ALTDEV ALT OVERSHOT IN DSCNT ALT XING RESTR NOT MET. FO RPTR CITES ACFT EQUIP PROB WITH AUTOPLT DISCONNECTING AND NOT AUTOCAPTURING THE ALT SET IN FLT MODE ANNUNCIATOR.

Narrative

ATC FACILITY: SOCAL APCH. LOCATION: LAX 069/42. FO FLYING ARR INTO LAX. SPD CHANGED BY ATC FROM 300 KTS TO 250 KTS AND 210 KTS. 14000 FT RESTR AT CIVET INTXN COMPLIED WITH ON AUTOPLT; DSNDED TO 12000 FT AT 40 DME. AT 42 DME WE HEARD THE ALT WARNING AND I LOOKED AT CTR ALTIMETER AND SAW 11700 FT. I IMMEDIATELY ADDED PWR AND LEVELED AT 12000 FT. ATC WAS CALLING AT SAME TIME SAYING FLT XY YOU MUST CROSS 40 DME AT 12000 FT. I SAID ROGER. THE AUTOPLT WAS NOT ON AS I CLBED BACK UP TO 12000 FT. I TURNED IT BACK ON; ARMED 10000 FT FOR THE 30 DME RESTR; AND WAS STARTING THAT DSCNT WHEN ATC SAID FLT XY YOU CAN NOW START DOWN TO 10000 FT. I AGAIN SAID ROGER. THE CAPT WAS TALKING TO LAX RAMP CONCERNING INTL CONNECTIONS AS WE LEVELED AT 12000 FT AT 40 DME. I FLEW THE REMAINDER TO THE ILS TO LAX. WE DEBRIEFED WHAT HAD HAPPENED AND DECIDED TO FILL OUT A FORM EVEN THOUGH ATC DIDN'T SAY ANYTHING ABOUT THE INCIDENT. LESSONS LEARNED INVOLVE AM45 BODY TIME WAKEUP -- 8 HR FLYING DAY; 1 1/2 HR SIT AT DFW FOR ATC FLOW INTO LAX. I DON'T REMEMBER DISCONNECTING AUTOPLT ON THE ARR. IT WAS OFF AS WE DSNDED THROUGH 12000 FT. SO WITH 1 PLT OUT OF THE LOOP AND TIRED; DOUBLECHK EVERYTHING; ESPECIALLY AUTOPLT. SUPPLEMENTAL INFO FROM ACN 348320: ALT CAPTURE WAS ANNUNCIATED BUT THE ACFT ON AUTOPLT FLEW THROUGH 12000 FT. HE DISCONNECTED THE AUTOPLT AND THE DSCNT WAS STOPPED AT 11700 FT.