A B737-300 PILOT FLYING A LOC-BC SELECTED VOR/LOC WITH THE FRONT COURSE SELECTED AND THE FLT DIR INDICATED AN OPPOSITE DIRECTION TURN AWAY FROM THE LOC.

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WE WERE FLYING THE LOC BACK COURSE TO THE RWY. I WAS PF. I HAD BRIEFED THE LOC BACK COURSE TO RWY XX. AS WE WERE COMMENCING THE APCH WE WERE TOLD THERE WERE 2 PILATUS ACFT IN THE APCH PATTERN. I HAD SLOWED TO 170 KTS AND WAS ON THE LOC. I WAS TOLD TO FOLLOW ONE OF THE PILATUS ACFT AND COULD NOT DSND UNTIL THE OTHER PILATUS HAD PASSED THE APCH PATH. WHEN I WAS CLRED THE APCH I WAS DISTR AND PRESSED THE VOR/LOC BUTTON. I IMMEDIATELY HIT THE HDG SELECT BUTTON AND STEERED THE JET BACK TO THE LOC COURSE. I WAS NOT SURE IF THE FRONT COURSE WAS IN BUT AFTER CHKING AGAIN WE HAD 100 COURSE IN THE SELECTOR. I COULD NOT UNDERSTAND WHY THE JET HAD TURNED AWAY FROM THE COURSE AND HAD LOST CONFIDENCE IN THE ABILITY OF THE FLT DIRECTORS TO CORRECTLY FOLLOW THE LOC; SO I BEGAN TO FOLLOW THE LNAV COURSE. AS WE APCHED THE DECISION ALT WE BROKE OUT AND HAD GND CONTACT. WE WERE ABOUT 1 1/2 DOTS OFF THE LOC TO THE L OF COURSE. I SAW THE APCH LIGHTS AND STEERED THE JET TO THEM AND DISCONNECTED THE AUTOPLT AND LANDED UNEVENTFULLY. AFTER WE LANDED AND DEBRIEFED; I KNEW THAT I HAD MADE A MISTAKE AND SHOULD NOT HAVE SELECTED THE VOR/LOC BUTTON THAT THE JET WOULD FLY TO THE FRONT COURSE AND TURN IN THE WRONG DIRECTION. THE FO DID NOT UNDERSTAND UNTIL I EXPLAINED IT TO HIM. I ASKED WHY THE ILS WAS NOT BEING USED IN THE LOW VISIBILITY; AND THE TWR STATED THAT THERE WAS A LARGE WINDSHEAR IN THAT DIRECTION.