B737 FLT CREW MISSED TURN ONTO ILS BECAUSE OF EQUIP MALFUNCTION.

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ARR TO CVG ON RADAR VECTORS FOR ILS TO RWY 18R. DSCNT AND APCH CHKLISTS COMPLETED; ILS FREQS IDENTED; APCH CTL CLRED US TO DSND TO 2500 FT MSL (THIS WAS AN UNUSUALLY LOW ALT FOR APCH INTO CVG) APPROX 23 NM FROM THE ARPT ON AN INTERCEPT HEADING FOR THE LOC. WE WERE THEN CLRED TO JOIN THE LOC AND FOR THE APCH. AS WE PROCEEDED WE NOTICED FLAGS IN THE LOC AND GS INDICATORS ON THE ADI AND WE BOTH ASSUMED OUR DISTANCE AND LOW ALT WERE PREVENTING US FROM RECEIVING A GOOD SIGNAL AND WE EXPECTED THE SIGNAL TO COME UP AGAIN ANY SECOND. AT 18 NM APCH CTL ASKED US WHY WE DIDN'T JOIN THE LOC AND GAVE US A 220 DEG VECTOR BACK TO REJOIN RWY 18R LOC. APCHING 12 NM; WE STILL HAD FLAGS IN LOC AND GS AND INFORMED ATC WE HAD EQUIP PROBS AND WENT MISSED APCH. ON NEXT APCH WE DISCOVERED THAT WHEN VOR/LOC BUTTON ON MODE CTL PANEL WAS SELECTED; THE ACFT WAS DROPPING THE ILS SIGNAL TOTALLY AND THE IDENT HAD A LOUD PITCHED TONE AND NO IDENT. WE CONTINUED AND LANDED USING A MANUALLY FLOWN APCH; WITHOUT EVACING THE APCH MODE OF THE B737-200 FLT CTL COMPUTER. THE ACFT WAS WRITTEN UP ON ARR CVG. 1) THE ACFT MALFUNCTION LED TO AN AIRSPACE VIOLATION; WE CROSSED RWY 18R LOC AND WENT INTO RWY 18L EXTENDED CTRLINE AT 18 NM. 2) WE DID NOT HAVE ANY CONFLICTS THAT WE WERE AWARE OF. 3) WE MADE ASSUMPTION THAT LOW ALT ON VECTOR (2500 FT) AND LONG FINAL 23-18 NM WAS REASON WE HAD FLAGS. WE SHOULD HAVE ASKED ATC IF ILS WAS MONITORING CORRECTLY. 4) RWY 18R DOES NOT HAVE ADF AND VOR IS NOT LOCATED ON FIELD SO IT IS VERY DIFFICULT/NOT POSSIBLE TO TELL YOU ARE XING LOC. SUPPLEMENTAL INFO FROM ACN 594273: AFTER OUR SHUTDOWN AT THE GATE MAINT WERE SUMMONED TO INVESTIGATE. THEY WERE ABLE TO EMULATE OUR PROB. WE DISCOVERED WE WERE COMPLACENT DURING THE INTERCEPT SINCE THERE ARE NO OTHER REF NAVAIDS (ADF'S; VOR FIELD LOCATED). THE ONLY PERSON WITH ANY 'BIG PICTURE' WAS CVG FINAL RADAR CTLR.