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Air duct weirdness

8K views 21 replies 4 participants last post by  lefty louie 
#1 ·
Climate control stopped functioning correctly this weekend on my 2007 C350. Unable to get air through the ducts. Heat is there, and I hear the fan blowing, but air does not get through any of the ducts (top, dash, or footwell), save for the first few seconds (more below).

Searched the forum here for a while and got many good tips. I tried the test/reset by depressing the Front Defog-Recirc buttons simultaneously. At first it did not pass, but I think I was not simultaneous enough in my pressing. After sorting that out, it consistently passes: I hear the dampers gently (more or less) activating, and the flashing lights go out after 15 sec or so. After doing this, I am still unable to get air to flow through ducts; if I turn the fan up to its higher ranges, I can hear the air rushing to get out, but it has no open duct to leave from.

One more bit of info: Starting from fan off, turning fan on will cause air will flow from the selected duct for 3-4 sec, and then it fades out. As I mentioned above, if the fan is running at high speed, after 3-4 sec I hear the air trying to get out of closed ducts. I can repeat this 3-4 sec of operation if I first reset by turning the fan off. I can also get this 3-4 sec of operation out of the top, dash, or footwell vents, according to which one is selected. Given this, and the fact that it passes the test/reset procedure, it seems the stepper motor lever arms on the ducts are not broken.

So, is this what a bad Climate Controller Unit does? If so, I read in other posts here that this is an easier repair than getting at the broken stepper motor lever arms, but I did not find any further info on how to replace it. If an SDS readout is required to further diagnose this, I have a good German car indy I can go to, but if the symptoms are fairly straightforward, I am happy to proceed without it. Any advice is appreciated.
 
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#2 ·
Changing the temperature or duct settings have no effect? I have no clue. I guess you could get a scan via SDS to see, but my understanding is that if the self-test passes, there are no codes - but I could be wrong!
 
#3 ·
Thanks, Rodney.

Yes, that is correct, changes in temp setting (manual left and right dials in this car) or in duct selection (a rotary knob) made while the fan is already on have no effect on the (apparently) closed ducts. It is only if the fan is off, and I then turn it on, that I get an initial flow through whichever is the selected duct, but then that stops after 3 sec . The fan is still running, and I can hear the air flow trapped behind closed ducts.

I get the same behavior with the front defogger: starting with the fan off, when I activate the front defogger I get an initial burst of air for 3 sec through the top vents, and then it quickly but smoothly fades out and I hear the powerful air flow trapped behind closed ducts.

It is as if the individual top, dash, and footwell ducts are working, but another damper upstream from those ducts, while initially open, closes soon after the system is activated by turning on the fan. Is there such a master damper, located after the fan but before the distributory ducts; perhaps one whose job it is to close off the system to drafts when the fan motor is set to Off?

It would be easier to understand that, rather than 3 individual ducts simultaneously malfunctioning in the same manner, plus the front defogger acting the same way, when it is independent of the panel settings for fan and ducts. Looks like this is going to turn out to be a bit of a head scratcher.

One other thought, though it seems rather unlikely: I don't think this behavior started until after I disconnected the two connectors to the fuel sender to work on it (my other recent thread). I did not notice any other electronic weirdness from disconnecting/reconnecting those two, and I did not disconnect the battery, but is there a procedure for resetting things that may have been affected? I know of the procedures for resetting window and sunroof movement, and the lock-to-lock on the steering. Are there others that might be relevant to climate control, other than the Front Defog/Recirc test/reset we have already mentioned?
 
#4 ·
There's no reset that I know of other than the test procedure, and I doubt the work on the sender had anything to do with this. Remove the cabin air filter (under the hood, over the battery) to see if that makes any difference.
 
#5 ·
That is an interesting idea, and it sent me on a little journey.

I removed the filter, and lo and behold, air flow was working. Even the front defog function. I shut the car off and fitted a new filter (had one in my spares). Odd thing is the filter I removed looked only modestly soiled, and I had installed it 2.5 years and 35K miles ago; service interval for this filter is 5 years or 52K, and the car has not be operated in a dusty environment. The cavity with the squirrel cage behind the filter was exceptionally clean. I use the charcoal version of the filter, by Mann.

