ACR MD88 FLC TRACKING INBOUND ON CVG ILS RWY 18R FINAL APCH COURSE RECOGNIZES A WEAK BUT READABLE LOC SIGNAL. TRACON INTERVENES WITH NEW CLRNC AS ACFT IS NOT OBSERVED TRACKING ON FINAL. AFTER LNDG; FO OBTAINS INFO FROM ANOTHER PLT THAT MAINT ALLEGEDLY WAS WORKING ON NAVAID AT THE TIME ACFT WERE USING LOC FOR NAVING.

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MD88 ACFT YUL-CVG. VECTORS TO FINAL ON CVG RWY 18L. HAZY DAY ABOUT 4 MI VISIBILITY. CLRED TO INTERCEPT LOC AT ABOUT 15 MI. FIELD WAS NOT IN SIGHT DUE TO LOWER VISIBILITY. WE INTERCEPTED THE LOC WITH THE CORRECT FREQ DIALED IN. IDENT WAS WEAK BUT READABLE. WE WERE THEN CLRED FOR THE APCH FROM 6000 FT MSL. PASSING THROUGH 5000 FT AT ABOUT 10 MI; APCH CTL DIRECTED US TO MAINTAIN 5000 FT. WE QUERIED THE CTLR TO MAKE SURE WE WERE CLRED FOR THE APCH. HE REPEATED 'MAINTAIN 5000 FT.' WE DID OUR BEST BUT BOTTOMED OUT AT 4600 FT AND CLBED BACK UP TO 5000 FT. HE TOLD US WE WERE R OF COURSE WHICH PROBABLY PUT US CLOSE TO THE FINAL OF RWY 18R. HE TURNED US TO 090 DEGS AND WE CAME BACK AROUND FOR AN UNEVENTFUL APCH AND LNDG. WE RECHKED THE ILS FREQ; COURSE; AUTOPLT (SAID LOC TRACK; GS TRACK); SCOREBOARD AND FLT DIRECTOR. ALL WERE CORRECT AND ARMED APPROPRIATELY. EARLIER IN THE DAY; OUR HDG SYS PRECESSED BUT CORRECTED ITSELF. WE DECIDED TO WRITE THIS UP ON LNDG IN CASE THE HDG SYS MAY HAVE CONTRIBUTED TO THIS EVENT. AFTER LNDG THE PLT OF 1 OF 3 ACFT AHEAD OF US TOLD US HE GOT AN UNRELIABLE SIGNAL ON GLIDE PATH AND HE HAD TO TURN OFF ALL AUTOMATION AND FLY VISUALLY; BUT HE HAD RWY IN SIGHT. WE DIDN'T. HE MENTIONED TWR TOLD HIM THEY WERE WORKING ON THE LOC EVEN THOUGH WE NEEDED IT FOR COURSE GUIDANCE. IT WORKED OUT AND THE CTLR DID AN EXCELLENT JOB BY MONITORING US ON FINAL AND DIRECTING US BEFORE A CONFLICT DEVELOPED. I THINK THAT'S THE MOST IMPORTANT JOB A CTLR HAS IS HIS ROLE AS SAFETY OBSERVER.