B737-300 CREW OPERATED THE APU WHEN THE APU OVERSPD LIGHT CAME ON; UPON APU SHUTDOWN. MAINT DID NOT DEACTIVATE THE APU; AND HAD THE WRONG PLACARD ON THE APU.

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I ARRIVED AT ACFT 30 MINS PRIOR TO THE SCHEDULED DEP TIME OF XA30. DURING REVIEW OF THE ACFT LOGBOOK; ON THE FIRST PAGE I FOUND THAT THERE WAS A PLACARD FOR AN MEL ITEM. THE PLACARD READ: 'APU OVERSPD ON SHUTDOWN.' THE MEL ITEM FOR THIS DISCREPANCY IS 49-00-01A. THE FO WAS COMPLETING THE ACFT EXTERIOR INSPECTION. I REFED THE COCKPIT MANUAL; FOUND THE MEL ITEM: 49-00-01A; APU. THE REMARKS OR EXCEPTIONS SECTION STATED; 'MAY BE INOP PROVIDED;' AND THEN STATED 2 CONDITIONS. AS I CONTINUED TO READ THIS PAGE; THE AREA FOR PLACARD TEXT STATED THE WORDING TO BE USED FOR THIS PARTICULAR MEL ITEM: APU INOP. THIS WAS NOT WRITTEN ON THE PLACARD IN THE ACFT LOGBOOK. AS I WAS NOT SURE OF THE TRUE STATUS OF THE APU; I CONTACTED COMPANY MAINT THROUGH THE LCL COMPANY RAMP FREQ. I EXPLAINED TO THE MAINT TECHNICIAN OVER THE RADIO THE PLACARD WRITE-UP AND QUESTIONED WHETHER THE APU WAS OKAY FOR USE OR NOT SINCE THE PLACARD DID NOT SAY APU INOP PER THE MEL. THE MAINT TECHNICIAN RESPONDED THAT THE APU WAS 'OKAY TO USE.' BY THIS TIME THE FO ARRIVED IN THE COCKPIT AND ALSO HE SAID THE APU WAS GOOD TO USE BASED UPON HIS EARLIER RADIO CONVERSATION WITH COMPANY MAINT PRIOR TO MY ARR TO THE COCKPIT. BASED UPON BOTH RADIO CONVERSATIONS; WE STARTED THE APU NORMALLY. PRIOR TO START; DURING AND AFTER THIS INITIAL START; WE NOTICED NO ABNORMAL WARNING LIGHTS FOR THE APU. THIS WAS THE FIRST FLT OF A SCHEDULED 5 FLT DAY IN THIS ACFT. IF THE PLACARD HAD SAID APU INOP OR MAINT EXPLAINED THAT THE APU WAS NOT GOOD TO USE; THROUGHOUT THE DAY OF FLYING; WE WOULD HAVE STARTED ENGS USING ALTERNATE PROCS AND AN EXTERNAL AIR SOURCE AS OUTLINED IN THE MEL. DURING THE DAY; THE APU FUNCTIONED NORMALLY. NO WARNING LIGHTS OR FAILED APU STARTS. DURING PUSHBACK FOR THE 5TH FLT OF THE DAY; WE STARTED BOTH ENGS. FOLLOWING THE ENG STARTS; WE SHUT DOWN THE APU AND AT THAT TIME WE HAD OUR FIRST ABNORMAL INDICATION FOR THE DAY; THE APU OVERSPD LIGHT ILLUMINATED. THE ILLUMINATED OVERSPD LIGHT INDICATES: OVERSPD SHUTDOWN PROTECTION FEATURE HAS FAILED A SELF-TEST DURING A NORMAL APU SHUTDOWN. SINCE THAT HAD BEEN WRITTEN UP IN THE ACFT LOGBOOK; WE KNEW THE APU OVERSPD LIGHT WOULD NEED A POSSIBLE RESET AT OUR DEST PRIOR TO STARTING THE APU. UPON ARR AT THE DEST; A FLT OPS LINE CHK AIRMAN MET US; AS HE WAS TAKING THE FLT OUTBOUND. IT WAS AT THIS TIME DURING A VERY SHORT CONVERSATION; THAT I REALIZED I SHOULD NOT HAVE OPERATED THE APU DURING THE DAY'S FLYING. DURING A CONVERSATION WITH MY CHIEF PLT ON A SEPARATE MATTER; I EXPLAINED TO HIM WHAT OCCURRED WITH THE APU. I SAID I WOULD FOLLOW-UP WITH MAINT THE NEXT DAY. THE NEXT DAY; MAINT AGREED WITH ME THAT THE PLACARD TEXT WAS INCORRECT AND THAT IT WAS NOT 'OKAY' TO USE THE APU WITH THE OVERSPD LIGHT WRITE-UP. FOR MYSELF; I THOUGHT SINCE MAINT HAD SAID THAT THE APU WAS OKAY TO USE; THAT THERE WAS NO PROB USING IT FOR NORMAL PROCS. I BELIEVE I FOLLOWED THIS LINE OF THINKING BECAUSE DURING MY ACFT CHKOUT; COMPANY LINE CHK AIRMAN WOULD DISCUSS THIS PARTICULAR APU MALFUNCTION AND SHOW ME THE APU RESET BUTTON; LOCATED IN THE E&E COMPARTMENT; USED TO RESET THE APU SYS. PERHAPS BECAUSE OF MY RECENT TRAINING EXPERIENCE WHERE THE OVERSPD LIGHT WAS ILLUMINATED; AND RESETTING THE APU PRIOR TO START; I BECAME DESENSITIZED TO THIS PARTICULAR MALFUNCTION. NOT TO MENTION I AM NEW TO THIS ACFT TYPE. ADDITIONALLY; PLTS AND MAINT ARE GAINING NEW EXPERIENCE WITH THIS PARTICULAR ACFT TYPE AS THE COMPANY BRINGS THE ACFT OUT OF THE DESERT TO FLY OPS THAT REQUIRE A HIGH DISPATCH RELIABILITY. THE LESSON LEARNED HERE FOR ME IS; IF THERE IS A PLACARD; I SHOULD INSIST THE PLACARD HAS THE CORRECT PLACARD TEXT FOR THE LISTED MEL ITEM TO AVOID ANY CONFUSION AND RESTR USE OF AN ACFT SYS.