A FLT CREW APCHING DFW BRIEFED FOR RWY 18R BUT WAS ASSIGNED RWY 13R. THE LOC FREQ WAS NOT CHANGED AND THE ACFT FLEW THROUGH THE LOC COURSE.

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I CALLED FOR THE APCH CHKLIST AFTER BRIEFING THE ILS RWY 18R APCH SO WHEN THE CHANGE TO RWY 13R OCCURRED WE HAD ALREADY RUN THE CHKLIST. IF WE HAD CHKED THE MORSE CODE IDENT WITH THE RWY CHANGE; WE WOULD HAVE CAUGHT OUR ERROR. THE FO AND I DEBRIEFED THIS AFTER THE FLT IN ORDER TO AVOID THIS IN THE FUTURE. APCHING DFW ON THE DUMPY 2; APCH TOLD ME TO EXPECT BEING TAKEN OVER THE TOP OF THE ARPT FOR A W SIDE RWY. I ASSUMED RWY 18R AND SO BRIEFED THAT APCH. OVER THE TOP OF THE ARPT APCH GAVE ME RWY 13R. I THEN BRIEFED THAT APCH. I ELECTED TO HAND FLY WITHOUT THE FLT DIRECTOR. APCH CLRED ME TO DSND TO 2300 FT AND TURNED ME TOWARD LOC. THEY WERE KEEPING US PRETTY CLOSE IN. I LEVELED AT 2300 FT AS ATC GAVE ME AN ADDITIONAL TURN TO JOIN THE LOC AND CLRED ME FOR THE ILS RWY 13R AND TO MAINTAIN 2300 FT UNTIL ESTABLISHED. I HAD THE FEELING WE WERE VERY CLOSE TO THE LOC. JUST AS I FINISHED THE TURN AND WITH THE LOC NEEDLE STILL AT FULL DEFLECTION; ATC GAVE ME ANOTHER TURN TO REJOIN. THAT'S WHEN I REALIZED WE HAD THE WRONG LOC FREQ. WE STILL HAD RWY 18R LOC IN THE ACTIVE WINDOW WITH FUZ FOR THE MISSED IN STANDBY. WE QUICKLY PUT THE CORRECT FREQ IN AND I SAW ABOUT A 1/2 SCALE DEFLECTION FOR RWY 13R. HOWEVER; I ALLOWED THE AIRPLANE TO DSND TO 2000 FT. I PROMPTLY CLBED BACK UP TO 2300 FT JOINED THE LOC AND THEN CAPTURED THE GS AND COMPLETED THE APCH. GETTING A HDG TO REJOIN THE LOC WHEN IT DIDN'T LOOK LIKE WE HAD EVEN REACHED THE LOC YET ALERTED ME TO THE FACT THAT I LIKELY HAD THE WRONG FREQ. ONCE WE HAD THE CORRECT FREQ AND WHILE STILL TURNING TO THE REJOIN HDG ASSIGNED BY ATC; THE LOC SHOWED US AROUND 1/2 SCALE DEFLECTION. WHEN I HEARD THE ALT ALERTER; I IMMEDIATELY BEGAN A CLB BACK TO 2300 FT. BASIC AIRMANSHIP; WHENEVER YOU CHANGE NAV FREQ ALWAYS CHK THE MORSE CODE IDENT EVEN IF THE APCH CHKLIST HAD BEEN PREVIOUSLY COMPLETED CORRECTLY. THE FO AND I DEBRIEFED THIS ITEM TO AVOID A FUTURE OCCURRENCE.