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

Paul Waterloo pwaterloo at compuserve.com
Fri Jun 15 20:43:40 EDT 2001


I don't know anything about A/C (or that much) but I did note in the Bently that when charging the system the low pressure switch must be disconnected and jumped shut. I would start by making sure the switch was plugged back in, if they disconnected it at all, it's the one by the fender on the passenger side.

Paul Waterloo
Applied Energy Services
Phone 708-524-9464
Fax 708-524-0079
Cell 312-961-2523
pwaterloo at compuserve.com

  ----- Original Message ----- 
  From: Steve Crosbie 
  To: 200q20v at audifans.com 
  Sent: Friday, June 15, 2001 5:13 PM
  Subject: [Fwd: 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>
      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.


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