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

Dale McCormack dale at themccormacks.org
Sat Jun 16 09:14:57 EDT 2001

Before you go too much further with your climate control head, if you're
getting cold air all the time, your cabin temperature sender may not
"know" it's as cold as it is.  This usually isn't a function of  a bad
sender, but rather of the small, dedicted-use fan which circulates air
over the temperature sender.  These units are located behind the dash,
to the left of the glove box.  

A simple test is to turn the ignition on, hold a lit match over the
sensor inlet (slotted 1.5" square on upper dash, right of center) and
watch to see if the flame is drawn toward the slot.  No movement (or fan
noise) no functional fan=bad fan motor or zero electricity to the motor,
probably the latter.  

This fan is accessed while sitting in the passenger seat without too
many contortions, but you have to loosen the allen-wrench machine screws
that affix the wood dash trim from passenger door to the ignition
switch.  A 1/4" socket w/extension worked well; still, be prepared for
hand cuts when working in the confined spaces behind the dash. 

In two different 100K + Type 44's I've found "climate discomfort" was
due to this little, but not cheap, item.  Be sure to test for voltage at
the motor as it usually doesn't go bad, but for some reason I've had to
get switch-controlled voltage from other sources to power it in lieu of
the original wiring.  

Good luck,

Dale McCormack > --__--__--
> Message: 3
> Date: Fri, 15 Jun 2001 17:13:07 -0500
> From: Steve Crosbie <scrosbie at uslink.net>
> To: "200q20v at audifans.com" <200q20v at audifans.com>
> Subject: [Fwd: Re: [200q20v] A/C Clutch Suppression, was Bose intermittent pops]
> <html><head></head><body>Bernie,<br>
>  &nbsp;&nbsp; Thanks for the diode tutorial.&nbsp; Not sure where the F12 went, but this
> may be the first re-charge this car has had (not good medical records).&nbsp;
> 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.&nbsp; Looks like Paul W. had the same experience with loss of&nbsp;
> popping after re-charge (i got a deal - $12 dollars cheaper for AC re-charge).&nbsp;
> 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.&nbsp; 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.&nbsp;
> Is it possible that when charge is very low that this system responds with
> the errant pulse, but not when fully charged&nbsp; (other as yet undiscovered
> gremlins - possibly bad low pressure sensor)?&nbsp; Acts as though the relay and
> diode are now working perfectly.&nbsp; 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. &nbsp; 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. <br>
>  &nbsp;&nbsp; My relay is #295 and I think I can tell which is the diode&nbsp; .. a very
> small resistor looking thingie that is slightly larger on the bottom end
> (only one round fixture near the base).&nbsp; 2 more questions.&nbsp; 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!).&nbsp; 2. Doesn't Fram suck
> (literally .. pieces of itself into Audi powerplant)?&nbsp; Humbly yours ...<br>
> Steve<br>
> <br>
> Bernie Benz wrote:<br>
> <blockquote type="cite" cite="mid:B74EEE41.6D9E%25b.m.benz at prodigy.net">
> First question: Wonder where the F12 went? &nbsp;Did they find a leak and fix it,<br>
> or just recharge it?<br><br>
> Second question: Have you pulled the failure codes from the AC control head?<br><br>
> Refer to your 200 Bentley, pg 590. The clutch relay, -578F or H, marked<br>
> on top #295 or #306 respectively, is powered thru the AC thermostat sw, <br>
> which cycles the compressor re. evaporator temp, from the AC progranmmer.<br>
> As I recall, this relay control power is supplied in all modes excepting<br>
> Econ and Off.<br><br>
> AC clutch power is supplied from the X bat source thru the AC low ref.<br>
> sw. (closed if you have ref.) to term 2/30 of the AC relay. &nbsp;If the relay<br>
> is energized the power exits the 8/87 contact to the clutch. &nbsp;The supression<br>
> diode bridges the relay contacts, the annode from pin 8/87 to pin 2/30, the<br>
> cathode.<br><br>
> If the compressor runs in Econ or Off the diode is most probably shorted. &nbsp;<br>
> If the diode has failed open, you should be hearing the pops when the relay<br>
> opens the clutch circuit. &nbsp;If you determine that the problem is the diode,<br>
> it is &nbsp;a round black, axial lead component on the PC board very near the <br>
> base (one of two) and easily traced to these two terminals. &nbsp;<br><br>
> If this doesn't work, switch to Fram.<br><br>

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