UNABLE TO COOL THE AIRCRAFT AFTER TAKE OFF; B767-400 RETURNS TO DEPARTURE AIRPORT. MAINTENANCE DETERMINES AN UNSECURED CARGO PALLET ROLLED AFT ON TAKE OFF AND ITS IMPACT WITH THE AFT BULKHEAD RENDERED TEMPERATURE CONTROL INOPERABLE.

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ON INITIAL APPLICATION OF TKOF THRUST; A SLIGHT THUMP WAS FELT IN THE COCKPIT; AKIN TO RUNNING OVER A HOLE IN THE RWY. AS ALL COCKPIT INDICATIONS APPEARED NORMAL; WE CONTINUED THE TKOF. DEP THROUGH 7000 FT WAS UNEVENTFUL. PASSING ABOUT 7000 FT FOR 10000 FT; WE NOTICED A LACK OF COCKPIT AIR CONDITIONING FLOW AND WARM COCKPIT TEMP. AT 10000 FT; WE MONITORED THE AIR CONDITIONING MAINT DISPLAY PAGE. TEMPS THROUGHOUT THE ACFT WERE IN THE MID 80'S; AND THE COCKPIT TEMP WAS 93 DEGS. PRESSURIZATION AND ALL OTHER INDICATIONS APPEARED NORMAL. WE ESTABLISHED COM WITH MAINT; BUT EVEN WITH THEIR ASSISTANCE WE COULD NOT DETERMINE CAUSE. THE COCKPIT TEMP WAS NOW 95 DEGS. I DECIDED TO RETURN TO ZZZ AND NOTIFIED FLT CTL. ACFT GROSS WT WAS 385000 LBS; WITH 16000 LBS IN THE CTR TANKS. I DECIDED TO DUMP FUEL DURING RETURN. WHEN CTR TANKS WERE EMPTIED; ACFT GROSS WT WAS 368000 LBS. I DECIDED TO EXERCISE EMER AUTH AND LAND OVERWT. I DID NOT DECLARE AN EMER AFTER CONSIDERING ACFT CONFIGN; VISIBILITY; RWY CONDITIONS AND FAMILIARITY WITH THE FIELD. AFTER LNDG AND TAXI TO GATE; A LOGBOOK ENTRY FOR OVERWT LNDG WAS MADE. THE THUMP TURNED OUT TO BE CARGO PALLET CONTACTING REAR BULKHEAD OF FORWARD CARGO BIN; WHICH INTERRUPTED AIR CONDITIONING TO ACFT CABIN.