We show that given a general a priori unstable Hamiltonian $$\begin{aligned} \frac{1}{2} p^2 + V(q) + G(I) + \epsilon h(p, q, I, \varphi , t), \end{aligned}$$ 1 2 p 2 + V ( q ) + G ( I ) + ϵ h ( p , q , I , φ , t ) , where h is a generic Mañé analytic function and $$\epsilon $$ ϵ is small enough, there is an orbit for which the momentum I changes by any arbitrarily prescribed value. We call this phenomenon as global diffusion since the size of the change in I is independent of both $$\epsilon $$ ϵ and h. The fact that the pendulum and rotor variables are uncoupled is used essentially in our proof. The proof is based on simple and constructive geometrical methods, carefully studying the reduced Poincaré functions of the problem which generate the corresponding scattering maps.