1989 300TE wagon. The center vent only works occasionally with the AC on. No rhyme or reason that I can discern. Any thoughts or guidance would be appreciated.

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1989 300TE

I would think the vacuum pods that open and close the center vents are failing. Known problem on these vehicles.
Do a search, LOTS of information on this topic.

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

Yeah, failing actuators/elements/pods - whatever you want to call them. The above is actually a very nice description of a failing system with an intermittent vacuum leak. When it works, the cold air comes out the dash vents. When it fails, there is a big vacuum leak and the cold air comes out the defrost vents (default position of the HVAC system). I've looked at - and passed on buying - cars that operate in that manner.

Per the 124 climate control manual, the center dash vents are open anytime the system is in the cooling mode. This is true whether the air conditioning is running or it's in EC mode. Whenever the system is working to cool the car, the center vents are open.

The footwell vents open whenever the system is working to heat the car.

The defrost vents open when the defrost button is pushed, when the hilo mode is selected, and for the first ~30 seconds after you start the car (to clear any accumulated condensation).

It's fairly easy to diagnose failing vacuum elements. Pull the glovebox and right there are the seven vacuum lines attached to a manifold. Pull them off one by one and test with a handheld vacuum pump. Each should hold vacuum. If not - there's your problem.

I bet at least three of the seven are bad on your car

- JimY