What was found
IN FLIGHT DURING CLIMB CHECK WITH APU RUNNING, F/O SHUT DOWN APU AND HEARD A DISRUPTION IN POWER AND GOT AN AC SERV BUS CAUTION FOLLOWED BY L/R WINDOW HEAT AND FDR2 FAIL/AC UTIL 1+2 STATUS MESSAGES ON, RETURNED TO DTW, STARTED APU. WHEN APU CAME ONLINE. FOUND NR 2 GENERATOR INOP. PER AMM 24-51-00 THESE ELECTRICAL BUS MESSAGES ARE NORMAL WITH ONLY GEN 1 ON. IDG 1 OP CHECKS GOOD IAW AMM 24-11-00. UTILITY BUSES OP CHECK GOOD IAW AMM 24-51-23. SERVICE BUS OP CHECKS GOOD IAW AMM 24-51-25.