We construct a new isomorphism between the endomorphism algebra of an induced cuspidal character sheaf and the group algebra of the relative Weyl group involved. We show it differs from Lusztig one by a linear character, and we relate this linear character to some stabilizers. Some consequences on characteristic functions of character sheaves are obtained. In the forthcoming part II, we will apply these methods to obtain a precision on Digne, Lehrer and Michel's theorem on Lusztig restriction of Gel'fand-Graev characters.