40 ILCS 5/20-125
If a retired employee returns to employment which is covered by a system from which he is receiving a proportional annuity under this Article, his proportional annuity from all participating systems shall be suspended during the period of re-employment, except that this suspension does not apply to any distributions payable under the self-managed plan established under Section 15-158.2 or under a defined contribution plan established under Article 2, 14, 15, or 16 of this Code.
The provisions of the Article under which such employment would be covered shall govern the determination of whether the employee has returned to employment, and if applicable the exemption of temporary employment or employment not exceeding a specified duration or frequency, for all participating systems from which the retired employee is receiving a proportional annuity under this Article, notwithstanding any contrary provisions in the other Articles governing such systems.
40 ILCS 5/20-125