ACR X FAILED TO TURN XPONDER ON. HAD LTSS FROM Y AT SAME ALT ASSIGNED.

Narrative

ON THE BEFORE TKOF CHKLIST; PRIOR TO OUR DEP FROM JFK; I INADVERTENTLY FAILED TO TURN ON OUR ACFT'S XPONDER. THIS WAS MOST LIKELY THE FIRST LINK IN A CHAIN OF EVENTS THAT LED TO A POTENTIAL CONFLICT. IN OUR ACR X TKOF FROM JFK'S RWY 31L; WE WERE CLRED THE KENNEDY 6 DEP CANARSIE CLB UP TO 5000 FT. TWR SWITCHED US TO THE DEP FREQ. I WAS FLYING THE ACFT AND THE CAPT WAS WORKING THE RADIOS. THE CAPT CHKED IN WITH DEP WITH CURRENT ALT AND CLRED ALT 'OUT OF APPROX 2000 FT FOR 5000 FT. DEP ACKNOWLEDGED OUR CALL. THERE WAS NO MENTION OF OUR XPONDER NOT BEING ON. A SHORT WHILE LATER DEP CALLED US AND CLRED US UP TO 7000 FT. WHILE PASSING APPROX 6000 FT THE CAPT SAW AN ACFT Y IN FRONT OF US. HE LOOKED DOWN AT OUR TCASII DISPLAY TO FIND OUT HIS DISTANCE AND ALT. IT WAS AT THIS POINT WE REALIZED THAT OUR XPONDER WAS NOT ON AND HENCE HAD NO TCASII INFO. HE THEN TURNED ON OUR XPONDER/TCASII AND THE ACFT SHOWED UP ON OUR DISPLAY AS BEING AT 3 MI IN FRONT; 300 FT HIGHER THAN US; AND MOVING L TO R. I STOPPED OUR CLB AT 6700 FT. WE THEN GOT A TCASII TA ALERT 'TFC; TFC.' SHORTLY AFTER THAT WE GOT AN RA ALERT 'MONITOR VERT SPD' WITH THE TA VSI SHOWING NOT TO CLB. DEP THEN CALLED AND GAVE US AN IMMEDIATE 90 DEG L TURN. HE ALSO GAVE THE OTHER ACFT A CLB. DEP THEN CLRED US TO OUR FLT PLAN RTE AND THANKED US FOR OUR HELP. WE ESTIMATE OUR DISTANCE WAS 2 1/2 MI AT THE CLOSEST POINT. OUR TYPE OF ACFT USED TO HAVE A FEATURE IN WHICH THE XPONDER CAME ON AUTOMATICALLY AT LIFTOFF. THIS FEATURE WAS TAKEN OUT WITH THE NEW TCASII MODIFICATION. WITH THE ALL TOO OFTEN CALL 'CHK XPONDER ON;' I WOULD LIKE TO HAVE THAT FEATURE BACK.