In a gearwheel mechanism for driving a secondary unit such as a fuel pump driven via the gear wheel mechanism by a primary unit, a driven gear mounted on the secondary unit is provided with a positioning element including sections covering teeth areas of the driven gear so that those sections cannot be engaged with a driving gear of the primary unit during mounting of the secondary unit, and in a pump having a gear provided with such a positioning element as wells as an internal combustion engine including a pump with a gear provided with a positioning element.