ACR Y ISSUED VISUAL APCH CLRNC TO FOLLOW WRONG ACFT HAD LTSS FROM ACR X WDB. LESS THAN HVY JET SEPARATION FOR WAKE TURB. SITUATION: ATC EQUIP PROBLEM RADAR NON DETECTION OF ACFT BY TRACON RADAR TRACKING COASTING.

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I WAS WORKING APCH AT THE O'HARE TRACON; RWY 27R; SIMULTANEOUS VISUALS. THERE WAS A LOT OF COASTING AND TARGET LOSS OCCURRING; BOTH PRIMARY AND SECONDARY TARGET LOSS. ACR X WAS ONE OF THOSE COASTING AND LOSS OF TARGET PROBLEMS. I HAD ALREADY CLRED HIM FOR THE VISUAL APCH AND WAS WORKING WITH MODERATE TO HVY TFC. ACR Y WAS TOLD HE WOULD FOLLOW ACR X. AS I WAS WORKING WITH OTHER ACFT TO SET THEM UP FOR THE APCH; ACR X HAD COMPLETELY DROPPED OFF; BOTH DATA AND TARGET. WHEN I DID TURN ACR Y ON BASE; THERE WAS ONLY ACR MLG; THE PREVIOUS ACFT AS A REF POINT. SO I TURNED TO FOLLOW HIM; NOT REALIZING ACR X WAS STILL THERE. AFTER WORKING WITH OTHER DUTIES; ACR Y ASKED ME IF HE WAS REALLY FOLLOWING ACR X. I DID NOT SEE ANY TARGET AT THAT TIME UNTIL THE SECOND OR THIRD SWEEP. WHEN I DID SEE AND REMEMBER ACR X; IT WAS A VERY SMALL PRIMARY TARGET JUST INSIDE THE MARKER. ACR Y WAS APPROX 2 1/2-3 MI BEHIND HIM. I THEN PULLED ACR Y OFF OF THE APCH FOR ANOTHER SEQUENCE. IN MY OPINION; THIS SITUATION OCCURRED BECAUSE OF EQUIP PROBLEMS. HOW CAN I WORK WHEN I DO NOT SEE TARGETS. HOW ABOUT UPDATING EQUIP AT THE BUSIEST AND MOST COMPLEX ARPT. I HAVE TAKEN MY OWN ACTIONS OR TECHNIQUES TO HELP PREVENT THIS FROM HAPPENING OR GOING THIS FAR AGAIN. SUPPLEMENTAL INFO FROM ACN 199105. PARALLEL VISUALS IN USE. RADAR BEACON SYS WAS CONTINUOUSLY COASTING. WHEN THE SUPVR SWITCHED BEACON CHANNELS; ACR X DID NOT HOLD HIS TAG. THUS; THE ARR CTLR TURNED ACR Y INTO THE SAME GAP. ALT AND VISUAL SEPARATION WERE APPLIED AFTER THE SECOND ACFT QUESTIONED THE OP. ULTIMATELY ACR Y WAS TAKEN OUT AND RESEQUENCED.