90q20v heater valve kablooey

[kf21857 at comcast.net]

Hi, I am a former list member but haven't been subscribed in a while because my car hasn't broken in a long time.

My 90q20v with climate control has a vacuum actuated valve with a water temp sensor in the heater hose a few inches from where it enters the firewall.  Yesterday the two halves of the valve separated while I was stopped in traffic, causing the cooling system to empty itself rather quickly.  Fortunately I was able to find a parking spot, remove the two pieces from the heater hose, unload the mountain bike I had on the roof and cycle to a hardware store to find a suitable part to splice the hoses back together.

Here are my questions.  I am planning a parts run to the local Audi dealer to order a replacement today, but I have spent quite a bit of time searching for the vacuum hose that is supposed to control this valve and it simply isn't anywhere to be found.  I had noticed the valve before and wondered why there was no hose going to it but had concluded that there just wasn't supposed to be one.  The valve defaults to open position when there is no vacuum applied so the heater works OK.  Exactly what is the function of this valve and when is it supposed to be closed?  Could this be the reason why my A/C has never worked worth a darn?  And where is the hose supposed to originate?  The Bentley shows a schematic with all of the vacuum operated controls on the heater but I haven't found anything telling me where to find it.  I am guessing that it is under the dash somewhere.  Do I really need to replace this thing?  Is it vitally important?  It looks rather expensive and I have made a semi p
ermanent repair by clamping a pipe nipple in place of the valve.
thanks,
Dave