B737-800 PACK light
 

Dear All
A couple of questions about the PACK light on the B737-800
As far as my understanding of the system goes the PACK light will come on in case of an overtemp - at the compressor outlet, at the turbine inlet and at the pack output - and/or if both the primary and the stby pack/zone controllers fail.
During normal ops the controller controls the onside temperature control valve supplying bypass air that mixed with the cold air from the pack gives conditioned air. Should the primary onside controller fail, the cross-side temperature controller will operate the stby temperature control valve providing bypass air.
Failure of a single controller will be indicated by the respective PACK light illuminating upon Recall only together with both MASTER CAUTIONS and the AIR COND annunciator on the 6 pack.
Failure of both the primary and the stby controller will cause the light PACK-light to illuminate steadily - the temperature control valve will freeze in the last attained position and the pack will continue to operate normally unless an overheat will arise.
The question that bugs me is
1 if you loose both controllers shouldnt both PACK lights to come on at the same time? - the rqh does not list a dual PACK light procedure
2 if you have one PACK light only doesn t that mean that the only plausible cause is an overheat condition as the chances of loosing the primary onside controller and the stby function in the cross side controller are pretty slim?
3 in case of an overheat the qrh never prompts you to check the pack temperature with the temperature selector but it instructs you to select a warmer temperature to reduce the load on the pack in the attempt to cool it off. Why?
4 then you are instructed to resume normal ops if the light extinguishes - i guess the overheat is gone cos you can not reset the controller(s), can t you?
5 to isolate the affected pack if the light stays on after a reset - i guess cos the cause for the light to come on is an overheat
6 And to descend to a lower altitude as both pack valves may have closed as a result of an overheat - how do you know that the pack has overheated as opposed to both controllers have failed?

Many thanks