Then I went out on errands, and tested it by starting it and running for a few minutes several times. It worked some of those times, and I could change the ducts the air was directed to at those times, but on other tries, it reverted to blocked ducts on start up.

At my destination I removed the filter again and when I drove back, it would work at times, but not others. I refitted the new filter, and got the same behavior. One thing that would consistently not work (except that first time in the driveway), was the front defog. I could start it up, and then I would hear a small mechanical noise in the dash (somewhere between a tick and a gentle clunk — must be a damper closing) and the air flow through the top vents stops but the fan continues.

I also performed the test/reset procedure, and it would always successfully complete. I could hear the normal noises of dampers activating. The only thing that seems different from the way things behaved before all this starting happening is on shutting the fan, I can hear a slow and sometimes intermittent creaking in the dash for about 5-8 sec, which I take to be a damper sealing off the air path.

Last night, I got to thinking that this kind of squirrelly, intermittent functioning probably isn’t a purely mechanical issue, but possibly of something that Rodney has raised in many many threads: that a battery problem can cause odd behavior in electrical systems. My notes from his posts say you should have:

At rest: -- 12.6v
Key in position -- No less than 12.5v
Starter engaged -- No less than 11v

I get:
At rest: -- 12.0 V
Key in position -- 11.8 V
Starter engaged -- 9-10 V (only for a split second on my digital meter, as car starts so fast)
While running -- 14.2 V

While idling and continuing to measure the voltage, I activated the climate system, including the front defog with the high fan setting, but did not see any change in voltage.

This car is now 11 years old (April 2006 production, June 2006 delivery) with 120K miles and still has the original Varta battery. I don’t think this battery owes anybody anything. I would have never suspected a battery issue as the car cranks so willingly, even in cold weather, but Rodney’s posts on this matter suggest that weird electrical things can happen in these cars before a battery weakens to the point where it fails to crank.

Based on this, it seems my next step should be to replace the battery, for reasons beyond the climate control system itself. I have to confess that there is still a part of me that is a bit skeptical that this is the root cause of the climate control issue, but I probably do not appreciate how complex and interconnected the control systems are in these cars. Additionally, I don’t know what else to try, beyond getting codes read by SDS, which would probably cost half of what a new battery would.
 
#6 ·
Even though the alternator is putting out enough voltage, a bad battery can act like big resistor and suck current from the system. I'd start there with those voltage readings. Also look to see if you have a "smog sensor" for the ACC system. I can't remember if the W203 had that or not - I know the W209 did. If so, it's there in the intake where the cabin filter is located. Maybe its dirty or bad - if you find it, try disconnecting it. I have heard of these causing odd problems, but it's usually making it kick into RECIRC mode by itself.
 
#7 ·
I got a new battery installed late yesterday (the Platinum AGM from Advance Auto that Rodney has recommended in other threads; it is marked as made in Germany, supporting Rodney's guess that it is made by Varta). That evening, behavior was the same. This morning, I had better success, but through more playing around, I have come to realize that the temperature setting on the dash plays a central role in this.

Rodney, you asked me about this earlier, and I responded that the behavior was the same regardless of temp setting. That is what I believed my experience was, but I did not play around with the temp settings that much, and may not have used extreme enough settings to discover what I have now. Or, perhaps, things are somewhat different after the battery replacement, and maybe with it being warmer today today (52° today), I tried more different temp settings. I just don't know.

But that said, today it was clear that temp setting plays a central role in the distribution of air. If I set the temp to the lower end of the settings, I get strong air flow from the appropriate vents; I get the strongest flow (an extra boost) if I turn the knobs as far CCW as they will go (64°) and get a tactile click in the knob. As I raise the temp (e.g., say middle of the dial, 72°), I get a much reduced flow, and if I raise it to the top (80°; my wife's usual setting, and she is the one who first identified the problem), nearly no flow or no flow. And whatever air flow I get at these settings, it is not warm.

Interestingly, the driver's side and the passenger side are completely independent: if I set the driver's side to 64 and the passenger side to 80, I get lots of flow out the driver side, but none out the passenger side. And vice-versa.

A corollary to all of the above is that I can not get heat in the cabin, even when the car is fully warmed up. I can feel traces of heat in the system drifting out when I shut the fan, but with the fan running, there is no heat.

