The heat is stuck on in my wife's 74 450slc (with the old manual HVAC system with the sliding levers). I have done quite a bit of diagnosis, and what I have found is that the heater (hot water) valve is always in the open position - that is, it is always flowing heated water. This valve is operated via a vacuum line coming from a metal vacuum switch (perhaps called vacuum solenoid) on the firewall. The valve operates as it should - open (heat) without vacuum and closed (no heat) when vacuum is applied. The vacuum switch that controls the flow of vacuum to the valve also operates as it should, switching vacuum flow to the valve on and off based on an electrical signal. The problem is that this switch is always seeing 12 volts, and hence keeping the heat on at all times. If I disconnect the electical plug from the vacuum switch, the heat goes off (valve shuts) but if I plug the electrical plug in, heat is always on (regardless of lever position). Does anyone have ANY idea where to look or what contols the electical side of the heating system? I looked at the levers, but they appear to be connected to cables only. ANY help would be GREATLY appreciated.

Thanks,

Alex