A MECHANIC IS INFORMED HE IMPROPERLY PLACARDED AND DEFERRED A B757-200 WHEEL WELL FIRE WARNING SYSTEM AFTER THE AIRCRAFT REQUIRED AN AIR TURNBACK FOR A WHEEL WELL FIRE INDICATION.

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ACFT RETURNED TO GATE AFTER AIR INTERRUPTION WITH WHEEL FIRE INDICATION. WITHOUT DOCUMENTING THAT THE SYSTEM WAS INOP; SYSTEM WAS PLACARDED INOP. TAKE MORE CAUTION IN DOCUMENTATION OF WORK ACCOMPLISHED. IMPROPER PLACARDING OF SYSTEM WITHOUT DOCUMENTING SYSTEM ACTUALLY INOP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE AIR TURNBACK OF THE B757-200 WAS DUE TO AN INDICATION PROBLEM. THIS WAS AN ETOPS AIRCRAFT AND A DEFERRABLE ITEM PER THE MEL. NO OTHER PROCEDURE WAS REQUIRED; SO HE PLACARDED THE WHEEL WELL FIRE WARNING SYSTEM. HE DID CHECK BOTH MAIN WHEEL WELLS FOR ANY EVIDENCE OF FIRE. THERE WAS NONE. THAT'S WHY HE FELT COMFORTABLE WITH DEFERRING THE INDICATION SYSTEM. REPORTER STATED HE WAS LATER INFORMED HE DID NOT ACCOMPLISH A FAULT ISOLATION MANUAL (FIM) TEST OF THE WHEEL WELL FIRE WARNING SYSTEM; AND DID NOT DOCUMENT IN THE LOGBOOK OR ON THE MEL RELEASE PROCEDURES; NO EVIDENCE OF FIRE WAS FOUND. ALTHOUGH NONE OF THIS WAS REQUIRED IN THE MEL PROCEDURES; THE FAA BELIEVED HE PLACARDED AS INOPERATIVE; A FIRE WARNING SYSTEM THAT WAS OPERATIVE AND SHOULD NOT HAVE BEEN DEFERRED. REPORTER STATED IF HE HAD PERFORMED THE FIM TEST; AND THE SYSTEM TESTED OK; AND HE SIGNED OFF THE WRITE-UP RELEASING THE AIRCRAFT; ONLY TO HAVE THE SAME INDICATION PROBLEM HAPPEN AGAIN WITH ANOTHER AIR TURNBACK; HE WOULD STILL BE CORRECT IN DEFERRING THE WHEEL WELL FIRE WARNING PER THE MEL. REPORTER STATED THAT PART OF THE ETOPS PREFLIGHT WALK AROUND IS THE REQUIREMENT TO CHECK THE WHEEL WELLS FOR CONDITION.