Based on today, I am confident that the vents and dampers, etc. are all fine. They do work.

So, my question is -- and perhaps Rodney, this is the issue that you were trying to get at when you asked in post #2 -- is if there is a temp sensor that controls delivery of air through the climate control system such that if heat is being asked for in the cabin but the sensor knows (or thinks) there isn't heat available from the coolant, it shuts off air flow until such time it determines that heat is available (to avoid the indignity of blowing cold air on the occupants). If so, that would be a good description of my problem. Presumably this means that sensor is dead or disconnected (it does not register that heat is actually available from the coolant) and consequently never permits air flow to the cabin when heat is being called for. I will note that my coolant temp gauge in the dash reads the proper 83°C when the car is warmed up.

Is that temp sensor the likely problem? And if so, is that sensor replaceable?

Thanks again for your guidance on this, Rodney, and I apologize for not exploring the temp setting issue more thoroughly when you first asked about it.

Oh, and I did look carefully in the cavity behind the air filter and could not identify any smog or other sensor back there. The only obvious things were the fan itself, the damper that comes down over it (I verified that the damper worked properly when activating the recirc button, and otherwise that it only opens when the fan turns on or the ignition is shut), and a sealed blue plastic port on the right side of the cavity (when facing the cavity from the front).
 
#8 ·
I have seen the humidity sensor in the overhead control panel fail and cause weird issues like this, but it's usually the opposite - you get heat, unless you turn the temp all the way to the coldest setting. Have you tried the ACC test/reset since replacing the battery?
 
#9 ·
I did do at least one ACC reset yesterday, and another 2 today, spaced out by 30 mins. No changes from that.

I dug up several other threads on that sensor in the OCP. The little grill for the small fan is clean (neither my wife or I or the previous owner were smokers). I have not opened the panel yet to get at that thermistor, but I did try opening the windows and sunroof to see if that would bypass the problem, as per info in other threads. It did not, but it does seem now that I get better air flow at higher temp settings on the ACC, but as before, (1) this is not accompanied by any heat and (2) if I go the final click at the CW end of the dial (80°), that does shut the air flow down (I can hear a click in the dash right before it does); if I only turn one side to 80°, it is only that side that looses air flow.

After closing up the windows and sunroof, the same behavior continued (reasonably strong flow out of the vents until hitting the 80° setting), whereas previously the temp setting was modulating the amount of air flow. But under no circumstances over the last several days have I gotten any heat out of the system.

I will look at prying off the OCP and seeing if I can get at the thermistor in the fan housing. I realize, as you have said and as the other threads discuss, it seems these thermistors either have broken leads or solder joints or otherwise fail open and result in infinite resistance and a constant call for heat.

After that I will try a little compressed air in the small fan in the dash (also looks clean externally). Failing that, I suppose it will need to be an SDS session to get a full reading for where the problem lies.
 
#11 ·
Thanks for this and the other guidance, Rodney. I greatly appreciate it.

It does seem that a failed-closed damper that isolates the heater core from the air plenum, along with a diverter controlled by the temp setting, could explain my issues.

If the diverter controls the fraction of the total air flow that is sent to the heater core, but that pathway is a dead end, it would explain:

-- why the temp dial seems to control the amount of air flow I get through the vents; lower T settings give me more flow, higher settings less flow (this behavior was suspended with sunroof open, but returned on a restart of the car).

-- why I get an extra boost in air flow when I turn the temp setting to that last click at the lower extreme (presumably the diverter now seals off flow towards the heater core entirely by closing an internal duct, and all air goes to the plenum)

-- why I get no air flow when I turn the temp setting to that last click at the upper extreme (presumably the diverter now seals off flow to the rest of the plenum and only feeds air flow towards the heater core, which is a dead end). Hence I still hear a rush of air, but no air flow through the vents. Ditto for the behavior I get out of the front defog button.

This is a job I will likely take it to a shop for. Nonetheless, I am curious if the iCarsfot i980 tool would be able to read codes that would help in the diagnosis of issues like this, even if it does not have all the diagnostic horsepower built into SDS. I feel I need something better than the simple OBD II tool I have now. I understand that OBD tools can only sense electronic faults and not purely physical failures.

