A Line Mechanic reports incorrectly deferring inoperative ground power lights on an A319 aircraft; minutes before departure; using a company operating manual instead of the MEL. Deferring of ground power lights in MEL; also required that Maintenance verify the avionics vent fan warning horn was operative on the ground.

Narrative

I received a gate call at XA58 for an XB00 trip (flight) concerning inoperative ground power lights. The grip came over as three of four ground power lights were inoperative on test. I immediately starting looking through the MEL for ground power lights; I used our Operating Manual; which at the time I thought (the power light) was cosmetic and serviceable; since only the light function was out. The aircraft was dispatched. I was informed after the aircraft left that the MEL card was located in Chapter 24 and it dealt with the ground power lights and an avionics vent fan light that was also not functioning. The avionics vent fan required that Maintenance verify the horn worked on the ground. This was not completed and the aircraft was dispatched with an illegal deferral. Once I was informed of my mistake; I immediately informed Maintenance Control and told them I had made an illegal deferral and could they have the item corrected at the next station downline. They complied with my request; assigned the item to the next station and told them to apply the correct MELs if they could not correct the problem.