B737-300 GOT A TCASII RA WITH A C182RG WHILE FLYING S OF THE ASSIGNED RTE. THE CTLR WAS MORE CONCERNED WITH GETTING THE B737 BACK ON COURSE THAN WITH THE CONFLICT. B737 DSNDED BELOW ASSIGNED ALT TO SATISFY THE RA.

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I WAS FLYING WITH A BRAND NEW FO (3RD TRIP). WE WERE CLRED TO FLY THE 'ZIGGY' ARR INTO ONT. I INSTRUCTED THE FO TO START THE R TURN TO THE INBOUND RADIAL; BUT HE WAS CONFUSED WHILE USING THE LNAV FUNCTION OF THE AUTOPLT. I STARTED THE R TURN FOR HIM AND AT ABOUT THE SAME TIME WE RECEIVED A TCASII ALERT AND THE APCH CTLR ADVISED US WE HAD MISSED THE TURN ONTO THE COURSE. HE HAD A PERTURBED SOUND IN HIS VOICE (AS EXPECTED). AT THAT MOMENT WE GOT THE RA ALERT TO 'DSND.' HE THEN CANCELED OUR APCH RWY 26R CLRNC AND INSTRUCTED US TO MAINTAIN 5000 FT. WE HAD BEEN CLRED TO 4000 FT SO WE WERE DSNDING ALMOST 2600 FPM. AT THAT MOMENT I WAS NOW MORE CONCERNED ABOUT MISSING THE TFC THAT; ACCORDING TO OUR TCASII; WAS INDICATING 5000 FT I DON'T KNOW WHY THE CTLR ASSIGNED US THE SAME ALT; THAT PLAINLY COULD HAVE CAUSED AN ACCIDENT. THE CTLR NEVER DID ACKNOWLEDGE THE FACT THAT THE SINGLE ENG WAS EVEN THERE! DID HE KNOW?? IF SO; WHY DIDN'T HE ISSUE IT AS ADVISORY TFC?? IF SO; WHY DID HE ISSUE AN ALT THAT WOULD CAUSE MORE CONFLICT? WE DSNDED DOWN TO ABOUT 4600 FT MSL TO COMPLY WITH THE RA ALERT. I TOLD THE CTLR WE HAD TFC TO AVOID; AND ALL HE SAID WAS; 'WELL YOU'RE 4 MI S OF THE ARR!' HE STILL DID NOT MAKE MENTION OF THAT ACFT. WE REJOINED THE LOC AND LANDED UNEVENTFULLY. I WILL BE MORE AGGRESSIVE IN TAKING THE ACFT SOONER IN THE FUTURE. SUPPLEMENTAL INFO FROM ACN 365031: WE RECEIVED A TA/RA AND VISUALLY ACQUIRED A C182RG AT 2 O'CLOCK POS.