A MECHANIC REPLACING AN AIR CONDITIONING PLENUM ASSEMBLY ON A B737-700 IS LATER INFORMED HE DID NOT INSTALL THE DIFFUSER SECTION WITH THE PLENUM ASSEMBLY.

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I REMOVED AND REPLACED THE PLENUM. THE PLENUM I REMOVED DID NOT HAVE A DIFFUSER AS PART OF THE ASSEMBLY. THE PLENUM I ORDERED FROM STOCK DID NOT HAVE A DIFFUSER ASSEMBLY. THE MAINT MANUAL REFERRED TO THE PART AS A PLENUM/DIFFUSER ASSEMBLY. THE PLENUM HAD A SERVICEABLE TAG. I THOUGHT THE PLENUM I HAD RECEIVED FROM STOCK WAS A COMPLETE ASSEMBLY AND I INSTALLED IT IN THE ACFT. THERE WAS NO INDICATION THAT I HAD AN INCOMPLETE ASSEMBLY. THE PART FROM STOCK WAS IDENTICAL TO THE PART I REMOVED FROM THE ACFT. THE PART NUMBERS WERE IDENTICAL AND THERE WAS A SERVICEABLE TAG ATTACHED. THE ONLY WAY I COULD HAVE AVOIDED THIS SITUATION WAS IF I HAD PRIOR KNOWLEDGE OF THE ASSEMBLY. TO AVOID THIS IN FUTURE MAINT INCLUDED A NOTICE ON THE COMPUTER PARTS ORDER PAGE THAT STATES IF ORDERING A PLENUM; TO ORDER A DIFFUSER ALSO. I WOULD PREFER THE PLENUM AND DIFFUSER BE ASSEMBLED PRIOR TO GOING INTO STORES AND HAVE A SERVICEABLE TAG THAT COVERS THE ENTIRE/COMPLETE ASSEMBLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THE PLENUM ASSEMBLY ATTACHES TO THE PRIMARY AND SECONDARY HEAT EXCHANGERS IN THE AIR CONDITIONING BAY. THE B737-700'S PLENUMS ARE SLIGHTLY DIFFERENT AND EASIER TO REMOVE AND REPLACE THAN THE OLDER CLASSIC B737-300/500 ACFT. THE DIFFUSER IS INSTALLED WITH THE PLENUM AND HELPS TO REDUCE THE FORCE OF THE AIR FLOW. REPORTER STATED IT APPEARS THE PLENUM HE HAD REMOVED; MAY ALSO HAVE HAD THE INNER DIFFUSER NOT INSTALLED AND THAT MAY HAVE BEEN CONTRIBUTING TO THE AIR CONDITIONING FLIGHT WRITE-UPS THAT HAD BEEN PLAGUING THAT PARTICULAR ACFT.