Your best bet would be to look on the Pelican parts site.
They have both parts diagrams so you can locate the valve, and also a
forum to post your question.
http://www.pelicanparts.com/bmw/catalog/shopcart/BE30/POR_BE30_VENTac_pg3.htm#item15
http://www.pelicanparts.com/bmw/techarticles/101-Projects-97-AC/101-Projects-97-AC.htm
http://www.pelicanparts.com/bmw/techarticles/JF-Tech/AC_Recharging.htm
http://forums.pelicanparts.com/101-projects-discussion-forum-bmw-3-series-e30-e36-1982-2000/

The E30 BMWs are great cars, and if you had an 325is you would
definitely have something worth hanging onto.
However, they get very expensive to fix if maintenance is ignored or you
have to depend on the dealer.

Here is a write up on changing the expansion valve:

http://forums.bimmerforums.com/forum/showthread.php?t=921770

Todd (Used to have a '93 E36 325I)

On 7/1/2011 8:15 PM, John Rodgers wrote:
>  My buddy here has a 1990 325i BMW with an A/C problem. When the system
> is charged and turned on, it runs for 4 or 5 minutes and can be felt
> blowing cold air. Then there is a loud click-click up under the dash and
> the air stops being cooled. I suspect this is the expansion valve acting
> up, but don't know for sure. I have been told that is where the valve is
> located, and if so - it appears to be a beast to get at - like the
> heater motor on the Vanagon - the dash has to come out.
>
> Can anyone confirm this? or tell me what they think of the situation and
> what steps he might take to remedy the problem?
>
> Thanks,
>
> John
>
> --
> John Rodgers
> Clayartist and Moldmaker
> 88'GL VW Bus Driver
> Chelsea, AL
> Http://www.moldhaus.com
>