What was found
INFLIGHT, LOWER AFT BODY OVERHEAT LT ILLUMINATED. ACCOMPLISHED QRH PROCEDURES AND DETERMINED THAT THE LEAK IS BETWEEN NR 3 BLEED VLV AND PACK VALVE. HAD TO DESCEND TO FL250 TO CONTROL CABIN ALTITUDE AT 10000 FT. RAN ENGINES AT H/P. NO OVERHEAT LIGHT CHECKED PLUMBING IN AFT STAIR AND FOUND CLAMP LEAKING AT AIR CLEANER. TIGHTENED CLAMP. OHMED OUT SENSORS, CHECKING FOR SHORTS TO GROUND, NONE FOUND. CHECKED ALL DUCT WORK INSIDE AND OUT INCLUDING KEEL BEAM WITH HAND HELD FLIR IMAGING SYSTEM. ALL TEMPS WELL BELOW TRIGGER THRESHOLD OF OVERHEAT SYSTEM IAW TSM 26-14-000. REMOVED AND REPLACED CONTROL UNIT. OPS CK GOOD IAW AMM 26-14-000-5.