A Maintenance Controller describes why he continued an MEL deferral of a B737-500 with a chronic cabin 'auto pressurization system' problem.

Narrative

Maintenance employee sent a deferral on aircraft 'automatic pressurization system.' Per the maintenance manual 21-30-00 troubleshooting the system tested normal. Fault code 30 showed low flow/high leak. He was unable to perform a pressure health check due to not enough personnel. Without looking at the maintenance manual; troubleshooting reference and the fault code given along with the 3 questions on the deferral: 1) the original discrepancy of the MEL 'DID NOT' change; 2) the aircraft 'WAS' still configured per the MEL Dispatch Deviation Procedures; 3) Troubleshooting 'WAS' performed; I felt the MEL was correct and just needed further troubleshooting. I will request and confirm any maintenance manual troubleshooting and fault codes given on any deferrals. Supplemental information from ACN 817068: Inbound (flight) report: automatic pressurization failed at FL360 regained control of cabin in alternate mode at FL210. I placarded automatic mode inoperative. Reference was the MEL 21. Callback conversation with reporter ACN 817069 revealed the following information: Reporter stated the Manual AC; Manual DC and Standby position pressurization controls worked fine on the B737-500. But in the auto mode position; the 'auto fail' light illuminating was a chronic problem. The aircraft was grounded at a downline station; where the Pressurization Health Check was accomplished. Reporter stated he was told a plastic tie wrap was found restricting the main (aft) outflow valve. Reporter stated that didn't make any sense; because the pressurization system worked fine in the manual and standby modes and was originally deferred because they were working fine; except for the position. Reporter stated the main (aft) outflow valve was replaced and dirt was found in the aft cargo drain valve.