Car is a W126 1984 300SD. Here is what it does:

Heater only blowing hot air through defrost vents. I know that the system is not designed to blow heat out of the center vents. But it should blow heat out of the side vents and it is not. I also notice that sometimes the heat coming out the defrost vents is very good and sometimes it is just marginal. The A/C system is blowing A/C out of all the vents as designed.

Here are the systems I have tested with 100% confidence for proper function: (a) Pushbutton unit (b) Temperature regulator - silver box under passenger footwell (c) Vacuum pods and vacuum pressures (d) Monovalve - entire unit and diaphram (e) Fuses.

The only thing left that I can think of is the heater core. I've read one post on the forum where similar symptoms were believed to be caused by the part.

Here is why I am suspicious though:

When I used to turn the heater on I heard a flap reversal noise or something inside the black HVAC assembly box. I don't hear that anymore. I've confirmed this by testing in other similar models. If you turn the A/C on high and then reverse the system to heat you hear a flap movement inside the box. Maybe this is only the center air vent flap but I don't think so because on my car the air is working like a champ. On other W126s the sound is not too loud but certainly audible. I remember in past seasons that when I switched from A/C function to heater function in this car that the noise was very loud...enough so that a passenger might ask where the noise came from.

Any thoughts?

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1984 300 SD 384,000 Miles