Will adjustment on the cartridge size

AIR-CON AMPLIFIER affect how long the dash centre vent is active?

 

Yes, indirectly.

The dashboard center vent opens in cooling mode. It is closed in heat, defrost, and blend modes.

The amp adjustment is used to correct a difference of selected temp versus provided temp. For example, if the selected cabin temp was 75 but the system cools the cabin down to 68 degrees you'd use this adjustment to correct the problem.
If a mis-adjusted amplifier was over-cooling the cabin, the center vent would be open for a longer period of time. Adjusting the amp for correct performance would reduce the amount of time the center is open....as the system would be in cooling mode for a shorter duration

Cheers
DD

 

However, my center vent has Never opened. Ever! Nor has it since I've owned the car.
I know the actuator is good from exhaustive testing.
So in spring I prop it open for AC, and in fall (pretty soon now) I let it close for winter heat.
(';')

 

The defroster vent and center vent are interconnected mechanically. If the defroster vent doesn't close, the center vent cannot open.
The vacuum source for the center vent and the defroster vent are controlled by solenoids. So you need to check operation of the solenoids, the vacuum source and lines to the solenoids, and the electrical signals to the solenoids. Any faults will prevent proper operation of the center vent.

Jon