Thanks again for your help.
 
#13 ·
Thanks, Rodney. I have ordered one.

To round out the description of behaviors in this thread, in case it is useful to anyone in the future:

With weather that was decidedly colder yesterday (mid 30s), varying the temp setting in the middle ranges did not modulate air flow through the vents as it did during warmer weather: flow was always meager or absent (except for clicking to the low end). I am pretty sure this lack of modulation is because with colder weather, any temp setting (other than at the low extreme) caused the diverter to deliver nearly all of the air to the (blocked) heater core.

This just highlights that this problem (presumably a frozen damper to the heater core) can be expressed with changing symptoms, depending on ambient conditions and how wide a range of temp settings are tried.
 
#14 ·
I got the iCarsoft i980 meter last week, and it may provide a clue on the heat/fan issue.

For context, when first connected, it returned 30 or so codes, many of which seemed to be a recounting of issues over the past several years. I wrote them down, and then cleared all codes in each category, figuring I would wait for any to return. Two days later I reread, and there were no new codes. Now it is several days since that, and I have two codes showing, one a repeat (0060, more on that below) and one a new one, B1003, that may be related to the ACC problem.

Meter reading from: SAM F - Signal Acquisition and Actuation Module Front
B1003 - Component M13 (heating water circulation pump) or the line to component M13 has a short circuit to ground or open circuit. If component M13 is not installed…

I opened the front SAM to have a quick look; it was clean and no obvious problems showing such as broken or corroded wires, but I did not start pulling fuses as it was getting dark and I did not have a chart of the locations with me. I will return to it tomorrow with the chart, and I will also look around outside the front SAM for broken or frayed cables, but if the B1003 code and Component M13 short/open are meaningful clues on the heat/fan problem and point to further tests that should be done, I would be grateful to hear of them. If my VIN is relevant (e.g., to knowing if M13 is present), it is WDBRF87H77F839147.

If I can hijack my own thread, the 0060 code (Continuous camshaft adjustment (right). Incorrect position of exhaust camshaft (P0015) appeared without the CEL being lit. Perhaps it is just a matter of time. This is the 3rd instance of these things (the magnets?) failing for this car, under “Scenario 2“ of the balance shaft document; I should replace it and the last remaining original one as well. From what I have read here it seems a simple enough job, just unclipping the electrical lead, unbolting, cleaning the gasket area, and bolting in the new one and reconnecting it. I also take it, from reading, that “right” is USA passenger side (I ask because it seems other cars I have worked on have referenced left and right from a position standing in front of the engine bay; either that, or I am starting to go crazy).
 
#15 ·
M13 is the electric pump that sits in front of the coolant reservoir. Some cars have it, some do not. If yours does not, then you can ignore that error.

AS for the camshaft adjuster solenoids, I'll be that not all four were replace. Often times, they only replace the ones that give the error. A'l four need to be replaced with the revised part. You can get all four for about $100 and it's an easy job - connectors and bolts like you said. However, one may be behind the oil cooler which has to be removed so that adds a little complexity, but not much.

Oh, and left/right is ALWAYS relative to sitting behind the steering wheel (regardless of which side the steering wheel is on).
 
#17 ·
I do have a small pump mounted in front of the coolant reservoir. It taps into the heater hoses, and is fed by an electrical connection off the harness (1st photo below).

I pulled off the connector and opened a little flap on it, exposing the tubular sockets that slide onto the power pins on the pump (2nd photo). There was some corrosion on the two leads (see 3rd photo too). I cleaned those up inside and out with some fine steel wool, using a tooth pick to rub the steel wool on the inside. They and the pins on the pump body cleaned up well; they look like they may be made of a silver alloy (4th photo). I followed that cleaning with some contact cleaner and a bit of DeOxit D5 (from the things I use to work on my aging stereo equipment). I reassembled everything and gave the car a spin to get everything warmed up, but no change in climate system behavior. I did another test/reset, and it passed that again, but no changes in behavior following that.

