B727 CREW TAKES EVASIVE ACTION AT MDW WHEN ON VECTORS IN THE ORD CLASS B AIRSPACE BASED ON A TCASII RA WHEN ATC SAID THERE WAS NO TFC.

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DURING ILS RWY 4 TO MDW; TWR SENT US AROUND ON SHORT FINAL DUE TO SPACING WITH DEP TFC. COMPLIED WITH INSTRUCTIONS OF L TURN HDG 250 DEGS; CLB AND MAINTAIN 3000 FT. CONTACTED APCH AFTER WE WERE LEVEL AT 3000 FT AND FLEW HDG 220 DEGS. NEXT THING WE KNOW; OUT OF THE BLUE WE GET AN RA TO DSND AT 1000 FPM. FO WAS FLYING AND COMPLIED WITH RA AND I ADVISED APCH WHO SAID THERE WAS NOBODY OUT THERE. THE TCASII DISPLAY WAS ON 10 MI RANGE. THERE WAS NO RED DIAMOND WHICH SEEMED ODD; BUT THERE WAS SOMETHING IN RED WRITING IN THE UPPER R CORNER OF THE DISPLAY WHICH I DID NOT COMPREHEND IN THE HEAT OF THE MOMENT; BUT WHAT I ASSUMED AT THE TIME WAS A DESCRIPTION OF A TARGET BEYOND THE RANGE OF THE CURRENT DISPLAY. AFTER DSNDING TO APPROX 2600 FT THE RA STOPPED AND WE CLBED BACK TO 3000 FT. UNEVENTFULLY FLEW ANOTHER ILS RWY 4 AND LANDED. IN RETROSPECT; I SUSPECT THE TCASII MAY HAVE MALFUNCTIONED SINCE WE DID NOT GET A TA FIRST OR A CLR OF CONFLICT AFTER; COMBINED WITH THE FACT APCH SAID THERE WAS NOBODY THERE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THAT THE TCASII UNIT WAS NOT WRITTEN UP FOR MAINT FOLLOW-UP. HE INDICATED THAT THE INDICATION WAS SIMILAR TO WHAT WOULD OCCUR IF THE TARGET WAS OFF SCALE OF THE TCASII DISPLAY BUT STILL REQUIRED AN RA BUT THAT THE CTLR HAD NO OTHER TFC IN THE IMMEDIATE AREA.