1989 420SEL Climate Control - Pain

[Piddo]

Problem:


Climate controls respond to cold/hot setting.


The fan works (variable speeds at both auto and overide settings) and hot/cold settings are fully operational.



With the exception of the windscreen (defrost setting), there is no air from any front vent. However, the rear passenger vent is fully operational at all fan speeds (auto and manual) and temperature settings.


In pursuit of the problem source, I have replaced the Change Over valve (MB 126 800 0978) and tested the operation of the push button control unit against a spare - no difference.


It is possible that I have a problem in one of the vacuum operating units. But which one (MB # ) would be helpfull and were is it located ?