SD climate control
 

I've got the SD back on the road after 18 months and am enjoying driving it.

During it's hiatus, it received a new auxiliary water pump and it's functioning properly.

The climate control system is also working properly with respect to all of its air doors..........all the pods are functional.

The issue is the temperature at the defrost outlets above the dash and the temperature of the center vents.

When the engine is fully warm.........90°C.........the temperature of the air at the footwells is very warm.........and comfortable.

However, the leak air at the defrost outlets (using the economy button) is best described as cold.........not quite as cold as ambient (25°F) but noticeable colder than body temperature.

The air from the center vents (vacuum on all the time) is also the same..........cold. I close the dash sliders to eliminate most of it.


The question that I have is for those who are intimately familiar with the airflow of these units. How can the system provide hot air to the footwells, hot air to the side window defrost outlets, cold air to the defrost outlets at the windshield and cold air to the center vents?

Appreciate it if WAG's are not part of the responses...........

I understand the nature of your question - How does cold air route through some parts of the system while hot/warm air is routed through other parts? You've caught me out of town without access to my manuals, and you don't want any WAGs, but how about a SWAG - a WAG of the Scientific variety?

My understanding is that the center vents are part of the A/C system, and the airflow through them is not routed across the heater core. Same with the vent to the rear seats. When the middle button is pressed, I believe air is routed through the center vents when the A/C is active, and through the side vents and leg flaps when the heat is active. The thing about the defrosters has me puzzled though. Most modern systems turn on the A/C when defrosting to remove humidity. I'm not saying the A/C is on, just that this is the portion of the system that would appear to be common to both. In Defrost mode, does the A/C'd air mix with heated outside air, or does the A/C'd air flow across the heater core? Not sure.

I'm thinking there may be a flap with a bad seal allowing A/C air or outside air into the ducting system, and with the flaps positioned for heat, the other vents are not sealed off from the outside air.

Put the temp control to max heat. Do the defrost vents blow hot in Defrost Mode? Do the center vents blow hot in A/C mode? From your description, I'd assume they are operating at least nominally. And what happens when the center button is pressed, do you get the same symptoms? Does the phenomena present itself only while you are moving or also while you are standing still?

Is there a diagram somewhere that shows the air flow ducting and the flaps controlling air flow through them?

My apologies if this is too much a WAG. It's always good to have someone to troubleshoot with and I hope my questions provoke responses that cause you to think in directions you may not have on your own.

Isn't it that the MB engineers designed the climate control system so that the center vents only blow cold air, the side vents only blow warm/hot air, and when the side vents are blowing warm air but the system is not in defrost, then the defrost vents only blow lukewarm air (if at all)?

I have the same symptoms but attributed it to the fresh/recirc door not closing fully. A portion of the pod(short throw) is leaking so the line is capped off. I've tried to match this symptom to what is shown on the schematic but there must be some differences in actual "physical" ductwork than what is shown. With that in mind, have you verified the fresh/recirc door is closing fully?

Also, what are you getting T/V mode?

Serendipity
 

Funny I was just thinking about center vents in my TD while driving home from work. Sometimes I feel air blowing strongly through the center vents. Sometimes not at all. The side vents however always blow air no matter what button or setting of the CC. People speak of the"pod" that controls these center vents. Can said pod function well intermittently? I'm not really sure when the center vents are supposed to blow as strongly as the side vents. Seems to me that they should be the same as the side vents at all times, but reading here indicates that isn't true. Has anyone done a write up on CC button selection vs. center vent "blowing"?I'm not really sure if I need a pod or if I just don't understand when the center vents are supposed to match the side vents.

In general, the center vents are only supposed to blow cold air. When you are in "heat" mode the center vents will be isolated. Some people have bypassed that function so the center vents are always open.

MBeige is correct and, regardless of CC or manual, the center vents are for fresh or conditioned air only.

fresh or conditioned air only
 

Then can "fresh" air be warmed air? Should in theory only A/C or ambient outside air blow through the center vents? I have felt warm air through the center vents with the heater on, even though it was cold outside.Has anyone done a "map" of the climate control settings, and what vent is supposed to be doing what, at each button setting, and fan speed? I might be having a stupid attack, however, I'm never real sure, in detail, what each button setting is supposed to do, when it's functioning properly. I use the "EC" setting mostly, and the defrost. I'm sure there was heh, "intelligent design" going on in the german engineer's mind, however I'd kinda like the center vents to do whatever the side ones are doing, at all times.

Yes. It's in the FSM. That said, I gotta load the FSMs onto my wife's laptop.me tooAs I've been told, the Mercedes engineers have millions of miles of experience with (your favorite topic here). Best not mess with it.

Now get off the damn computer and spend some time with your families. Merry Christmas!

If you are feeling warm air thru the center vents when the heat is on, some warm air is naturally raising from the housing of the heat exchanger, which is below the fresh air duct, but if the air flow is measurable thru the center vents, then the vent flap is opening when it shouldn't be. And yes, I did some mental mapping the system many years ago when I replaced the CC for a manual system in my W123. Unfortunately, I forgot most of it when I said, "Good riddense" to the CC.

How 'bout if the thread gets hijacked instead?

I want to correct myself, the side vents blow hot and cold air. Center vents, only cold air. I was thinking, in heat mode, the side vents should blow heated air, while the center vents, if opened, blow cold air, even in heat mode.

Agreed

Yes, I am one of those. The center vents in my W123 blow the same type of air the side vents blow out. Cold when cold on the side, hot when hot on the side. It has worked this way since the day I removed the center vent actuator or pod and routed the vacuum lines to each other (rather than to a 3-way connector that goes to the actuator/pod).

Removal of the center vent was necessary because the spring in the actuator kept the center vent flap closed (default). Removing the actuator/pod + spring allowed me to set the center vent flap to open.

Brian...i had that problem on an old 126 I no longer have...put in a different controller and it was fixed.
I do have a 126 climate control manual...and at this time only one 126 thats apart for an engine swap....so if you want to borrow the manual youre welcome to it. Let me know.