VFR ACFT Y ERRONEOUS XPONDER MODE C ACR X TCASII RA NON RESPONSE.

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TCASII MAY BE A BONE OR A BLESSING DEPENDING ON WHICH SIDE OF THE FENCE YOU'RE ON. I HAVE READ MANY ARTICLES TOUTING HOW TCASII SAVED MANY ACFT FROM IMPENDING DOOM. TCASII MAY BE A GREAT TOOL IN PREVENTING ACCIDENTS; BUT THE SYS IS ONLY AS GOOD AS THE INFO PROVIDED BY XPONDER EQUIPPED ACFT. THE FOLLOWING SIT OCCURRED JUN/XX/93; 20 NM FROM ATL ARPT. I WAS WORKING THE ARR 'A' SECTOR AT ATL TRACON. MY WORKLOAD WAS HVY AS WE WERE IN THE MIDDLE OF AN ARR RUSH. ILS APCHS WERE IN USE AS THE VISIBILITY WAS POOR DUE TO THE HAZE AND THE SETTING SUN. ATL WAS LNDG ON RWY 26R AND 27L. THE FINAL TO RWY 27L WAS OUT TO 20 MI E OF ATL. MY SUPVR SHOWED ME A VFR TARGET OPERATING 20 MI ENE OF ATL SBOUND. MODE C READOUT OF THIS ACFT WAS 4000 FT. I WAS VECTORING AND DSNDING MY ACFT TO 3500 FT FOR THE APCH TO RWY 27L. THE VFR ACFT WAS CONTINUING S AND I ISSUED TFC TO ALL AFFECTED ACFT. ACR X WAS ON THE DOWNWIND AT 7000 FT AND WAS INSTRUCTED TO DSND AND MAINTAIN 3500 FT. I TOLD THE PLT TO EXPEDITE DSCNT TO GET BELOW THE VFR TFC AT 4000 FT. THE VFR TFC WAS AT ACR X'S 10 O'CLOCK POS AND ABOUT 10 MI. I KEPT UPDATING THE VFR'S POS TO ACR X. THE SIT EVENTUALLY DEVELOPED TO A POINT WHERE THE VFR ACFT WAS AT ACR X'S 10 O'CLOCK POS AND 2 MI AND BOTH ACFT WERE INDICATING 4000 FT ON MODE C. I ISSUED ACR X A TURN TO THE R TO INCREASE SPACING WHILE ACR X CONTINUED HIS DSCNT. ACR X THEN RPTED THE ACFT IN SIGHT 'BELOW' HIM. THE MODE C READOUT OF THE VFR ACFT DID NOT CHANGE. IT REMAINED CONSTANT AT 4000 FT THROUGHOUT THE ENTIRE EVENT. I ASKED ACR X IF HE HAD THE ACFT ON TCASII. HE TOLD ME THAT THE ACFT APPEARED ON TCASII AT 4000 FT. I THEN ASKED THE PLT IF TCASII GAVE HIM AN RA AND HE SAID YES. IT TOLD HIM TO DSND. HAD THE PLT NOT SIGHTED THE ACFT AND ACTED ACCORDING TO TCASII RA; I MIGHT BE FILLING OUT DIFFERENT PAPERWORK. TCASII RESPONDED AS IT WAS SUPPOSED TO; NO DOUBT ABOUT IT. BUT THE TCASII SYS WAS ONLY SEPARATING THE 2 ACFT BY DATA; AND WHEN THE DATA IS IN ERROR; AS IT WAS IN THIS SIT; THE RESULTS COULD BE A TCASII INDUCED DISASTER.