Busy Weekend: 928S4 HVAC

Tackled the 928S4 this morning that was having a full heat issue in the cabin.  After some searching, I discovered that there was no vacuum to the heater control valve (defaults to full heat when not working), which was the root of our problem.  Next I had to figure out why there was no vacuum there when the heater knob was turned to “cold.”  I started by taking the mityvac to all the vacuum lines to try and track down any leaks.  Ended up finding a broken check valve, and a couple of loose connectors, so replaced those, but still had the same issue.

Heater control valve.   Note the arm is in the full hot position.

Location of the busted check valve.  I bypassed it for the time being, as it will most likely have to be ordered directly from Porsche.  I think I will stock up on them as they appear to be brittle in most of the cars I see.

Since the vacuum lines in the engine bay appeared to be good, next was to tackle the unit in the center console.  I pulled that apart and started testing components.  Nothing conclusive.

Next I went to pull the selector switch assembly apart to ohm out the temp sensor in the alternator duct (yes, you read that correctly, there is a sensor in the fresh air duct to the alternator that affects the HVAC system….a typical 928).  What I found when I pulled the connector off was a bunch of electrical corrosion.  I cleaned the contacts, and all of a sudden had working vacuum at the heater control valve.


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