B737-800 Captain raises some interesting points about allowing the crew to placard a single pack controller failure prior to takeoff. This pack light on test could also indicate a ram air door actuator failure which has a performance penalty.

Narrative

Departing on an out and back the First Officer tested the Master Caution system and received a PACK light. The PACK light went out on reset. With the information we have this indicates a single pack controller failure. This is crew placardable with MEL 21-20a. Although I do not know how to tell if we use A) primary or B) secondary for the placard. We completed the turn and were to keep the aircraft for our third leg. Maintenance came to the plane to make a balancing entry and performed a bite check. The bite check indicated a pack temperature controller failure which was placarded with 21-20a. My problem now is that although we are given the ability to placard; the systems knowledge from the MEL book is not comprehensive enough to account for the other problems that can be revealed during a bite check. Without being able to diagnose the problem; I do not believe we should be placarding the pack light that resets. I have also learned that when Maintenance does a bite check that it can be a ram air door actuator problem. This would be MEL 21-04 that requires a weight penalty correction.