as
additional metal will likely make its way to the Orifice tube at a later
date. Back in the days when Freon R12 was readily available, there also was
a freon based solvent that could be used to flush out A/C lines, condensers
etc, but during these times, I don't know what is being used to flush out
A/C systems.

HTH
Scott (too cool) Mockry