PNEUMATIC DUCT FAILURE DURING INITIAL APU START RESULTS IN A L WING-BODY OVERHEAT LIGHT AND DUST AND SMOKE IN THE GALLEY OF A B737-300. QUESTION WAS RAISED AS TO PROPER DEFERRAL BY MAINT.

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ON ORIGINATOR; WHEN APU BLEED AIR AND R PACK WERE TURNED ON; FLT ATTENDANT SAID THERE WAS A LOUD HOWLING NOISE AND SMOKE/DUST IN AFT GALLEY. SHORTLY THEREAFTER THE L WING-BODY OVERHEAT LIGHT CAME ON; THE QRH CHKLIST WAS COMPLETED AND ABOUT 1 MIN AFTER APU WAS SHUT DOWN; THE LIGHT EXTINGUISHED. I NOTIFIED DISPATCH AND MAINT CTL OF THE SIT. CONTRACT MAINT WAS CALLED AND WAS ADVISED OF THE SIT. HE TALKED WITH MAINT CTL AND WAS DIRECTED TO INSPECT THE APU AND DETERMINE ITS STATUS. THE APU WAS OPERATING NORMALLY AND CLRED BY MAINT; HOWEVER; MAINT CTL DECIDED TO MEL THE APU (MEL 49-1) TO ALLEVIATE ANY SUBSEQUENT PROB. LET ME SAY AT THIS TIME THAT ALL PARTIES HAD BEEN ADVISED OF THE WING-BODY OVERHEAT LIGHT; AND THE LOGBOOK WRITE-UP INCLUDED WING-BODY OVERHEAT AND SMOKE/DUST IN THE CABIN. WHILE DISCUSSING THE WRITE-UP AND MEL; DISPATCH AND I BOTH WERE UNEASY ABOUT HOW THE WING-BODY OVERHEAT DISCREPANCY HAD BEEN ADDRESSED AND IF IT HAD BEEN CLRED. MAINT CTL ASSURED US BOTH THAT THE MEL ON THE APU CLRED THE ACFT FOR FLT. WE PROCEEDED ON FLT TO BOISE; WHERE I AGAIN TALKED TO DISPATCH ABOUT THE SIT AND HE ADVISED ME THE PLANE WAS TO BE REMOVED FROM SVC IN LAS VEGAS. UPON ARR IN LAS VEGAS; IT WAS DECIDED BY DISPATCH; CHIEF PLT; PLT SAFETY REPRESENTATIVE AND MYSELF THAT THE CORRECT COURSE OF ACTION TO BE A COMPLETE INSPECTION OF THE APU AND ALL BLEED DUCTING TO FIND THE CAUSE OF THE PROB (WING-BODY OVERHEAT LIGHT) PRIOR TO CLRING THE ACFT FOR FURTHER FLT. THIS WAS ACCOMPLISHED BY MAINT IN LAS VEGAS AND A BLOWN BLEED DUCT SEAL OR CLAMP WAS DISCOVERED TO BE THE SOURCE OF THE PROB; HOT AIR HAD BLOWN OFF THE WING-BODY OVERHEAT SENSOR CAUSING THE LIGHT TO ILLUMINATE. THIS ALSO CAUSED THE SMOKE/DUST TO ENTER THE CABIN THROUGH THE AFT LAVATORY AREA. THIS WAS REPAIRED; TESTED NORMAL AND ALL SYS CLRED FOR FLT. ALTHOUGH; WHEN WE LEFT SEATTLE THE FO AND I BOTH BELIEVED THE PLANE WAS AIRWORTHY AND ALL PROPER CHANNELS AND CLRNCS HAD BEEN COVERED. LATER DISCUSSION WITH DISPATCH; CHIEF PLT AND SAFETY REPRESENTATIVE CONCLUDED THAT THE INITIAL LOGBOOK WRITE-UP INCLUDING THE WING-BODY OVERHEAT LIGHT PROBABLY HAD NOT BEEN SUFFICIENTLY CLRED WITH MEL 49-1. IT APPEARS THAT PREVIOUS SIMILAR INCIDENTS HAD GOTTEN THE ATTN OF FAA AUDITORS AND THEY INTERPRETED IT DIFFERENTLY THAN MAINT. I WOULD SAY WE ALL NEED TO GET ON THE SAME PAGE AND NOT PUT THE PLTS AND DISPATCH IN AN AWKWARD SIT IN THE CLRING OF THIS TYPE OF DISCREPANCY. I ALSO COULD HAVE UTILIZED THE CHIEF PLT CHANNEL AT AN EARLIER STAGE OF THE DISCUSSIONS FOR HIS INPUT.