Back home, I pulled the connector off, started the car, and measured the voltage across the socket connectors on the harness. I got 12.8V whether the heating system was turned on or off. In any case, clearly power is making it to the pump pins, so my harness and Front SAM look ok. While the connector to the pump was off, I also measured the resistance across the power pins on the pump. Initially it read 6.1Ω or so, but over 10 sec climbed slowly to 7.5Ω. It stayed more or less constant after that, for the additional 10 sec I looked at it. I don’t know if that is a proper reading for a healthy pump, but it is neither open or shorted.

Following that, I cleared the B1003 code and will have to wait to see if it comes back. If it doesn’t, then corrosion on the connector was the reason for the code, and my problem lies elsewhere. If the code returns, I suppose I need to replace the pump; Is that a proper diagnosis?

I took a stab at looking up the part online, and I found it described as either “recirculator pump” or “auxiliary water pump”, part 203-835-01-64. Is this the right part?

Re-circulator Pump for 2007 Mercedes Benz C350|203-835-01-64 : Mercedes-Benz Parts - Genuine OEM | MBOEMParts.com

I don’t know if a failed pump could be the sole cause of my problem, or one in addition to a problem with a damper on the heater core. If it is the former, that would mean (1) I save some time and money because I only have to replace the pump, which looks extremely simple, and (2) there must be some kind of interlock in the heating system that closes off air flow out of the heater core if the appropriate temperature is not detected there.

My hunch is I will not be so lucky.
 

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#19 ·
I am having a problem identical to Louie's. We have communicated via PM.
This weekend as I was doing some spring prep on my ALFA Spider I came across my endoscope in my tool chest and decided to take a look inside my C230 vents. I was not able to fit it into the dash vents, but I did get a good look into the defroster vents and footwell vents. In both cases with when I set the dial to direct air to either place I can see the flaps fully opening, regardless of the temperature settings, but the airflow remains restricted to the driver side. The system is very clean and does not show any water intrusion or debris of any kind. As Louie reported it functions normally when in full cold AC mode. When I set it on AUTO or MANUAL and have the thermostat set for heat I get heat on the passenger, but only fresh air on the driver.
I cannot reliably direct the endoscope further down the system and I don't have a good picture of how the flow is diverted side to side. Are there more flaps and diverters deeper in the system? Is there a diagram or has anyone take photos of these parts outside of the car? Thanks!!
 
#20 ·
I too would be interested in seeing a diagram that shows the damper/divertor that is upstream from the dash distribution flaps. The closest I have seen to that is what is in the EPC, which has almost no detail.

Interesting that you get any heat, even if it is restricted to the passenger side. I do not, which, combined with Rodney's earlier comments, have made me think there is one damper/divertor that controls access to the heater core.

My situation is that setting different temps for driver and passenger side can modulate the air flow delivered to each side, depending on details of the setting and the ambient temperature, but there is never any heat delivered whatsoever. I figured that that meant all driver vs. passenger air distribution differences were controlled downstream from the damper/divertor at the heater core, but the more I think about it, the less likely this seems.

Your condition of getting heat on one side makes me wonder if that damper/divertor is two-sided. If that is true, it can be helpful to resolving your issue, and it would also help me, as I would know that both sides of mine are frozen shut.

One other thing I might add is that since the time I acquired this car (about 4.5 years ago now), there was a faint sweet odor in the cabin. My daughter noticed it, asking "Why does this car smell like crayons? I did not think much of it, figuring it was residual perfume from the previous owner. But I have since noticed it appears only after the car is fully warmed up, and after reading another post on here about the smell of coolant, I began to wonder if it might be a small weep of the special MB coolant, a weep that, over time, might have some connection to the failing of the damper/divertor at the heater core. That is, maybe an accumulation over time of drops of dried coolant on, say, the damper's bushing (can't be a lot, as I have no evidence of leaks underneath the car and I have never had to top up the coolant), has gummed up that damper's operation.

Anyway, back on topic, I too would love to know exactly where that damper/divertor is and see a diagram of it.
 
#22 ·
No, car still behaves the same. I thought I had mentioned here (but apparently not) that when I took the car to an excellent local German-car shop for state inspection, I had him look at the problem. He has an SDS and it came back with nothing. As far as it being the damper, he said it's deep in the dash and he could start digging, but it would come to several hours of labor and no guarantee he'd find anything. Wish I had better news. I now have the balance shaft issue starting to appear, at 136K, so no investments in this car, unfortunately.
 
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