  The dynamical Jahn--Teller effect on fullerene sites in insulating
Cs$_3$C$_{60}$ is investigated fully ab initio. The vibronic excitations of
rotational type are at $\ge$65 cm$^{-1}$ while the net kinetic contribution to
the Jahn--Teller stabilization energy constitutes ca 90 meV. This means that no
localization of distortions by intermolecular interactions is possible in these
fullerides, therefore, free rotations of deformations take place independently
on each C$_{60}$. The latter destroy the orbital ordering and establish a
conventional exchange interaction between $S=1/2$ on fullerene sites. The
corresponding exchange model is derived and predicts the N\'{e}el temperature
for A15 Cs$_3$C$_{60}$ close to experiment.
