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A-Class 2011 Heating and ventilation

Discussion in 'Electrics, Vacuum, Ignition and ECU' started by aclassdriver, Nov 20, 2012.

  1. aclassdriver

    aclassdriver New Member

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    Location:
    Leicestershire, UK
    Your Mercedes:
    A Class / 2011 / 180CDI
    I'm having trouble with the heating system. It seems way too sensitive.

    I bought the car in March and reported the problem to the dealer, who took a look and said they couldn't find anything wrong. The problem went away during the summer (didn't need the heater), but has now returned as follows.

    If the car is driven away from a cold start, with the cabin temperature control set to 20 deg C with the fan on low (1), the following happens.

    After about ½ mile the heating system starts pumping very hot air into the car.

    This continues for the next 4-5 miles until the cabin is very warm. Then, suddenly, the system starts pumping cold air into the car. Then after a mile or so more the system settles down.

    If I set the control at say 18 when setting off, pretty much the same thing happens, although it settles down after a shorter distance.

    In engineering terms the heating control system appears to have far too much “gain”. Even after the system has settled down, turning the temperature control up or down by just 1 degree can cause the temperature of the air coming into the cabin to change significantly (from cold to hot or vice versa).

    I believe the car has a cabin air temperature sensor in the courtesy lamp housing and that the heating system is trying to control the air coming into the cabin so that the cabin reaches temperature in a timely fashion, but it seems to be far too aggressive. If the cabin temperature is only a little bit too low, then the system pumps in very hot air, and vice versa. The motorised flap controlling the mixing of hot and cold air coming into the car seems to be over-reacting.

    The only way to warm the car up comfortably in winter is to set the temperature control to the minimum (i.e. Off) when setting off. Then slowly over the next 5-10 miles turn the control up in 1 deg steps.

    I also notice the car windscreen sometimes suddenly mists up (even in dry weather) if I don't run with the AC on.

    Is this normal for the heating control system on a Mercedes A class?

    If it’s not normal, are you able to suggest what I ask the dealer to adjust/check/replace?
     
  2. whitenemesis

    whitenemesis Senior Member

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    Your Mercedes:
    CLS55 AMG '05
    Can you set the climate control to fully "Auto" and leave the car to set the fan speed? I would recommend to run with the a/c on permanently, summer or winter. Especially in winter as it will dry the damp air as it enters the car.

    Must admit my 203 climate had a large hysteresis, temps would swing from hot to cold. Swapping out the analogue controller for the digital (luxury) unit helped a lot. The CLS appears to be much better controlled, very little hysteresis in the system when left on fully Auto
     
    Last edited: Nov 20, 2012
  3. OP
    aclassdriver

    aclassdriver New Member

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    Your Mercedes:
    A Class / 2011 / 180CDI
    Unfortunately it doesn't have a fully Auto setting.
     
  4. npuk

    npuk Guest

    Weird seeing as you dont have the luxury climate control option which does have a fully auto fucntion and will adjust temp and speed automatically.
     
    Last edited by a moderator: Nov 20, 2012
  5. television

    television Always remembered RIP

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    Location:
    Daventry
    Your Mercedes:
    2002 SL500, 216 CL500, all fully loaded
    This is an easy one, the heater change over motor that sits under the steering column is at fault, they are only cheap little motors as on the 230,, give the thing a small whack and it may come back to normal. They only cost £35 plus fitting
     
  6. OP
    aclassdriver

    aclassdriver New Member

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    Your Mercedes:
    A Class / 2011 / 180CDI
    Thanks for your replies. The car is booked into the dealer again tomorrow, I'll let them know your suggestion. I hope they find the problem this time.
     
  7. FiveAlive

    FiveAlive New Member

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    Your Mercedes:
    2009 C220CDI
    Did you get a useful response from the dealer? My partner's A150 has a heating system that does the same thing as yours, as did another one that we test drove; I suspect it is normal behaviour but that doesn't stop it being annoying.
     
  8. OP
    aclassdriver

    aclassdriver New Member

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    Your Mercedes:
    A Class / 2011 / 180CDI
    Car was booked in for 2 days "investigation" after which they concluded the motorised flap was defective but they had to order the part from Germany. So, a few days later it went back to the dealer to have the flap replaced under warranty. The bad news is that it's made absolutely no difference! It's now booked in for further investigation in early January - arrghh!

    I'm beginning to wonder if the cabin air sensor (inside the ceiling interior light housing) is responding much too slowly. I've checked if the little fan it contains (which draws cabin air in over the sensor) is operating but have found it's very, very weak and will barely hold the tiniest, thinnest piece of tissue over the grill. Not sure if this is normal or not, but it can't be helping.
     
  9. television

    television Always remembered RIP

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    Daventry
    Your Mercedes:
    2002 SL500, 216 CL500, all fully loaded
    I still say the heater change over motor valve
     
  10. whitenemesis

    whitenemesis Senior Member

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    Your Mercedes:
    CLS55 AMG '05
    Does it have one?
     
  11. television

    television Always remembered RIP

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    Joined:
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    Location:
    Daventry
    Your Mercedes:
    2002 SL500, 216 CL500, all fully loaded
    Its my eyes again I saw 2011 and thought 211:(
     
  12. OP
    aclassdriver

    aclassdriver New Member

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    Your Mercedes:
    A Class / 2011 / 180CDI
    But isn't that what they've replaced already?
     
  13. OP
    aclassdriver

    aclassdriver New Member

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    Your Mercedes:
    A Class / 2011 / 180CDI
    The car has now been back to the dealer three times. