A DHC8-102 WAS DISPATCHED IN NON COMPLIANCE WITH THE INCORRECT RUDDER HYD PRESSURE SWITCH INSTALLED.

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ON JAN/XA/00 AN ACFT CAME IN WITH A LOGBOOK ENTRY THAT THE #2 RUDDER HYD CAUTION LIGHT CAME ON AND STAYED ON INFLT. THE LEAD MECH TOLD ME WHAT PART NEEDED TO BE CHANGED AND HAD GOTTEN THE PART FROM THE PARTS ROOM. NEXT; HE SHOWED ME WHERE TO PUT THE PART IN THE ACFT; WHICH WAS IN THE TAILCONE (THE TAILCONE IN THE DHC8 IS A POORLY LIT AREA WITH THE EXHAUST PIPE FROM THE HEAT EXCHANGER RUNNING THROUGH IT). AFTER I STARTED TO INSTALL THE FIRST SWITCH (HOLDING A FLASHLIGHT IN ONE HAND AND REMOVING AND REPLACING WITH THE OTHER HAND) THE LEAD MECH FOUND OUT THAT BOTH SWITCHES WOULD HAVE TO BE REPLACED. HE ADVISED ME THAT I WOULD HAVE TO CHANGE BOTH PRESSURE SWITCHES. THE LEAD MECH THEN ASKED ANOTHER MECH TO GET ANOTHER SWITCH AND TO USE THE PART NUMBER FROM THE BOX WHICH THE PART I WAS INSTALLING CAME IN. NORMALLY; WHEN TASKED WITH A JOB; I GO TO THE ILLUSTRATED PARTS CATALOG AND LOOK UP THE PARTS MYSELF; BUT SINCE HE HAD LOOKED UP THE INFO ALREADY; I THOUGHT HE HAD THE CORRECT PART NUMBERS. THE OTHER MECH CAME BACK WITH THE PART AND I INSTALLED IT. I DID NOT KNOW AT THE TIME THAT THE SECOND SWITCH I INSTALLED SHOULD HAVE BEEN A DIFFERENT PART NUMBER AND SINCE THE PARTS LOOK ALIKE I DID NOT NOTICE THE DIFFERENCE IN THE POORLY LIT TAIL CONE. THE LEAD MECH CAME TO ME AFTER I HAD COMPLETED INSTALLING BOTH SWITCHES AND CLOSING THE ACCESS PANEL TO ADVISE ME THAT THE SWITCH THAT NEEDED TO BE CHANGED WAS ON THE RUDDER ACTUATOR AND THE SWITCHES I HAD REPLACED WERE TO THE PWRED FLT CTL SYS. I THEN WENT AND LOOKED UP THE PART NUMBER FOR THE PRESSURE SWITCH ON THE RUDDER ACTUATOR THAT NEEDED TO BE CHANGED AND INSTALLED IT. I THEN DID AN OPS CHK; WHICH WAS GOOD. WHEN THE ACFT WAS TAXIED TO THE GATE; I NOTICED THAT THE RUDDER PRESSURE LIGHT CAME ON AFTER A DELAY OF ABOUT 5-7 SECONDS. WHEN I DID THE OPS CHK; I DID NOT WAIT THAT LONG BEFORE MOVING TO THE NEXT STEP AND IT WAS NOT NOTED ON THE WORK CARD. THE RUDDER PRESSURE LIGHT WAS MEL'ED (DEFERRED FOR LATER MAINT) AND 3 DAYS LATER WAS FERRIED TO A MAINT BASE TO BE FIXED. THAT'S WHEN IT WAS DISCOVERED THAT THE SECOND SWITCH THAT I INSTALLED WAS THE INCORRECT PART. THE CORRECT PART WAS THEN INSTALLED.