A B737-300 RETURNED TO THE FIELD 3 TIMES DUE TO GEAR GND SENSING PROBS. FIRST INCIDENT WAS AURAL WARNING; SECOND GEAR HANDLE WOULD NOT RELEASE; THIRD; GEAR HANDLE WOULD NOT RELEASE DUE TO SAFETY SENSOR INSTALLED WRONG.

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I WAS ASSIGNED TO ASSIST IN TROUBLESHOOTING A LNDG GEAR PROB ON A COMPANY B737-300. THE ACFT HAD ALREADY MADE 2 AIR TURNBACKS EARLIER IN THE DAY -- FIRST FOR AURAL WARNING AFTER TKOF; AND SECOND BECAUSE GEAR LEVER WOULDN'T MOVE OUT OF THE 'OFF' POS AFTER ROTATION. MY CREW DETERMINED THE CAUSE TO BE A FAULTY MAIN GEAR AIR SENSE PROX SWITCH. MY CO-WORKER REMOVED THE SENSOR AND SPLICED IN A NEW ONE; THEN I COMPLETED THE INSTALLATION BY MOUNTING THE SENSOR; WHILE HE BEGAN GIVING A 'TURNOVER' TO THE ONCOMING CREW. I HELPED SET THE SENSOR TO TARGET GAP PER THE ACFT MAINT MANUAL. WE THEN COMPLETED OUR DAY BY TURNING OVER THE SYS CHKOUT TO THE RELIEF CREW. THE FOLLOWING DAY I WAS INFORMED THAT THE ACFT HAD MADE A THIRD AIR TURNBACK ON ITS FIRST FLT ATTEMPT FOLLOWING OUR MAINT. INSPECTION REVEALED THAT I HAD IMPROPERLY MOUNTED THE NEW SENSOR. THE FACE WAS INADVERTENTLY POSITIONED 90 DEGS FROM THE TARGET. SOMEHOW I HAD STILL BEEN ABLE TO SET THE PROPER GAP -- BTWN THE TARGET AND THE SIDE OF THE SENSOR HOUSING! WHILE I WAS MOUNTING THE SENSOR THE PREVIOUS EVENING; I WAS CONVINCED THAT I WAS DOING IT CORRECTLY; SO DID NOT BOTHER TO LOOK CLOSELY AT THE ILLUSTRATION IN THE MAINT MANUAL. THIS HAD BEEN POOR JUDGEMENT -- ESPECIALLY SINCE I HAD NOT ACTUALLY OBSERVED THE REMOVAL OF THE SENSOR. I COULD IMMEDIATELY SEE MY MISTAKE; AND THAT THERE WAS AN ENTIRELY SEPARATE FLANGE AND SET OF MOUNTING HOLES ON THE SENSOR -- CLRLY NOT 'DUMMY PROOF!' OF COURSE; THIS MISTAKE WOULD HAVE BEEN CAUGHT PRIOR TO RETURNING THE ACFT TO SVC; HAD OUR RELIEF CREW PERFORMED THE SYS CHKOUT CORRECTLY PER THE ACFT MAINT MANUAL. APPARENTLY; THEY WERE ABLE TO RETRACT THE GEAR WITH THE ACFT ON JACKS; BUT THEY INADVERTENTLY USED THE GEAR LEVER SAFETY RELEASE TRIGGER; WHICH COMPLETELY BYPASSES THE AIR/GND SENSE SYS THAT THEY WERE SUPPOSED TO BE TESTING.