HVAC/Heater Problem

[mbzjag8090]

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?

[Holmesuser01]

I think if you have a heater core problem, you will be smelling antifreeze in the car, like mine is doing.

I think you have a heater box door that is not opening. Could be something as simple as a vacuum line disconnected.

I'm preparing to tear into my 190E and replace the heater core. I have the new one ready to go.

Good luck with your project!

Bruce

[mbzjag8090]

Is it necessarily the case that I will smell antifreeze if there is a problem with the heater core?

All vacuum lines and pods are in place and functioning.

With regards to the heater box door is anyone aware of the internal workings of the HVAC black box? Is there such a thing as a heater box door? No mechanic I have spoken to knows what to tell me.

[Holmesuser01]

One of the doors in the heater box may not be moving properly. I'm sorry, as I didnt mean that this would be the official name for this part.

The heater core simply has antifreeze moving through it, and the fan blows outside air across it, heating the air. The core doesent control where the air flows.

[mbzjag8090]

Okay, thanks. I suppose the entire HVAC box might need to be replaced in that case? I haven't taken it out of the car but suspect that getting inside the unit/taking apart to see what is wrong would be impossible. Thoughts?

[Holmesuser01]

The box can be opened up. I will be cleaning 25+ years of crud out of mine while I have it apart doing the core replacement. The service manual gives fairly decent details on how to get the dash out of the way, along with most of the console.

[mbzjag8090]

Thanks. Any other thoughts on this?

[Holmesuser01]

If you unplug something... tag it so you will know where it goes. Same for vacuum lines... Colored tape is a nice idea.

[mbzjag8090]

You were right...one of the door bindings was broken. The leftmost vacuum actuator arm near the driver's footwell was unattached from the plastic clip which is on the rod running through the airbox. Unfortunately the plastic clip which secures it is broken off. Ordered a "flap repair kit" from MB. Hopefully it will include the necessary items to fix.

A couple of the plastic clips in the airbox are extremely poorly designed for this auto. Should be steel bindings. They are very small, would add little overall weight to the car and are of too critical an importance for HVAC function. Not to mention the time and effort involved in taking the whole thing apart to do the fix.