[V8] Missfire with A/C on

[dsaad at icehouse.net]

There are two things you can do for cheap that will probably fix your problem 
if it is indeed caused by the A/C clutch cycling. (A common problem btw)
All you need to do is place a (fast) diode across the clutch terminals, reverse 
biased or anode to ground, cathode to +12v terminal. (could this be 
the "harness" that Audi retrofitted?) This will supress any negative 
transients. If that does not solve it then place another diode inline with the 
12v terminal - cathode to the clutch terminal, anode to the +12v wire. This 
diode must have enough forward current capacity to carry the full load of the 
clutch coil + a generous reserve. A diode from an alternator would work fine 
here. This will supress positive transients larger than battery voltage + the 
drop across the diode.

If you wanted to get really fancy, there are power transistors specifically  
designed for this application. This is probably what Audi used in their revised 
A/C relay.

What is happening here is your clutch is working just like an ignition coil 
with breaker points. When the clutch turns off there is a large ammount of 
energy dumped back into the electrical system. This is what creates the spark 
energy in an ignition coil. So the fix is to suppress that energy with diodes 
and in severe cases use a "slow" transistor switch that prevents the buildup in 
the first place by switching the coil off slowly.

Dave


Quoting Kneale Brownson <knotnook at traverse.com>:

> On my 200q20v I had the same issue that was the subject of an Audi
> recall
> fix way back when.   Seems the clutch in the A/C compressor kicks up a
> jolt
> of power that gives the ECU  a shock.  Another symptom common to this
> problem is a loud pop heard on the radio when the jolt occurs.  The fix
> was
> to replace the A/C relay and install a little wiring harness insert into
> the wiring to the compressor clutch.  There's discussion of this issue
> on
> Scott Mockry's website:  http://www.sjmautotechnik.com/elec.html#click
> That little harness and relay were pretty pricey from the dealer.  Scott
> may have a better price.
> 
> At 12:23 PM 2/19/2004 +1000, greg.moyle at mainroads.qld.gov.au wrote:
> >
> >
> >
> >
> >Help,
> >My 90 V8Q auto has developed a misfire or more a total cut-out but only
> >when the A/C is running. It seems to only be a problem above 2000rpm so
> >cruising around town isn't to much of a problem. When highway cruising
> the
> >motor seems to cut out momentarily every 40 seconds or so but sometimes
> >will run for a couple of minutes before doing so. The cut-out is so bad
> >sometimes the box will drop into limp mode but a restart will fix that.
> >Without the A/C on no problems even under heavy acceleration. This
> weekend
> >we have 41 degrees Celsius so the Black Audi will be staying home I
> think,
> >not much fun in a black car in that heat with no air.
> >
> >Any advice would be greatly appreciated.
> >
> >Cheers
> >Downunder Greg
> >
> >
> >
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