ACFT EQUIP PROB. ACR TURNS THE WRONG DIRECTION TOWARD THE NEW HDG.

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OUR FLT WAS BEING VECTORED OFF THE BOIDS 4 ARR TO A FINAL FOR RWY 18R. WE WERE GIVEN SEVERAL VECTORS TO FIT INTO A NORTHERN FLOW FOR THE DOWNWIND FOR RWY 18R TYPICAL FOR THE BOIDS 4 AND WE DISCUSSED THIS POINT AFTER THE CLRNC. OUR INCIDENT OCCURRED AFTER SEVERAL VECTORS CLOSE TOGETHER TO FIT INTO THE RWY 18R FLOW. WE REMEMBER BEING ON A 360 DEG HDG. WE INITIATED THAT TURN AND THE CTLR GAVE US 350 DEG HDG AND A DSCNT TO 3000 FT. CAPT X WAS IDENTING THE ILS FREQ AND CHANGING THE FMS TO REFLECT THE FIX YOHAN FOR A LONG FINAL. FO Y SELECTED 360 DEGS AND 350 DEGS IN THE HDG SELECT MODE AND THOUGHT HE WAS TURNING TO THIS HDG AFTER PULLING THE KNOB. AT THIS MOMENT; CAPT X LOOKED UP AND NOTICED THE AIRPLANE IN A R TURN INSTEAD OF A L TURN TO 350 DEG HDG. FO; Y; STARTED AN IMMEDIATE CORRECTION WHEN THE CTLR CALLED TO INSTRUCT US TO KEEP TURNING TO 090 DEGS TO AVOID THE E TFC FLOW. WE CONTINUED THE TURN TO 270 DEGS AND WENT BACK AROUND TO THE RWY 18R FLOW OF TFC. WE DO NOT KNOW WHY THE AIRPLANE DIDN'T TURN L TO 350 DEGS IN HDG SELECT. WE DID NOT NOTICE THAT THE AIRPLANE HAD STARTED TURNING R INSTEAD OF L UNTIL APPROX 060 DEGS. WE CONFIRMED WITH THE CTLR THAT NO TFC CONFLICT WAS CREATED AND PROCEEDED BACK AROUND FOR THE ILS FOR RWY 18R WITH A NORMAL LNDG IN DFW.