A B737-500 ON CLBOUT AT 1000 FT CREW WAS UNABLE TO PRESSURIZE ACFT. RETURNED AND LANDED OVERWT. FOUND ELECTRONICS COMPARTMENT DOOR OPEN.

Narrative

SHORTLY AFTER TKOF; THE PRESSURIZATION AUTOFAIL LIGHT CAME ON. FO WAS FLYING; SO CAPT TOOK OUT FLT MANUAL TO TROUBLESHOOT THE PROB. WE FOLLOWED THE BOOK PROCS AND WERE UNABLE TO SOLVE THE PROB. THE ACFT WAS PRESSURIZED AND CLOSING THE OUTFLOW VALVE MANUALLY WAS NO HELP. WE CALLED DISPATCH AND MAINT TO DISCUSS THE PROB AND LOOK FOR OTHER SOLUTIONS; BUT ALL IDEAS RESULTED IN NO IMPROVEMENT. NO OTHER LIGHTS ACCOMPANIED THE AUTOFAIL LIGHT; SO WE DID NOT KNOW WHAT WAS CAUSING THE PROB. WITH DISPATCH CONCURRENCE WE DECIDED TO RETURN TO ORD AND LAND. WE WOULD BE A BIT OVERWT; BUT THE LOUD NOISE ALONG WITH THE UNKNOWN CAUSE OF THE PROB LED US TO BELIEVE THAT WE SHOULD GET THE ACFT ON THE GND. WE LANDED AT 115000 LBS WITH 40 DEG FLAPS ON RWY 9R. EQUIP WAS REQUESTED TO STANDBY DUE TO THE UNKNOWN NATURE OF THE PROB AND THE LOUD NOISE WE WERE EXPERIENCING. LNDG WAS NORMAL AND WE REQUESTED FIRE DEPT TO DO A ONCE AROUND THE ACFT. FIRE DEPT SAW NO PROBS SO WE TAXIED TO GATE. MAINT FOUND AFT ELECTRICAL EQUIP ACCESS DOOR OPEN; AND MICRO SWITCH STUCK; SO DOOR INDICATOR WAS INOP. MAINT LUBED AND REPAIRED MICRO SWITCH; AND CLOSED THE DOOR. WE REFUELED AND PROCEEDED WITHOUT PROB TO DEST. REF DOOR BEING LEFT OPEN; FO PERFORMED PREFLT IMMEDIATELY AFTER BLOCK-IN WHILE ACFT WAS BEING OFFLOADED.