Air Flow Problem- Cold air out defrost heat out floor. - PeachParts Mercedes-Benz Forum

Graplr · #1 12-19-2008, 09:15 PM

So after bearing with this problem since I've owned my car, I decided to again try and figure it out. I've asked here before and investigated some but can't seem to figure it out. In the summer when my AC (parallel flow condenser AND R12) is on the coolest I get is 45F at the vent. In the winter the hottest I get at the vent ~90F. I have come to the conclusion that there is some sort of airflow problem.

Symptoms-
on Defrost- I get warm air everywhere. Same temp out defrost and side vents.
on Defrost and any other button depressed simultaneously- I get warm air everywhere
on any middle button- I get warm air on the floor and cold air out the defrost

What I've done so far-
Monovalve rebuild
New Aux Water Pump
Been through 2 different CCUs- Same symptoms on both
Disable Fresh air flap pods (the ones behind the glove box) from flap to make sure it wasn't open all the time. I took the glove compartment and watched it, it never opened and still same symptoms.

Since I get warm air out all vents on defrost I usually drive with the defrost and another button depressed to get warm air at my feet (in the winter). In the summer the cooling was okay. I usually drove around with the AC on high. Better than it was when my AC was not working.

So all in all, this leads me to believe there is outside air mixing with the heated air in the winter and outside air mixing with the cooled air in the summer. It almost seems there is some air flowing through under the defrost somewhere.

So, does anyone have any thoughts?
Is there some other sort of fresh air flap coming in the defrost vent?

This is on my 84 300SD
Thanks!
Ryan

Lycoming-8 · #2 12-19-2008, 10:23 PM

Although pushing more than one button at once appears to give you a semi-acceptable work-around, I don't believe that these systems are supposed to be operated that way. It actually states in the owner's manual that the horizontal buttons are to be operated individually and the same goes for the fan control ones.

Believe I have read here that pushing multiple buttons at once can cause problems with the ACC module.

Graplr · #3 12-20-2008, 07:48 AM

I'm almost positive I got the idea of depressing multiple buttons simultaneously from the car's manual.

You may be correct that you should not push two arrow buttons at once, but I'm almost positive the manual says you can push the defrost and the button next to it simultaneously.

The manual is out in the garage, in the cold right now and I just woke up so I'll double check later today.

But regardless, this has nothing to do with my problems since when I push single buttons is when I have the problem. And the problem was present with both push button units.

Graplr · #4 12-20-2008, 08:09 AM

Okay, I just checked the manual. It does not mention anything about depressing multiple buttons simultaneously, positive or negative. So I'm not sure where I got the idea to do that.

I know that doing this is the only acceptable way I get heat to the floor. I used to press just the defrost and my feet would freeze. Or I would push just the middle button and my feet would be warm but I would get could air out the defrost.

Brian Carlton · #5 12-20-2008, 09:00 AM

I'm curious about this phenomenon since the SD acts in a similar way without the use of the aux. pump. The replacement pump was affecting the monovalve in a bad way and I disconnected it a couple of years ago. The results are such that the defrost air is the coldest of the group. For some reason, unknown to me, external air is getting to the defrost outlet and is not getting to the other outlets in the vehicle. When fully warm, the defrost outlets remain colder than the others but not full cold. There is still sufficient heat to keep the windshield defogged.

I was under the assumption that the lack of sufficient flow from the pump was the issue on the SD............but, maybe it's not.

Graplr · #6 12-20-2008, 09:25 AM

That is interesting Brian. How did the faulty pump affect the monovalve? Shouldn't the air flow temp be independant of the monovalve since I get warm air, just not out all vents?

When I first purchased the car, it had a faulty aux pump. I just took it out and put an elbow in. This past week I finally put a new one in and manually wired it (not to the CCU system so I could see what difference if any it makes).

My findings were that it did not affect any air flow anywhere. But it did get the air warmer faster and did keep it warm at idle whereas before the air would cool at idle.

It is quite a substantial difference between the temp of the air coming out the defrost when an arrow button is depressed. I don't recall putting my thermometer in there (I usually keep it in the side vent and check the floor periodically), but I would guess around 50F when the floor is around 80-90F.

Brian Carlton · #7 12-20-2008, 09:40 AM

The monovalve cycles repeatedly to maintain temperature. The cycling is normally silent to the driver. When this replacement pump was used, the cycling became quite loud.........to the point where I was a bit concerned about the CCU. So, it was disconnected and I haven't had time to figure out why a pump that's only six months old would create problems for the monovalve.

Graplr · #8 12-20-2008, 09:48 AM

Quote:

Originally Posted by Brian Carlton

The monovalve cycles repeatedly to maintain temperature. The cycling is normally silent to the driver. When this replacement pump was used, the cycling became quite loud.........to the point where I was a bit concerned about the CCU. So, it was disconnected and I haven't had time to figure out why a pump that's only six months old would create problems for the monovalve.

I guess I never considered the monovalve cycling. I think last winter I disconnected the wires to monovalve but can't remember. I'll do that next and see if it makes any difference.

Graplr · #9 12-20-2008, 09:49 AM

Brian, is the AC functional in your SD? If so, is it R12 or R134a and what temps do you see?