A Captain reports about uncontrollable air conditioning high temperatures at cruise altitude on a B737-500 caused by Maintenance error. Crew could not control pack temperatures with temperature selectors in auto or manual. Temperature controller had been replaced earlier; a Functional Check was not performed. New controller was found to be cross-wired.

Narrative

The aircraft had two recent write-ups for trouble controlling the aircraft temperature. Corrective action had been taken; including replacing a temperature controller. We also could not control the pack valves with the temperature controllers in auto or manual. At cruise altitude; the left pack ran full hot and resulted in a duct overheat. The overheat produced a strong hot ducting smell in the cabin before it tripped and cooled off. The passengers and Flight Attendants were briefly alarmed and both pilots donned O2 masks and accomplished the Smoke/Fumes Checklists until we were sure the incident was from the duct overheat. Good air quality was regained and we continued to our destination. Here is the error that led to the overheat and smell: We discovered in the next few minutes of flight that the temperature controllers were cross wired. The left [temp selector] controlled the right pack valve and the right controlled the left pack valve. We caused the duct overheat by moving the right [temp selector] to manual hot to unstick a full cold right valve; but this caused the left valve to run full hot. Attempts to cool the left side with the left [temp selector] only kept the right full cold. When Maintenance replaced the temperature controller they failed to verify it was hooked up properly. A simple check with the APU running would have allowed them to confirm left to left and right to right.[There was] uncontrollable air conditioning due to Maintenance error. When replacing systems that are identical and parallel; Maintenance needs to ensure they are not cross wired. A Functional Test of the system controller should be required before the item is signed off.