99 S Class Air Conditioning issues

Hi all,
I have recently bought this beast and although the air con works when the car is stationary, once I start the car moving the vents seem to shut off!
The center console vents close completely when the setting on the dial is auto. The side vents seem to let air in from outside, but not from the engine, for example; the air con is on but the heat/cold air is whatever is outside. Also if the air con setting is on auto, then the fans do not increase to try to get the temp requested?
any ideas please?
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sounds like a regas to me. sytem shuts down when pressures are out of range,

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