AN MD80 APCHING ORD RWY 27L ILS 4 MI IN TRAIL OF A B737 ENCOUNTERED SEVERE WAKE TURB AND DEVIATED FROM ITS ASSIGNED APCH COURSE DURING THE RECOVERY.

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DSNDING FROM 7000 FT TO 5000 FT AND 200 FT FROM 6000 FT WHEN WE WERE TOLD TO STOP THE DSCNT AT 6000 FT MSL; AND WE WERE SLOWING FROM 210 KIAS TO 170 KIAS IN TRAIL OF A B737 ABOUT 4 MI AND ALSO TOLD TO INTERCEPT THE LOC. SIMULTANEOUS ILS APCHS TO RWY 27R WERE IN USE. NUMEROUS LARGE ACFT IN THE AREA AS WELL. WE WERE GIVEN A TURN TO INTERCEPT RWY 27L LOC WHILE STARTING TO LEVEL AT 6000 FT; WITH AUTOPLT ENGAGED WHEN WE ENCOUNTERED A SEVERE WAKE TURB; ROLLING THE ACFT HARD R CAUSING THE AUTOPLT TO DISENGAGE. CAPT TOOK CTL OF THE AIRPLANE AND REQUIRED A L 90 DEG ROLL INPUT TO LEVEL; FOLLOWED BY A SEVERE WAKE TO THE L CAUSING SIMILAR INPUT TO THE R. IT WAS ALL WE COULD DO TO KEEP THE ACFT LEVEL. THIS LASTED ABOUT 15-20 SECONDS. UPON ROLLOUT; APCH BARKED; 'I NEED A HARD L TURN NOW!' IT TOOK A MOMENT TO REGISTER THAT DURING THIS WAKE ENCOUNTER; WE CONTINUED ON OR NEAR OUR HDG THROUGH THE ILS RWY 27L LOC AND INTO THE PATH OF THE RWY 27R APCHS. REALIZING WHAT WAS GOING ON; WE TURNED L AS INSTRUCTED AND WERE ADVISED OF TFC ON THE RWY 27R APCH AND HAD VISUAL CONTACT ON THE AIRPLANE. WE COULD NOT GET A WORD IN WITH ZAU. FINALLY; CAPT BARKED BACK AT ATC REGARDING OUR INABILITY TO COMMUNICATE WITH CHANNEL CONGESTION. WE TOLD ATC WHAT HAPPENED AND THEIR RESPONSE WAS; 'YOU ARE IN TRAIL OF A B737;' TO WHICH WE REPLIED THAT IT DID NOT MATTER WHAT WE WERE IN TRAIL OF -- WE WERE IN A WAKE TURB UPSET! WE INTERCEPTED THE RWY 27L LOC AND MADE AN UNEVENTFUL LNDG. BEFORE HANDING US OFF TO THE TWR CTLR; THE APCH CTLR DID APOLOGIZE. UPON LNDG; THE FLT ATTENDANTS KNEW SOMETHING HAD HAPPENED AND NUMEROUS PAX DEPLANING ASKED US WHAT THE ABRUPT TURNING WAS ALL ABOUT.