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

Bernie Benz b.m.benz at prodigy.net
Thu Jun 14 23:42:33 EDT 2001

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>
Date: Thu, 14 Jun 2001 23:02:39 -0500
To: Bernie Benz <b.m.benz at prodigy.net>, "200q20v at audifans.com"
<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.


From: Kneale Brownson <knotnook at traverse.com> <mailto:knotnook at traverse.com>
Date: Thu, 14 Jun 2001 08:45:31 -0400
To: "TM" <t44tq at mindspring.com> <mailto:t44tq at mindspring.com> , "Steve
Crosbie" <scrosbie at uslink.net> <mailto:scrosbie at uslink.net> ,
<200q20v at audifans.com> <mailto:200q20v at audifans.com>
Subject: RE: [200q20v] Bose intermittent pops

At 10:12 PM 06/13/2001 -0400, TM wrote:

I can't remember exactly what the problem was, but there was a TSB issued by
regarding a voltage spike when the A/C compressor turned on, which
apparently caused
a pop in the radio.

Maybe you could find it by searching the archives.


It's really important to fix this problem because every time you hear the
pop in the speaker, you're also giving the ECU the equivalent of a brain
electroshock treatment.   After a while, you start getting a violent jerk
in the engine. The info is on Scott Mockry's site at:

The harness installs easily into the wiring to the A/C magnetic clutch by
separating a connection,  plugging in the extra wire and grounding the
diode.  The relay replaces the A/C clutch relay in the panel below the
steering wheel.  I got mine last fall from Carlsen for a little over $100,
I think.  The harness and replacement relay  are dealer-only according to
the nondealer vendors I contacted last fall.

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