ACR FLC DSNDED BELOW IAP ALT. CAPT (PF) MISINTERPRETED DME SOURCE FROM NAV DISPLAY.

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CAPT WAS NOW ON THE 757 LESS THAN 100 HRS. WE WERE FLYING MARGINAL VFR (1200 FT BROKEN AND 5 MI) LOC 27 INTO SAN; CA. CAPT WAS IN LNAV. DURING DSCNT AUTOPLT DISCONNECTED BY ITSELF SO CAPT HAND FLEW APCH. I (PNF) WAS IN EXPANDED ILS MODE FOR DME ON APCH. IRS RELATIVE POS WAS OFFSET A LITTLE BIT TO THE L; SO CAPT ELECTED TO GO TO EXPANDED ILS MODE FOR ACCURATE LOC DATA; BUT DIDN'T TELL ME. HE STATED A LITTLE CONFUSION OVER WHERE HE WOULD BE RECEIVING PROPER LOC DME DATA. I STATED THAT IT WOULD BE ON THE R DME WITH AN 'L' IN FRONT. HE STARTED TO DSND FROM 2000 FT AT WHAT HE BELIEVED TO BE REEBO; BUT HE WAS LOOKING AT THE INCORRECT DME (THE L ONE INITIALLY) FROM MAP MODE WHICH CAUSED HIM TO DSND BELOW 2000 FT (REEBO) EARLY. ONCE I REALIZED THAT HE WAS ON ILS EXPAND ALSO; I RETURNED TO MAP MODE FOR SITUATIONAL AWARENESS. AT THIS POINT; TWR STATED THAT SHE WAS SHOWING US LOW AT REEBO. I CONFIRMED THAT WE WERE AT REEBO BUT OUR ALT WAS 1400- 1500 FT MSL. WE LEVELED OFF WITH RWY IN SIGHT UNTIL VASI LIGHTS GAVE US CORRECT GS INFO. LNDG UNEVENTFUL. AFTER LNDG; WE DISCUSSED HOW WE HAD LEFT REEBO EARLY. THE CAPT STATED THAT HE REALLY SHOULD HAVE BRIEFED APCH BETTER TO CLARIFY WHICH DME WOULD BE THE CORRECT ONE FOR LOC DATA. I WAS UNAWARE THAT HE WASN'T SURE. I WAS ALSO SOMEWHAT CONFUSED BECAUSE I BELIEVED HE HAD LEFT REEBO ACCURATELY. LOOKING FOR 6.4 DME TO DSND; I BELIEVE WE HAD; FROM THE EXPANDED ILS LOC DME? IT IS EASY TO GET HIGH ON THIS APCH; AND I'M SURE THAT WAS ON OUR MINDS AS WE WERE DSNDING TO 700 FT MSL. BEING NEAR DUSK WITH ALL THE CLOUD COVER ALSO MADE THE APCH MORE DIFFICULT THAN USUAL. SUPPLEMENTAL INFO FROM ACN 316110: WE PASSED SWATT AND BEGAN DSCNT TO 2000 FT MSL. BECAUSE THE LNAV WAS DISPLACED; I SWITCHED MY HSI TO EXPANDED ILS TO CHK THE COURSE. I SWITCHED BTWN MAP AND EXPANDED ILS SEVERAL TIMES. BECAUSE OF THIS SWITCHING OF HSI'S; I BELIEVE I STARTED A SIT. LOOKING BACK ON IT THE CONFUSION BEGAN ON MY PART IN REGARDS TO EXACTLY WHERE THE LOC DME WAS TO BE PRESENTED ON MY PANEL. IN THE FUTURE I MUST BE MORE SPECIFIC IN BRIEFING RADIOS; MAP; EXPANDED ILS. THEN IF CHANGES TAKE PLACE BOTH PLTS MUST KNOW AND KEEP UP.