B727 INITIATES TCASII MANEUVER WHEN VECTORS BY SJU FROM A B757 ARE INSUFFICIENT FOR STANDARD SEPARATION.

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ON DEP OUT OF TJSJ RWY 8. WE WERE CLBING OUT ON VECTORS TO INTERCEPT THE JAAWS 9 DEP PROFILE (326 DEG RADIAL OFF OF SJU VOR). WE HAD JUST INTERCEPTED THE JAAWS 9 326 DEG RADIAL AT APPROX 4000 FT MSL IN VMC WHEN DEP CTL GAVE US A TFC VECTOR OF 20 DEGS TO THE R. THIS VECTOR PUT US IN IMC. THE TCASII SYS WAS BEING DISPLAYED ON THE WX RADAR SCOPE; ON THE 10 NM RANGE; NO WX MODE. BECAUSE WE WERE ENTERING IMC THE RADAR SCOPE MODE WAS SWITCHED TO THE WX MODE AND THE RANGE WAS SWITCHED TO THE 30 NM RANGE. THESE SELECTIONS ENABLED THE RADAR SCOPE TO DISPLAY TCASII AND WX. AT THE SAME TIME THE ABOVE SELECTIONS WERE BEING MADE; DEP CTL GAVE US A TA. WE RPTED TO DEP WE HAD NO VISUAL CONTACT WITH THE TFC BECAUSE WE WERE IN IMC. WITHIN SECONDS OF THE TFC CALL; A TCASII TA WAS GIVEN BY THE TCASII SYS FOLLOWED SHORTLY BY AN RA WARNING. THE RA WARNING GAVE A 'DSND NOW' ADVISORY. THE DSCNT RATE DISPLAYED BY THE TCASII WAS 4000 FT DOWN. WE FOLLOWED THE TCASII ADVISORY UNTIL THE ALL CLR SIGNAL WAS GIVEN. WE ADVISED DEP CTL OF THE TCASII A WARNING AND THEN CONTINUED OUR CLB. DEP CTL APOLOGIZED FOR THE VECTORING PROBS THAT LED TO THE TCASII WARNINGS. CAUSE: POOR TFC FLOW AND VECTORING BY DEP CTL. PREVENTION: BETTER TFC MONITORING BY DEP CTL AND A REQUIREMENT FOR ALL PART 135 AND PART 121 CARGO CARRIERS TO BE REQUIRED TO INSTALL AND USE TCASII. NOTE: NORMALLY THE ACFT USED ON THIS RTE IS NOT EQUIPPED WITH TCASII. LUCKILY; WE HAD AN ACFT WITH A TCASII SYS THIS DAY.