Thanks to both of you for your answers.
According to my MB user's manual, when the ext. temp. grows rapidly, the indication stays on the last low measure. The user's manual doesn't says that the indication will disappear in that case.
But, if Steve is right, I hope that MB have think of this case. I'm still shure that an A/C sending suddenly VERY HOT air is not a normal situation on a so technicaly elaborated car. And I'm still shure also that there is a relation at least with the ext. temp. indicator and the A/C, because I repeated almost 20 times the failure, and EVERY TIME the A/C send hot air as soon as the indication disappeared.
If one of you can be of further help ...
Thanks by advance,
P.S. : My C240 is the french version, I'm living in Paris.