AN ACR B737 FLC DSNDS BELOW THEIR XING ALT WHEN THE CAPT SELECTS THE WRONG AUTOFLT SYS MODE AND SETS THE ALT ALERTER SELECTOR BELOW THEIR XING ALT.

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ZLA CLRED ACFT TO BANDS INTXN WITH MULTIPLE SHORT DSCNTS OF 1000 FT - 2000 FT. CAPT USED VERT SPD MODE TO ACCOMPLISH TASK. DURING TRANSITION TO SOCAL APCH; CLRNC ABOVE WAS RECEIVED. THIS WAS TO BE AN FMC FUNCTION. WE WERE ANTICIPATING A VISUAL APCH; SHORTCUTTING THE ARR. INSTEAD OF RE-ENGAGING VNAV; FIRST THE CAPT SET 2200 FT IN ALT WINDOW -- THAT WOULD HAVE BEEN OK; BUT VNAV HAS TO BE ENGAGED TO READ FMC ALTS. VERT SPD LET THE ACFT DSND BELOW (BANDS; 7000 FT). DUTIES OF CREW MEMBERS WERE DIVIDED. CAPT WITH VISUAL; FO WITH ARR; STAR AND APCH CHKLIST ITEMS. OTHER DISTRS: ATIS ONLY RECEIVES CLOSE IN AS DOES COMPANY FREQ. THE ABILITY TO BACK EACH OTHER UP WAS NOT THERE 'AS USUAL' -- XCHKS OF EACH OTHER NOT DONE DUE TO DIVISION OF DUTIES. ALSO TOO DISTR BY RA WHILE WITH ZLA BY KING AIR PASSING 500 FT OVER US. SUPPLEMENTAL INFO FROM ACN 386755: CLRNC: TO KAYOH; CROSS KAYOH AT 7000 FT; 190 KTS. THEN CLRED TO DSND EASTSIDE 2 ARR; TRANSITION TO RWY 19R. ACFT B737 FMC EFIS. DURING TRANSITION TO SOCAL APCH; CLRNC ABOVE WAS RECEIVED. THIS WAS TO BE AN FMC FUNCTION. WE WERE ANTICIPATING A VISUAL APCH; SHORTCUTTING THE ARR. INSTEAD OF RE-ENGAGING VNAV FIRST; THE CAPT SET 2200 FT IN ALT WINDOW. THAT WOULD HAVE BEEN OK; BUT VNAV HAS TO BE ENGAGED TO READ FMC ALTS. LET THE ACFT DSND BELOW (KAYOH INTXN 7000 FT).