HVAC vacuum climate control, 78-79

The early 928 vacuum-actuated climate control system can be complicated to understand. I found that the schematics from the 1978 Service Info Manual (pgs 130-131) were inaccurate and lacking clarity, which led to unnecessary confusion.

Here, you'll find a revised schematic intended to clear up issues found in the original publication. Primary focus is for 1978 with either an manual or automatic center vent, however other important information is included. And though 1979 was manual center vent only, this still may be a useful companion to the 1979 Service Info Manual.

Porsche 928 HVAC vacuum diagram 1978
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First, let's start with defining automatic and manual.  In 1977-78, Porsche was sampling options for a manual or vacuum actuated center vent.  You will find some with a knob on the center vent, and some without.

  • With a knob manually actuates the center vent and does not require vacuum.
  • Without a knob automatically actuates the center vent and does require vacuum.

For manual, you will not find an orange vacuum hose, so you may disregard its function.

For automatic, you will find the orange vacuum hose for the center vent flap is not routed for best function.  See, for AC and all heat settings (except for defrost), the original hose routing will close the center vent for all temperature slide control positions except full cold (all settings to the right from the left stop).  This shuts off crucial airflow to the center of the car and limits heat-blend AC.  This revised schematic shows where to re-route this hose to keep open the center vent for all temp settings and functions other than defrost.

Next, the climate controller has two internal differences which only affect hose routing.  Use this revised schematic to determine which one you have and where to correctly route hoses.

Lastly, the interior vacuum system is not only used for climate control, but for central locking as well.  Central locking was not included in the original schematic, but is included in this revised schematic and also separate documentation. Some early 928s do not have a check valve installed to isolate the central locking from the climate control.  Use this revised schematic to determine where to add a check valve P/N 92853736102.