What was found
MKE - FLT 891 - ON CLIMB-OUT, THE PRESSURIZATION AUTO FAIL LIGHT ILLUMINATED. CAPT TOOK OVER FOR THE FIRST OFFICER, WHO WAS FLYING THE A/C, AND THE FIRST OFFICER WENT THROUGH THE QUICK REFERENCE HANDBOOK PROCEDURES. SWITCHED TO STANDBY SYSTEM AND CABIN WENT TO 10,000 FT WITH WARNING HORN SOUNDING. OXYGEN MASKS DROPPED WHEN A/C WAS APPROX AT 12,000 FT. SWITCHED TO MANUAL CONTROL AND RETURNED TO FIELD AT MKE. FLIGHT LANDED WITHOUT FURTHER INCIDENT. NO EMERG WAS DECLARED. MX REMOVED AND REPLACED THE CABIN PRESS CONTROLLER AND THE CABIN PRESSURE SELECTOR PANEL (P/N 711002-5). OPERATIONAL CHECK NORMAL. PRESSURIZED A/C. OPS CHECK GOOD. REMOVED AND REPLACED PAX OXYGEN BOTTLE AND REPACKED OXYGEN MASKS. (M)