MD83 CREW WAS CLRED FOR A CAT 3 AUTOLAND APCH ON RWY 17L AT DFW WITH THE ILS TUNED TO RWY 35R.

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WE WERE FLYING FROM MEM TO DFW WITH DFW AT LOW MINIMUMS DUE TO FOG. DURING OUR TRIP; DFW WAS AT CAT 3 CONDITIONS AND WE BRIEFED THE CAT 3 (AUTOLAND) APCH. DFW WAS LNDG S. PASSING BYP ON THE ARR; DFW CAME UP TO 1600 FT RVR FOR RWY 17L. WE DECIDED TO CONTINUE THE AUTOLAND AS PLANNED. WX ON THE ARR WAS VFR WITH CLR SKIES AND 10+ MI VISIBILITY. THE PROB AT DFW WAS LOCALIZED GND FOG AND 100 FT CEILINGS; WITH TOPS AROUND 500 FT. ON BASE LEG TO ILS RWY 17L; WE WERE VECTORED AT 4000 FT IN CLR VFR ON TOP CONDITIONS WITH UNLIMITED VISIBILITY. THE FO RPTED THAT HE WAS NOT GETTING A GOOD IDENT ON EITHER ILS RECEIVER. I CHKED AND GOT A WEAK AND INTERMITTENT IDENT. WE DECIDED TO CONTINUE AFTER BOTH CONFIRMING THAT WE WERE BOTH TUNED TO 111.75 AND 174 DEGS FOR ILS RWY 17L. ALL TFC ON OUR FREQ WAS BEHIND US; AND I GOT THE IMPRESSION WE WERE THE FIRST TO FLY THE APCH TO RWY 17L. THE CTLR TURNED US TO DOGLEG TO INTERCEPT R AROUND DRAAK AT 4000 FT. THE IDENT STARTED TO GET STRONGER AS THE CTLR CLRED US TO INTERCEPT THE ILS; MAINTAIN 4000 FT UNTIL ESTABLISHED. FO RPTED HE WAS NOW GETTING A GOOD IDENT AND BOTH OUR 'OFF' FLAGS HAD RETRACTED. AS OUR DOGLEG APCHED THE FMS COURSE LINE; I TURNED TO 174 DEG HDG AND SELECTED LOC THEN AUTOLAND. THE FMA SHOWED 'LOC CAP' INDICATING IT WAS CAPTURING THE LOC. I WAS ALIGNED ON COURSE AND THE CDI WAS CTRED. THE GS WAS DSNDING AND FAILED TO CAPTURE; SO WITH ABOUT 1/2 DOT ABOVE GS I BEGAN A MANUAL DSCNT TO CAPTURE IT. WE WERE DSNDING OUT OF 4000 FT ABOVE CLOUDS IN VFR CONDITIONS. SUDDENLY THE AIRPLANE; STILL ON AUTOPLT; MADE A SHARP R TURN (W) AND BEGAN TURNING AWAY FROM THE COURSE ON THE NAV DISPLAY. I WAS LOOKING OUTSIDE AND CLRING AND DIDN'T NOTICE WHAT THE CDI DISPLAYED. I DISENGAGED THE AUTOPLT AND TURNED BACK TOWARDS THE FINAL APCH COURSE JUST AS APCH CTL CALLED US AND ASKED IF WE WERE ESTABLISHED. THE CTLR CLBED US BACK TO 4000 FT AND TURNED US E; AWAY FROM THE ARPT AND VECTORED US AROUND FOR ANOTHER APCH. WE TOLD HIM WE HAD TROUBLE WITH THE IDENT AND A FLUCTUATING COURSE AND GS. ANOTHER ACFT THAT HAD BEEN BEHIND US WAS VECTORED TO FINAL AND HAD NO TROUBLE. WE SUBSEQUENTLY INTERCEPTED WITH A MUCH STRONGER IDENT AND SHOT THE APCH FULL AUTOLAND WITH NO INCIDENTS. ON SHORT FINAL WE NOTICED A TRUCK AND WORKMEN ON THE APCH LIGHTS AND DECIDED THE APCH ZONE HAD NOT BEEN STERILE; POSSIBLY CAUSING OUR PROBS; BUT I WAS PAGED IN THE TERMINAL AND ASKED TO CALL MR X AT APCH CTL. MR Y IS A SUPVR CTLR AND APOLOGIZED TO ME. HE SAID THE ILS HAD BEEN PREVIOUSLY SHUT DOWN FOR WORK ON THE APCH LIGHTS; AND THE ILS HAD BEEN TURNED AROUND TO RWY 35R WHILE THE LIGHTING REPAIRS WERE MADE. THE ILS HAD NOT BEEN TURNED BACK TO RWY 17L WHILE WE WERE MAKING THE FIRST APCH. HE APOLOGIZED FOR THAT; BUT CHIDED ME FOR LEAVING 4000 FT BEFORE WE WERE FULLY ESTABLISHED. I EXPLAINED THAT WE WERE RECEIVING AN IDENT AND THOUGHT WE WERE ESTABLISHED (UNTIL THE RADICAL R TURN) AND MUST HAVE BEEN LOCKED ON THE BACK COURSE; BUT I DON'T THINK HE FULLY UNDERSTOOD ME. I THANKED HIM FOR HIS APOLOGY; AND HE REITERATED THE DANGERS OF DSNDING PRIOR TO BEING ESTABLISHED. LESSONS LEARNED: I THINK I GOT THE PROPER IDENT FOR THE RWY; IPPZ; BUT I AM NOT POSITIVE. IF THE ILS WAS TURNED AROUND AND WE WERE RECEIVING THE RWY 35R IDENT; IAJQ; WE FAILED TO NOTICE. I ONLY DSNDED FROM 4000 FT TO CHASE THE GS BECAUSE WE WERE IN VISUAL CONDITIONS AT THE TIME AND I THOUGHT WE WERE ESTABLISHED ON COURSE. BUT NEXT TIME I NEED TO BE MORE CERTAIN OF COURSE CAPTURE BEFORE STARTING DOWN. WE SHOULD ALSO HAVE BEEN MORE FORCEFUL WITH THE CTLR WHEN HE QUERIED ABOUT THE INITIAL BAD IDENTS. SUPPLEMENTAL INFO FROM ACN 632921: JUST PRIOR TO GETTING HANDED OFF TO THE TWR; WE ASKED THE CTLR WHAT HAD HAPPENED WITH THE LOC. HE DID NOT GIVE US ANY SPECIFICS; BUT SAID THAT THEY HAD TO DO 'SOMETHING' TO MAKE IT USABLE. IT WAS NOT UNTIL AFTER CAPT HAD CALLED THE CTLR ON THE GND THAT WE FOUND OUT THAT THE LOC HAD BEEN TURNED AROUND TO THE RWY 35R SIDE BECAUSE GND CREWS WERE WORKING ON THE RWY 17L APCH LIGHTS. UPON REFLECTION; I REALIZE THAT WE PROBABLY NEVER GOT AN ACCURATE IDENT ON THE LOC FOR RWY 17L; SINCE IT SEEMS THAT THE LOC WAS TURNED AROUND TO RWY 35R; WHICH HAS A DIFFERENT IDENT. I LISTENED TO THAT IDENT AT LEAST 5 TIMES PRIOR TO THE FIRST ATTEMPT AT INTERCEPT; AND I WAS SURE THAT I HAD HEARD IT CORRECTLY; ALTHOUGH IT WAS FAINT. THAT SHOULD HAVE BEEN A CLUE THAT SOMETHING WAS AMISS.