B737-300 FLT CREW VIOLATED THE CROSSING RESTRICTION ON A VISUAL APCH; FAILING TO REFERENCE THE VOR AS A DME REFERENCE FOR IDENTIFYING THE FIX.

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ARR INTO DTW AND PLANNING TO FLY VISUAL APCH TO RWY 21L (BACKED UP BY ILS); ESTABLISHED AT 5000 FT MSL; BASE LEG. CAPT FLYING AND MONITORING DTW (POSITIONAL AWARENESS) PRIOR TO SELECTING LOC FREQ; FO -- IN AN EFFORT TO PROVIDE DME -- SELECTS DTW. PROB OCCURS BECAUSE APCH DME IS BASED ON CRL VOR. I SHOULD HAVE CONFIRMED PROPER NAVAID SELECTION WITH FO. ROLLOUT ON EXTENDED FINAL APPROX 20 MI DTW/32 MI CRL. OPS NORMAL UNTIL I DEPART 5000 FT APPROX 2 MI EARLY (BELIEVING WE'VE REACHED ROBBI/25.2 CRL) FOR NEXT RESTR. DECISION BASED ON DTW DME. AT THE SAME TIME; FO NOTES THAT I'VE DSNDED EARLY. I SEE THAT FO IS MONITORING DTW AND XCHK FMC AND DETERMINE I HAVE DSNDED EARLY. I BREAK OUR DSCNT RATE AND WOULD HAVE CALLED APCH BUT WE WERE IN MIDST OF FREQ CHANGE TO TWR. AT THIS POINT; WE CROSS ROBBI (DON'T RECALL EXACT ALT) AND THEN CONTINUE DSCNT TO 4000 FT. CONTINUE APCH TO UNEVENTFUL LNDG. ON TAXI-IN; GND NOTIFIES US OF A PROB ON FINAL APCH AND INSTRUCTS US TO CONTACT TRACON. SPEAK WITH TRACON SUPVR. EXPLAINED MY ERROR AND HE CONCURRED. HE STATED THAT HE WOULD RESEARCH PROB TO SEE IF THERE WAS A CONFLICT WITH OTHER TFC. HE RETURNED A CALL APPROX 45 MINS LATER THAT THERE WAS A CONFLICT AND THAT HE WOULD BE FILING PAPERWORK. ON FINAL; WE DID NOT PERCEIVE (VISUALLY/TCAS) ANY TFC IN OUR IMMEDIATE AREA. FIRST; I SHOULD HAVE HIGHLIGHTED DME REF WHEN BRIEFING THE VISUAL APCH AND CONFIRMED WHAT NAVAID SETUP WOULD BE. MY NORMAL HABIT IS TO MONITOR VOR AT THE FIELD TO REMAIN POSITIONALLY AWARE. I SHOULD HAVE EMPHASIZED THAT IN APCH BRIEFING AND THAT THE FIELD VOR IS NOT THE DME REF FOR THE APCH. SECONDLY; BETTER XCHK OF FMC LEGS PAGE ON MY PART WOULD HAVE HELPED MY SITUATIONAL AWARENESS. I WAS REFING DME MORE THAN LEGS PAGE. THIRDLY; VERBALIZING WHEN I PLANNED TO DEPART 5000 FT WOULD HAVE ALERTED FO THAT SOMETHING WAS IN ERROR. ONE ITEM FOR ATC -- WE WERE SWITCHED TO TWR PRETTY EARLY (APPROX 18 MI). MAYBE THAT'S A FUNCTION OF CTLR WORKLOAD; BUT IF THERE IS A TFC CONFLICT; BELIEVE APCH WOULD BE BETTER EQUIPPED TO PROVIDE A WARNING CALL. OVERALL; THIS WAS AN INCIDENT THAT COULD HAVE BEEN PREVENTED BY A BETTER BRIEFING; MORE DISCIPLINED MONITORING AND COORD ON MY PART.