40 ILCS 5/8-194
A vacancy in the membership of the board shall be filled as follows:
If the vacancy is that of an ex-officio member, the mayor of the city shall appoint a person to serve until a person qualified as hereinbefore described shall assume the duties of member. If the vacancy is that of an elective office the remaining elective members of the board shall appoint a successor from among the employees who shall serve until an employee is elected and qualified for the remainder of the unexpired term. The employee shall be elected at a special election to be held concurrently with and in the same manner as the next regular election for an employee member.
Any elective member who leaves the service of the employer or becomes a member of any other annuity and benefit fund, or any pension fund, shall automatically cease to be a member of the board.
40 ILCS 5/8-194