4000 A/C idle boost valve installed

[Ti Kan]

Hi all,

This is the final report on my 4000 A/C project.  It is now finished.  I
plumbed in the idle boost valve and wired the valve to the compressor
clutch circuit.

My '80 4000 didn't have an idle boost valve for the A/C.  In fact the
only post-warmup idle control mechanism is a electronic gizmo wired
inline with the electronic ignition control module.  When the idle dips
below the 940rpm spec, it advances the ignition timing to increase the
idle speed.  For the most part it works well when the car is tuned
correctly and the idle control device doesn't go crazy trying to
compensate.  However with the A/C on, increased load from the A/C
compressor, as well as the alternator (additional electrical power is
being consumed by the radiator fan, A/C compressor clutch, interior HVAC
blower, etc.) causes the idle control device to go overtime.

While the ignition timing advance based idle control works, it does have
its peculiarities.  When I get going from a stop with A/C on, the
ignition advance provided by the idle control device suddenly goes away
as the engine revs up beyond idle, and it feels a bit discontinuous.  On
a manual trans car like mine, that is just at the moment when I am
clutching in.  As a result the engine tends to jerk a bit, and with my
cable operated clutch this sometimes causes the clutch takeup to be less
smooth from the engine motion (it sometimes even goes into a brief and
annoying oscillation).

Also, the A/C compressor on the 4000 is designed to cycle on and off
based on the temperature slider control position in conjunction with a
capillary tube that measures the evaporator's temperature.  The
compressor's load on the engine is present at all rpms, not just at
idle.  The timing advance idle control doesn't do anything to correct
for that at any rpm above idle, so while driving the car, the compressor
kicking in and out is noticeable.

The idle boost valve is a little electrically operated solenoid air
bypass valve and is plumbed around the throttle plates.  With A/C off
it's closed and no air flows through the valve.  When the compressor
kicks in, the valve opens and let additional air around the throttle,
increasing the engine speed slightly to compensate for the load.  This
device is standard on 82+ CIS-equipped Audis, and its function was
supplanted by the more sophisticated idle stabilizer valve found on the
CIS-E cars as well as the type-44 turbos.

It seemed like it should be a very easy and straightforward job to
install an idle boost valve, but several minor snags caused a lengthy
delay.  First, I needed a couple of T-pieces to plumb in the extra hoses
to the valve.  Some of you have read my query on this list for those
pieces and thanks to Avi Meron and Huw Powell who sent me stuff from
their collection, I was able to accomplish that.  Upon cutting the stock
air hoses where I was to install the T-pieces, I discovered that the
original rubber hoses were hard and brittle from age and basically fell
apart.  So, I decided to replace *all* the hoses in the intake/vacuum
system.  Also, I wanted to make the extra wiring from the compressor
clutch circuit to the valve look as professional as possible, so that
means wrapping the entire length of it in a loom and cable tied nicely. 
The wire has to go under various components in the engine compartment,
following existing wiring, so I wound up removing a bunch of stuff to
gain access.  All told, it took me more than 3 days of work to get the
idle boost valve installed and looking good.

The result?  Most satisfatory!  Now, there is no perceptible change in
engine speed with the A/C on or off.  The idle boost valve compensated
just about perfectly.  The jerkiness on takeoff is gone, and the whole
driving experience just seems smoother when the A/C is running.  I'm
sure the car doesn't accelerate quite as quick with A/C on but the
previous unpleasant anomalies with the driving characteristic is all
gone.  This is really quite amazing, how a little extra rpm compensation
could change the feel of the car.  I checked the ignition timing at idle
with the idle boost valve electrical plug in and unplugged and there is
most definitely a difference.  Without the valve, you can see the
ignition timing jump deep into advance when the compressor comes on.
With the valve enabled, there is only the occasional blip of advance,
and there is virtually no difference with or without the compressor
running.  Exactly how it should be.

The new rubber hoses probably fixed some minute vacuum leak too.  The
car idles more smoothly than before.

This concludes my A/C project, and everything is working great.  To
recap, I did a full R134a conversion by doing the following:

- Totally evacuated the system of R12 and old mineral oil.
- Dumped the original Mini-York compressor, installed a new Sanden SD-5
  R134a compressor using an adapter bracket.  New compressor comes
  pre-filled with PAG oil.
- Changed to a new, slightly longer drive belt due to change in engine
  pulley to compressor pulley distance.
- Replaced receiver/drier and expansion valve with new ones, both are
  R134a friendly.
- Had new compressor hoses custom made using barrier hose stock, with
  special 90-degree angle fittings with service ports.
- Changed to R134a-compatible green nitrile o-rings.
- Had the system professionally recharged with R134a.
- Upgraded to the dual radiator fan setup of the later year 4000s.
- Installed idle boost valve.

The A/C in my 4000 probably now works better than it ever did.  The
Sanden compressor is quiet and smooth, the dual radiator fans are also
quiet, the idle air valve makes the driving as smooth with A/C on as it
is without, and I get nice cold air in the car.  Perhaps not nearly as
good as the system on my S4, but considering that the 4000 is now 22
years old it's not bad at all!

Sorry for the lengthy writeup, but I am a happy camper.

-Ti
01 S4 2.7 biturbo quattro
84 5000S 2.1 turbo
80 4000 2.0
-- 
    ///  Ti Kan                Vorsprung durch Technik
   ///   AMB Research Laboratories, Sunnyvale, CA. USA
  ///    ti at amb.org
 //////  http://www.amb.org/ti/
///