[200q20v] A/C Clutch Suppression, was Bose intermittent pops

Bernie Benz b.m.benz at prodigy.net
Fri Jun 15 17:01:05 EDT 2001

Nice going Steve,

I like your "If it got fixed somehow, don't break it!" corollary to my
"Don't fix it if it ain't broke" philosophy.  Thanks.

Nothing beats Fram Orange, excepting maybe Penski Yellow!  Great products!
One never need change oil.  Black is for rice burners.

From: Steve Crosbie <scrosbie at uslink.net>
Date: Fri, 15 Jun 2001 16:31:42 -0500
To: Bernie Benz <b.m.benz at prodigy.net>
Subject: Re: [200q20v] A/C Clutch Suppression, was Bose intermittent pops

 Thanks for the diode tutorial.  Not sure where the F12 went, but this may
be the first re-charge this car has had (not good medical records). Mechanic
checked for leaks, found none and assured me that some leakage is
unavoidable, especially when cold winter temps shrink o rings and fittings -
claims that some brand new DORF motors products need a recharge after their
the first winter.  Looks like Paul W. had the same experience with loss of
popping after re-charge (i got a deal - $12 dollars cheaper for AC
re-charge). For some reason the AC compressor would not cycle off in AUTO
mode last night (very hot, w/ hot engine) but would go off in ECON and
BI-LEVEL modes.  This morning with cold engine and temps (low 60's in MN
this AM) the AC compressor cycled nicely as I raised and lowered the temp
... and NO popping was heard. Is it possible that when charge is very low
that this system responds with the errant pulse, but not when fully charged
(other as yet undiscovered gremlins - possibly bad low pressure sensor)?
Acts as though the relay and diode are now working perfectly.  I did a code
check and got only one questionable code; for channel #12 I got 0 volts for
total system interruptions (should be between 5 and 9.5 volts), less than 5
volts is suppose to be an open low pressure switch.   Began diagnosics in
Bently on low press. switch test D8-260, but gave up when I didn't
understand the temperature chart when I attemped cooling perf. test D8-330
(referenced in 1st test), and don't have an AC pressure guage ... may just
be easier to just replace this sensor.
 My relay is #295 and I think I can tell which is the diode  .. a very small
resistor looking thingie that is slightly larger on the bottom end (only one
round fixture near the base).  2 more questions.  1. If the popping has
indeed stopped, do I stop now? ( have a feeling your going to tell me
something like ... if it got fixed, don't break it!).  2. Doesn't Fram suck
(literally .. pieces of itself into Audi powerplant)?  Humbly yours ...

Bernie Benz wrote:
First question: Wonder where the F12 went?  Did they find a leak and fix it,
or just recharge it?

Second question: Have you pulled the failure codes from the AC control head?

Refer to your 200 Bentley, pg 590. The clutch relay, -578F or H, marked
on top #295 or #306 respectively, is powered thru the AC thermostat sw,
which cycles the compressor re. evaporator temp, from the AC progranmmer.
As I recall, this relay control power is supplied in all modes excepting
Econ and Off.

AC clutch power is supplied from the X bat source thru the AC low ref.
sw. (closed if you have ref.) to term 2/30 of the AC relay.  If the relay
is energized the power exits the 8/87 contact to the clutch.  The supression
diode bridges the relay contacts, the annode from pin 8/87 to pin 2/30, the

If the compressor runs in Econ or Off the diode is most probably shorted.
If the diode has failed open, you should be hearing the pops when the relay
opens the clutch circuit.  If you determine that the problem is the diode,
it is  a round black, axial lead component on the PC board very near the
base (one of two) and easily traced to these two terminals.

If this doesn't work, switch to Fram.


From: Steve Crosbie <scrosbie at uslink.net> <mailto:scrosbie at uslink.net>
Date: Thu, 14 Jun 2001 23:02:39 -0500
To: Bernie Benz <b.m.benz at prodigy.net> <mailto:b.m.benz at prodigy.net> ,
"200q20v at audifans..com" <mailto:200q20v at audifans..com>
<200q20v at audifans.com> <mailto:200q20v at audifans.com>
Subject: Re: [200q20v] A/C Clutch Suppression, was Bose intermittent pops

  Just got my car back from the shop - it needed 2 lbs of freon (mucho
dinero) and the car now is ice cold.  The popping radio syndrome is now
gone. But the AC compressor appears to keep running if set on auto ....
regardless of input temp.  Put it all the way to HI and heat was coming out
of the vents but the compressor was still on.  Only will come off when econ
or bi-level buttons are pushed.  There is no popping sound when the AC
clutch disengages when either of these buttons is pushed.  Is this normal or
should the AC compressor cycle on and off in the auto setting?  Per your
comment the diode in the relay may be shorted.  If so I am game to try for
the cheaper fix. Got out suspected relay (#6 on the under dash label -
"control unit f. magnetic clut. Relay f.  heating (GB)") that is Audi part #
443 919 578 F.  I take it that it is updated to the same # but ending in H.
I'm not an electronics expert, and I can tell a resistor from a capacitor
(rated in uF's), but have no idea what the diode looks like in this relay.
The only two thingies left (except for the relay) are a yellow rectangle
(labeled 100n J63) and a smaller red rectangle (labeled 0.22,   60,  WIMA,
A6  all on different lines). Is one of these a/the diode? I have little
confidence that the local Radio Shack  help can help me -  I often know more
than them (very little).  Will just replacing this diode fix what ever the
zenner diode wiring harness is made to solve ($75.95 retail).  The down side
if I screw this up is nothing since it appears I have to replace the relay
anyway ($95.5 retail) and the upside is I can invest difference in 68 Mann
filters!  Thanks once again in advance.

Bernie Benz wrote:
We've been here before, Guys.  The inductive energy stored in the clutch
coil is suppressed by a diode in the A/C clutch relay.  If this diode is
shorted, BTDT, the compressor runs all of the time.  If the diode is open,
BTDT, this dissipating stored energy causes the voltage spike upon the relay
opening.  So, you can order and install the the Audi money making fix, or
just replace the diode in the A/C relay.  50V, 2A or better diode, $0.20
from radio shack.


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