Found on a forum:
Ford Focus stalls when air conditioning is turned on

I have a friend of mine who yold me about a problem he is having with his Focus that he bought a while ago. 
The car has only about 5 thousand miles on it, but occasionally when he puts the air conditioning on, the
car dies, even at higway speeds. The car starts right back up when he pulls over and then runs fine. He put 
it in the shop for a couple of days and the dealership could find nothing wrong. The car has a manual
transmission.

Has anyone out there experienced this problem and how to resolve it? any help would be greatly appreciated.
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I would try one more time with your local dealer; make sure you ask for the chief mechanic, or the service 
department manager, or the dealership owner (although he'll likely give you to the service manager...).  If he
can't solve it, have him call "the Hotline", he'll know what that means. The Ford engineers at the other end 
will have many suggestions for your local guys, including, probably, something to do with the drag on the
serpentine belt caused by the clutching in of the AC compressor (which is what it sounded like to one of the
techs at the dealership I work for). There are other possibilities; AC could be overcharged or many others.
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Glad to have found a colleague who works for Ford.  Your comment is valid however, the Zetec engine is 
fitted with an autotensioner which should keep the drive belt under correct tension at all times.
Reprogramming the engine management module with WDS has fixed this problem for us in Europe.
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We have seen this problem on European Focus, and is easily solved by reprogramming the engine 
management module.  Your authorised Ford dealer will have the diagnostic equipment called WDS in order
to do this for you.

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