CLOSE PROX ACR-MLG SMT-HELI SUPPOSED TO BE OVER RWY 22R BUT IN FACT ON LOC TO 14R.

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DURING APCH RWY 14R AND INSIDE ROMEY; TCAS GAVE A TA; FOLLOWED BY AN RA TO DECREASE DSCNT RATE. FO WAS FLYING AND REDUCED DSCNT RATE FROM 800 FPM TO 200 FPM. SHORTLY AFTER THIS; WHILE PASSING 1800 FT MSL; WE PASSED 300 FT ABOVE THE INDICATED TARGET. WE HAD ALREADY CHKED IN WITH ORD TWR AT ROMEY; SO WE ASKED IF THERE WAS A HELI IN OUR AREA. CTLR'S REPLY WAS NO THAT THERE WAS A HELI WELL TO THE E OF US. I THEN TOLD THE CTLR WE HAD RECEIVED AN RA FROM THE TCAS. HE ASKED THE HELI PLT HIS LOCATION AND HE ADVISED HE WAS OVER BY RWY 22R. SUBSEQUENT CONVERSATION BTWN THE CTLR AND HELI PLT CONFIRMED THE HELI WAS R BELOW US WHILE WE WERE FLYING THE 14R ILS. AFTER LISTENING TO THE ORD TWR TAPES AND TALKING TO THEIR QUALITY ASSURANCE REP; I LEARNED THAT THE CTLRS HAD AND WERE STILL GOING THROUGH A SHIFT CHANGE. THE PLT OF THE HELI HAD ONLY BEEN ON THE JOB FOR 2 DAYS AS A VACATION RELIEF PLT. ALTHOUGH OUR TCAS SHOWED THAT THE HELI PASSED 300 FT DIRECTLY BELOW US WE NEVER SAW HIM AS WE WERE JUST COMING OUT OF THE CLOUDS BASES. ALSO THE ORD TWR COMPUTER PLOT OF THIS INCIDENT SHOWED THE HELI TO BE 400-600 FT BELOW AND 600 FT TO THE R OF US. I FEEL THAT WITHOUT THE TCAS IF WE HAD CONTINUED OUR 800 FPM DSCNT RATE IT WOULD HAVE RESULTED IN MID AIR COLLISION OR VERY CLOSE TO ONE.