A B737-800 ON PREFLT TEST FOUND #1 FUEL LOW PRESSURE LIGHT NOT ON WITH PUMP OFF. LIGHT PUSH TO TEST OK. REFUSED DEFERRAL OF PRESSURE SWITCH INDICATION.

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I WAS FACED WITH MAKING A DECISION ABOUT AN MEL ITEM. IT WAS 28-03; FUEL LOW PRESSURE LIGHTS. THIS ITEM HAD PREVIOUSLY BEEN PLACARDED AND SIGNED OFF. THIS WAS A REPEAT OF THIS SAME ISSUE. DURING PREFLT; WE DISCOVERED THAT THE #1 FUEL LOW PRESSURE LIGHT WAS NOT ON WITH THE FUEL PUMP SWITCH IN THE 'OFF' POS. THE LIGHT SHOULD HAVE BEEN ILLUMINATED SINCE THERE WAS A LOW PRESSURE CONDITION AT THE TIME. WE PRESSED THE LIGHT AND IT PRESSED TO TEST NORMAL (LIGHT WORKED). WE CALLED MAINT AND THE MECH ARRIVED AND TOOK THE LOGBOOK ONTO THE JETBRIDGE AND BEGAN TO PLACARD THE LOW PRESSURE LIGHT INOP AGAIN. I FELT THIS ITEM COULD NOT BE PLACARDED UNDER THE MEL BECAUSE THE ITEM (28-03) WAS THE LOW PRESSURE LIGHT AND IT WORKED NORMAL (PRESSED TO TEST NORMAL). I QUESTIONED THE WISDOM IN PLACARDING THIS AND WAS TOLD THE LIGHT AND THE LOW PRESSURE SWITCH WERE ONE IN THE SAME. I EXPLAINED TO THE MECH THE WAY THIS ITEM WAS TIED INTO THE MASTER CAUTION SYS AND HOW IT WAS SUPPOSED TO WORK AND IN FACT HOW; WITH THIS CONDITION; IT WAS NOT WORKING NORMAL. IF THE PUMPS ON THE L SIDE HAD BOTH FAILED THE MASTER CAUTION SYS WOULD NOT HAVE ALERTED US BECAUSE THE ACTUAL PROB WAS WITH THE LOW PRESSURE SWITCH AT THE PUMP (NOT SENSING LOW PRESSURE) NOT THE LIGHT. THIS COULD RESULT IN AN ENG FLAMEOUT. WE WERE FLT PLANNED AT FL390. I REFUSED TO TAKE THE AIRPLANE IN THIS CONDITION AND I ASKED FOR A SUPVR. AFTER EXPLAINING THIS AGAIN TO THE SUPVR; A NEW LOW PRESSURE SWITCH WAS ORDERED AND REPLACED. THIS FIXED THE PROB AND WE DEPARTED. I SPOKE WITH SOME FOLKS AT MAINT CTL AND THEY PASSED THIS INFO AROUND AND I WAS TOLD THAT THIS PRACTICE OF PLACARDING THE LIGHT INSTEAD OF CHANGING OUT OR CHKING THE LOW PRESSURE SWITCH WAS GOING TO CONTINUE. THE LOW PRESSURE SWITCH IS NOT IN THE MEL AND IS; THEREFORE; MANDATORY TO BE OPERATIONAL. HOW CAN WE PLACARD THIS ITEM UNDER THE LIGHT WHEN THE PROB LIES WITH THE LOW PRESSURE SWITCH? GIVEN THE OUTCOME; IF BOTH PUMPS FAIL AND THERE IS NO ALERT THROUGH THE MASTER CAUTION; WHY WOULD WE CONTINUE THIS PRACTICE?