FLC CHANGED FREQ BEFORE CLRED BY APCH CTL.

Narrative

CLRED FOR VISUAL APCH BY ORD APCH BUT TOLD TO MAINTAIN 250 KTS AS LONG AS POSSIBLE. FO SWITCHED TO TWR ABOUT 8 MI FROM MARKER FOR RWY 22R. TWR FREQ SO BUSY THAT COULD NOT GET WORD IN. APCH MEANWHILE PUT A 737 AHEAD OF US. WE PICKED UP TFC AND TRIED TO SLOW AND MANEUVER TO MAINTAIN DISTANCE WITH 737. TWR FINALLY CALLED US AND TOLD US TO GO BACK TO APCH. APCH CTL JUMPED ALL OVER US SAYING WE SHOULD HAVE STAYED WITH THEM UNTIL XING THE MARKER EVEN THOUGH CLRED FOR VISUAL APCH. CTLR STATED THEY NEVER SWITCH ANYONE TO TWR UNTIL XING THE MARKER. I FEEL COMMENT IMPLIED LCL KNOWLEDGE THAT OUR CREW COULD NOT HAVE KNOWN AS MOST OF US FLY INTO ORD ONCE A MONTH OR SO. PLUS; FAILURE TO INFORM US THAT THEY WERE GOING TO PUT ANOTHER ACFT IN FRONT OF US WHILE REQUESTING US TO GO 250 KTS AS LONG AS POSSIBLE IS SETTING UP A PROB. A CONTRIBUTING FACTOR WAS ACFT HAD A GENERATOR DEFERRED REQUIRING MONITORING OF ELECTRICAL LOADS BY MYSELF AND THE AUTO PRESSURIZATION WAS ERRATIC. SO I HAD TO MANUALLY PRESSURIZE ACFT; THUS DISTRACTING ME FROM MORE VIGILANT MONITORING OF APCH. FINALLY; I FEEL ORD APCH NEEDS TO BE AWARE OF THE RIGHTS IMPLIED WITH GIVING A VISUAL APCH WITH NO RESTRS/CLRNCS.