Passenger side hot, driver side ok.

Hi,

I have a 2000 SLK320, which is a pleasure to drive.
But the passenger airvent seem to be stuck on hot,
while the driver side is working fine, cold & hot.

I did find some info on the side about the duo valve, which I have checked and seems to be in order.
the 3 wires which feed this component has 13.5V on wires 2-3 but nothing on wires 1-2 (I assumed that the duo valve is controlled seperatly)??
One of the valves seem to be staying warm, while the other is working according to the contollers.

I have replaced the duo-temperature sensor, which is behind the radio, and supposedly provides temparate information for driver-and passenger side.

I am not out of idea's.

Could someone tell me what Voltage the duo-valve should have?
somehow I expect, 13.5V between wire 1 and 2 and 13.5V between 2 and 3, thinking 2 is neutral.

or any other idea's would be great.