An Avionics Mechanic reports he was so focused on verifying the DC-9 Take-off Warning System was operating correctly; that he failed to accomplish other required tests on the Switch Packs that were replaced in the Center pedestal. An air turnback was performed due to aircraft would not pressurize.

Narrative

Through others and my troubleshooting; the big picture to the problem that caused such air turnback was overlooked. The original problem was for a Take-off Warning issue. In the repair of such problem; by replacing of the left Switch Pack in the pedestal; another problem arose that was the cause of the incident in question. This Switch Pack was not replaced by myself and was worked on the ramp as I have been told. The aircraft came to the hangar where I work for further troubleshooting. In the short sign; the replaced Switch Pack was of no help and the other Switch Pack was suggested as to be the cause of the write-up. I could not verify through my troubleshooting that this was the problem; but I could not disprove the finding of earlier troubleshooting that the other Switch Pack was indeed the issue. So I changed the second Switch Pack as per earlier troubleshooting indicated. The test of the Take-off Warning System was successful with no other faults indicated. In regard as to such; I signed off the Log write-up as OK for service. In the meantime; forgotten were the other tests for the Switch Pack installation. Subsequently one of the systems that the Switch Pack also controls failed to function properly; which did not allow the aircraft to pressurize properly. Through my saying the Take-off Warning System was Ok for service; I was ultimately responsible to be sure that all systems on said Switch Packs that were replaced were in proper working order. The human factor of being focused on the Take-off Warning System operating correctly overshadowed the other issues involved and a rushed judgment occurred. As a result an air turnback occurred. I deeply regret others and my actions that allowed this to occur. I take pride in my work and work ethics and am deeply disappointed that ultimately I allowed this to occur.