A Line Mechanic reports on how distractions and assumptions led to his disconnecting the Nose cowl Anti-Ice valve connector; instead of the Pressure switch connector on the # 1 engine of a B737-800. An air turnback to departure airport was required.

Narrative

Received an ACARS message ahead of arrival and prepared for deferral of #1 engine Anti-Ice (A/I) valve. Ran overtime list and held one Mechanic to assist; then gathered required tools and equipment; including cowling pump; ladders. etc. Glanced over applicable MEL and associate Maintenance Procedures Manual (MPM). Encountered difficulty with locking feature of High Pressure (HP) Modulating valve; but eventually secured in the 'Closed' position. Continued on with the Engine Cowl A/I valve MPM procedure. Locked valve 'Open' and as I proceeded with the next step to remove the connector; was informed of another potential maintenance issue with an interior item to address on the same plane. Returned to Task and failed to completely read the step and pulled the wrong connector. In hindsight; I had two opportunities to read the procedure completely. I made the assumption that since I deactivated the valve; the connector I was removing was on the valve; similar to my experience with MD80's. After the air interruption; we focused our attention on the HP valve since it had given us some difficulty during the Lockout procedure. Eventually removed the valve; suspecting an internal failure causing the cockpit annunciation but found nothing suspicious. At this point I questioned Maintenance Control as to whether or not we were overlooking something obvious. We came to the conclusion that is was best to replace both valves and associated seals to eliminate the problem; placards; etc. Maintenance Control called around shift change at 3 PM; for an update on parts and to assess the scenario and after conferring on their end momentarily; asked if I pulled the connector on the valve or the pressure switch...problem solved; error uncovered. Don't make assumptions and also realized that as much as we strive to minimize delays; if awareness and attention to detail costs a few extra minutes... so be it. The alternative can be a mess that never needed to happen.