Since you are getting hot air then the water valve must be open. You really need a wiring diagram to deal with it but basically the valve is always powered and is grounded (to be closed) by the push button controller. The ground side of the circuit is where you usually find the problem but a bad fuse seven would do the same thing. It probably would shut down more than the water control though.
For diagnostic purposes I would block off the water flow by squeezing the hose or removing it and putting a plug in it. If this gets you good A/C follow it up.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
[This message has been edited by stevebfl (edited 05-26-2000).]