PVT SMT PLT READS WRONG DME DURING FLUCTUATING READOUTS AND TURNS 3 MI TOO SOON ON STAR ARR PROC. HAD SWITCHED #2 VOR FOR A XRADIAL READING AND READ THAT DME INSTEAD OF #1 DME. FAILED TO ADVISE ATC OF NAV EQUIP PROB.

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EVENT: PREMATURE TURN ON STAR (KNEAD 1 ARR -- DALLAS; FT WORTH). ATC VECTORED ME FROM S TO INTERCEPT R213 CVE ON KNEAD 1 ARR. I WAS TRACKING CVE R213 INBOUND WHEN #1 AND #2 DME READOUTS BECAME INTERMITTENT AT ABOUT KNEAD (47 DME CVE). SWITCHED #2 VOR DME TO MQP TO IDENT TILLA (MQP R119 AND 42 DME CVE) EXPECTING TO TURN TO 080 DEGS AS SHOWN ON STAR. WHEN MQP R119 IDENTED; DME READOUT SHOWED 40 DME. SUSPECTING I HAD NOW PASSED TILLA (42 DME); I MADE IMMEDIATE R TURN TO 080 DEGS AS SHOWN ON STAR. ATC CALLED TO ASK HDG. I REPLIED 080 DEGS. ATC REQUESTED TURN TO 040 DEGS. I SWITCHED BACK TO CVE AND SAW DME READ 45 DME; WHICH MEANT I HAD TURNED TO 080 DEGS TOO EARLY. ATC SOUNDED IRRITATED; BUT NO MORE WAS SAID. CORRECTIVE ACTION: ACFT GNDED AND REPAIRS TO DME MADE. HUMAN PERFORMANCE: IN HINDSIGHT; I SHOULD HAVE IGNORED ALL DME INDICATIONS AND RELIED ONLY ON XRADIAL IDENT OF TILLA. SIGHT OF 40 DME (ACTUALLY FROM MQP) MADE ME THINK I HAD OVERSHOT TILLA. PROBABLY SHOULD HAVE DECLARED DME INOP AND ASKED FOR RADAR IDENT OF TILLA ALSO.