ALTDEV ALT EXCURSION AFTER TCASII RA.

Narrative

WE WERE ON A DOWNWIND LEG FOR AN ILS 27L AT ORD. OUR ALT WAS 5000 FT MSL AND FLT CONDITIONS WERE IMC DUE TO AN OVCST CLOUD LAYER FROM 2500 TO 6000 MSL. THE TCASII DISPLAY SHOWED US 5 MI IN TRAIL OF ANOTHER ACFT AND NUMEROUS ACFT ON PARALLEL APCHS TO ILS 27R AT ORD. I WAS FLYING AND THE CAPT WORKED THE RADIOS WITH ORD APCH. THE CTLR GAVE US A L TURN TO A HDG OF 360 DEGS. ONCE WE WERE LEVELED OUT ON THAT HDG THE TCASII ISSUED AN RA TO CLB AT 1200-1500 FPM AND INDICATED A TARGET MINUS 300 FT CLBING. DUE TO THE IMC CONDITIONS AND OUR COMPANIES RULES I COMMENCED A CLB AT 1500 FPM. THE CAPT TOLD ORD APCH OF THE RA AND OUR CLB. ORD APCH TOLD US TO IGNORE THE RA AND STATED OTHER ACR TFC 2000 FT ABOVE US. THE RA CONTINUED UNTIL WE REACHED 5800 FT MSL AND AT ONE POINT INDICATED THE TARGET AS MINUS 100 FT CLBING. ONCE THE TCASII ISSUED 'CLR OF CONFLICT' WE DSNDED IMMEDIATELY BACK DOWN TO 5000 FT MLS. THE WHOLE EVENT TOOK LESS THAN 1 MIN AND WE (NOR THE APCH CTLR) NEVER SAW THE INTRUDER. WE SUSPECT A TCASII SYS ERROR. A NORMAL APCH AND LNDG AT ORD WAS COMPLETED.