A B737-400 WAS DISPATCHED IN NON COMPLIANCE WITH WING BODY OVERHEAT WARNING ILLUMINATED. WAS DEFERRED PER THE MEL 49-1 APU INOP. INCORRECT MEL REFED.

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AFTER ENG START; CONFIGURED PRESSURIZATION PANEL FOR TKOF (PACKS ON; ISOLATION VALVE AUTO; ENG BLEEDS ON; APU VALVE CLOSED). WE HAD NOT YET SHUT DOWN APU WHEN 'L WING-BODY OVERHEAT' LIGHT ILLUMINATED. THIS OCCURRED APPROX 2-3 MINS AFTER PRESSURIZATION PANEL WAS CONFIGURED. WE ACCOMPLISHED CHKLIST PROC AND LIGHT EXTINGUISHED. I PULLED OFF TO THE SIDE AND CALLED MAINT CTL. THEY ASKED US TO RECONFIGURE PANEL NORMALLY WITH APU SHUT DOWN TO SEE IF LIGHT WOULD ILLUMINATE AS WE RETURNED TO THE GATE. IT DID NOT. AT ZZZ1 OPS I CALLED MAINT CTL AND THEY SAID THEY WERE GOING TO MEL THE APU (49-1) AFTER AN INSPECTION OF THE AIR CONDITIONING BAYS AND THAT THE PLANE 'HAD A HISTORY.' I WAS TO FIND OUT THAT THE APU HAD BEEN MEL'ED THE NIGHT BEFORE AND THAT THE MEL HAD BEEN REMOVED PRIOR TO US LEAVING ZZZ FOR ZZZ1. I THEN CALLED THE FLT OPS DIRECTOR AND WE AGREED THAT MAINT'S EXPLANATION WAS ACCEPTABLE. UPON FURTHER CONSIDERATION AND GIVING THIS CONSIDERABLE THOUGHT; I AM UNSURE THAT THE MEL 49-1 IS APPROPRIATE TO THE PROB WE HAD. HERE IS MY REASONING: THE 'WING-BODY OVERHEAT' ON THE GND IS AN 'OBSERVED FAULT' AND THERE IS NO MEL TO ALLOW DISPATCH OF THE CONDITION. MERE EXTINGUISHING OF THE LIGHT DOES NOT CORRECT THE CONDITION. WHILE I UNDERSTAND THAT THERE MAY BE A BLEED AIR LEAK ALONG THE KEEL BEAM DUCTING IN THE VICINITY OF THE APU UPSTREAM OF THE KEEL BEAM 'CHK VALVE;' THERE IS NO MEL DISPATCH PROC THAT ALLOWS FOR DISPATCH OF A KNOWN OR SUSPECTED BLEED AIR LEAK IN THIS AREA. FURTHERMORE; MEL 49-1 APU IS INTENDED TO BE USED FOR APU FAULTS; ELECTRIC OR PNEUMATIC OR OTHER INTERNAL CONDITIONS; NOT FOR BLEED AIR LEAKS EXTERNAL TO THE APU UNIT. WHILE THE METHOD USED TO DISPATCH AN ACFT IN THIS CONDITION MAY BE LOGICAL AND SAFE; THERE IS REALLY NO WRITTEN GUIDANCE TO THE OPERATORS (TO MY KNOWLEDGE) TO DO THIS. I SUGGEST THAT IF THE METHOD OF DISPATCH DESCRIBED ABOVE IS ACCEPTABLE TO BOEING AND THE FAA; THAT WRITTEN PROCS BE DEVELOPED; APPROVED AND PUBLISHED IN THE MEL WHICH INCLUDES BOTH MAINT AND OPS STEPS THAT ARE CLRLY DEFINED. AS A SIDE NOTE; THIS ACFT CONTINUED WITH THIS MEL TO ZZZ2.