A DC9 FLC DSNDED BELOW THE ASSIGNED ALT NEAR AML.

Narrative

EVENT: WHEN DSNDING TO 12000 FT; THE MASTER CAUTION LIGHT ILLUMINATED AS I WAS LEVELING OFF. ATTN WAS DIVERTED TO THE OVERHEAD PANEL TO IDENT THE MALFUNCTION. (A PREVIOUS MALFUNCTION HAD OCCURRED AND CORRECTIVE ACTION PERFORMED.) BECAUSE OF WEAK LIGHTING; PROPER IDENT WAS DIFFICULT. AS I IDENTED THE MALFUNCTION; I ALSO OBSERVED THAT I HAD DEVIATED 400 FT BELOW THE ASSIGNED ALT. I IMMEDIATELY CORRECTED TO THE PROPER ALT. THE CTLR MADE NO CALL AND TCASII SHOWED NO ACFT WITHIN 20 MI. THE MALFUNCTION WAS WRITTEN UP AND THE SYS WAS DISABLED AND AN MEL WAS DOCUMENTED. CHAIN OF EVENTS: THE MALFUNCTION OCCURRED DURING A CRITICAL PHASE OF FLT WHICH DIVERTED BOTH PLT'S ATTN. TIMELY IDENT PREVENTED PF FROM RETURNING TO FLYING DUTIES. ONCE IDENTED; FLYING DUTIES CORRECTED DEV. HUMAN PERFORMANCE: PLT TENDENCIES ARE TO IDENT THE MALFUNCTION AND DIRECT CORRECTIVE ACTION. WHEN CAUTION SYS CANNOT READILY IDENT PROB; THEN BOTH PLT'S DIVERSION IS ALMOST IMMINENT. LACK OF SELF DISCIPLINE CAUSED ME TO FAIL TO LEVEL THE ACFT AT THE PROPER ALT. HAD THE CAUTION SYS ADEQUATELY ILLUMINATED THE MALFUNCTION; THIS WOULD NOT HAVE OCCURRED.