AN ACR B737 FLC STARTED THEIR SID TURN TOO EARLY AND WERE CUTTING OFF THE PREVIOUS DEP. THE DEP CTLR INTERCEDED VECTORING THEM OUT OF HARM'S WAY.

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CLRED FOR TKOF RWY 34R WITH MOUNTAIN 4 DEP. WE HAD ALREADY BRIEFED THE APPROPRIATE T-PROC (ENG OUT) AND DEP FOR RWY 34R SO WE EXECUTED OUR TKOF. THE CAPT CALLED FOR GEAR UP THEN PASSING 3000 FT FOR AFTER TKOF CHKLIST. AT THAT POINT HE AGAIN READ THE MOUNTAIN 4 DEP ALOUD AS I DID THE AFTER TKOF CHKLIST. AS HE READ CROSS D8 SEA AT OR ABOVE 4000 FT THEN R HDG 070 DEGS HE ASKED FOR 070 DEGS IN THE HEADING WINDOW AND SAID WE'LL HAVE NO PROB MAKING THE XING RESTR. SOON AFTER THAT HE STARTED A R HAND TURN. I CLRED HIM R. MOMENTS LATER DEP CALLED AND SAID WE HAD TURNED TOO EARLY AND CUT SOMEONE OFF; TURN BACK L TO 340 DEG HDG. AFTER FURTHER VECTORS WE WERE BACK ON COURSE WITH NO INCIDENT. I BELIEVE THE CAPT READ EVERYTHING CORRECTLY; HE JUST WAS THINKING 4000 FT AND OUR T-PROC TURN AT 4.3 DME SOMEHOW TOOK THE PLACE OF 8 DME. I WAS PREOCCUPIED WITH RADIO CALLS AND AFTER TKOF CHKLIST AND JUST HEARD HIM READ EVERYTHING CORRECTLY BUT DO THE WRONG OR EARLY TURN. AFTER A MORNING OF SOLID TSTMS AT LAX AND OUTSTANDING CREW COORD AND PROCS THERE MAY HAVE BEEN A LET DOWN WITH THIS VFR TKOF. SUPPLEMENTAL INFO FROM ACN 392984: AT 4000 FT ON 338 DEG RADIAL I SAID 'WE HAVE PASSED 4000 FT.' I THEN GLANCED AT SID AND THOUGHT IT READ AT 4000 FT TURN TO 070 DEGS. I REQUESTED 070 DEG HDG SELECT AND BEGAN TURN. AT THIS POINT I WAS PROBABLY 4-5 DME. AFTER TURN BEGAN; ATC SAID 'TURN TO HDG 340 DEGS.'