It stays on all the time once pressed in. The pump runs all the time once pressed in, so it has chilled air available all the time. If you dial it up to 28 it adds hot air to the chilled air so you get dry warm air.
If you dial it down to 16 on a hot day it just puts out the chilled air.
It doesn't regulate the cooling by dropping the pump in and out, it does it by blowing more or less hot air into the chilled mix, and running the fan faster when farther from the set point, slowing as it nears it.
If it was minus 20 outside, you would press the centre button in again to turn the pump off and get maximum heat, as the air coming in is dry anyway at minus 20.
Today once I got rid of the hot air from the sunshine, I pressed the centre button which put the light out and stopped the pump and everything worked fine, because the air from outside was at about 12 degrees, so it could maintain 19 all right.
If I press the right knob to get recirculated air, the centre lamp comes on and the pump starts to dry the air so I don't mist up. (the centre lamp won't go out again though until I press the centre button in again)
If I twizzle the left knob until it points at either of the two windscreen symbols the same thing happens.
This is how it is supposed to work.
Edited by: mnl