As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FLIGHT 2152 MKE-BWI THE APU GENERATOR OVER SPEED AND AUTO SHUT DOWN. THE CREW DIVERTED TO GRR SINCE THE RIGHT ENGINES DC GENERATOR WAS ALREADY ON MEL. MECHANIC WAS DISPATCHED AND THEY REPLACED THE APU DC POWER CONTROL UNIT. MECHANIC OPS CHECK THE APU DC SYSTEM WITH NO FURTHER DEFECT NOTED AND RETURNED THE AIRCRAFT TO SERVICE.