A B737-700 FLC EXPERIENCES A PAX OXYGEN DEPLOYMENT; A DIVERSION AND AN OVERWT LNDG AT BWI; MD.

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ACFT PAX OXYGEN DEPLOYMENT; DIVERT; OVERWT LNDG. AFTER 27 MIN SECURITY DELAY; ACFT PUSHED NORMALLY AND WAS CONFIGURED FOR A BLEEDS OFF MORE PWR TKOF FROM PVD RWY 23L ENRTE PHX. TKOF WAS NORMAL. CAPT FLYING AND PASSING 205 KIAS. JUST AFTER FLAP RETRACTION; STICK SHAKER ACTIVATED WITH MAX PWR STILL SET AND APPROX 9 DEGS NOSE-UP. BACK PRESSURE WAS REDUCED AND STICK SHAKER CEASED PASSING 210 KIAS. PASSING 4000 FT; CAPT ENGAGED AUTOPLT 'A' AND BEGAN USING VERT INDICATOR AND HDG MODES. CAPT'S AIRSPD INDICATOR BEGAN FLASHING 'SPD LIMIT' AND 'MASI' RINGS BEGAN FLASHING. 'B' SIDE AUTOPLT WAS ENGAGED AND APPEARED TO FUNCTION NORMALLY. WE DISCUSSED POTENTIAL OF ADA VANE STUCK OR REVERSED SINCE CAPT'S AIRSPD INDICATOR AGREED WITH FO'S AND STANDBY INDICATOR. PASSING FL240 IN STEEP CLB FOR TFC; HORN SIMILAR TO AUTOPLT DISENGAGE HORN SOUNDED. SUSPECTING MORE AUTOPLT PROBS; WE DISCONNECTED AUTOPLT AND DEPRESSED HORN SILENCE BUTTON. HORN CONTINUED UNABATED. NO MASTER CAUTION OR OTHER WARNING LIGHTS WERE OBSERVED AT THIS TIME. CLBING THROUGH FL280; 'MASTER CAUTION;' 'OVERHEAD' AND 'PAX OXYGEN ON' LIGHT ILLUMINATED. UPON ILLUMINATION; WE IMMEDIATELY DONNED OXYGEN AND RECONFIGURED TO NORMAL BLEED. CABIN PRESSURE ALT IMMEDIATELY BEGAN TO DECREASE FROM ABOUT 14000 FT. WE THEN STARTED AN IMMEDIATE DSCNT TO FL180 AND THE HORN FINALLY SILENCED AT A CABIN ALT OF ABOUT 9000 FT. DISPATCH AND MAINT WERE CONSULTED AND WE DIVERTED TO BWI FOR AN UNEVENTFUL OVERWT LNDG WITH MINIMUM BRAKING ON RWY 28. SUPPLEMENTAL INFO FROM ACN 556772: PASSING 4000 FT; I ENGAGED 'A' AUTOPLT AND SELECTED VERT SPD AND LEVEL CHANGE MODE. WOULD NOT STAY ENGAGED. ZNY ASKED US TO EXPEDITE CLB AND ABOUT FL240 A BEEPING HORN SOUNDED SIMILAR TO AUTOPLT DISCONNECT. WE COULD NOT SILENCE IT. WE BEGAN TROUBLESHOOTING. (I HAD THIS SAME HORN BEFORE FROM UNRESOLVED SOURCES.) AT ABOUT FL280; THE MASTER CAUTION ILLUMINATED WITH OVERHEAD PAX OXYGEN 'ON' LIGHT. WE IMMEDIATELY DONNED OUR MASKS; LOOKED AT THE PAX PANEL AND NOTICED WE HAD FAILED TO RECONFIGURE AFTER THE BLEEDS OFF TKOF.