ACR LGT FLC USES TCASII TO PROVIDE THEIR OWN SPACING WHILE ON APCH AT ORD WHICH MADE THE APCH CTLR VERY UNHAPPY.

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ON 22R ILS AT ORD; ATC ASSIGNED 170 KTS TO RIDGE OM. JUST PRIOR TO SALTZ INTXN (ABOUT 2 DME FROM RIDGE); VISUAL CONTACT WITH THE ACFT AHEAD INDICATED WE WERE CLOSING AND SPACING HAD BECOME MARGINAL. TCASII INDICATED DISTANCE AS JUST UNDER 3 NM. SPACING WAS MARGINAL AND DETERIORATING; IN ACCORDANCE WITH SAFETY WE SLOWED TO 140 KTS. ABOUT 1/2 MI FROM RIDGE; APCH CTL ASKED OUR SPD. WE STATED 140 KTS. APCH CTLR INSTRUCTED US TO GO R BACK TO 170 KTS AND HOLD THAT UNTIL RIDGE. WE BEGAN ACCELERATING AND NOTED THAT SPACING HAD BECOME A CONCERN AND THAT TCASII HAD SHOWED LESS THAN 3 NM SPACING AT ONE POINT. CTLR EMPHATICALLY TOLD US THAT SPACING WOULD BE DONE WITH HIS RADAR AND NOT WITH TCASII AND THAT OUR SPACING WAS NOW 5 NM. AT THAT POINT TCASII SHOWED 3.5 NM SPACING WHICH AGREED WITH VISUAL PICTURE. PASSED RIDGE; CONTACTED TWR. JUST PRIOR TO TOUCHDOWN; THE IN TRAIL ACFT CALLED 'RIDGE INBOUND' SO SPACING ABOUT 5 NM AND NO PROBLEM. WE SLOWED BASED ON VISUAL CONTACT AND CONFIRMED BY TCASII. WE WERE UNABLE TO ADVISE ATC PRIOR TO HIS INQUIRY DUE TO FREQ CONGESTION.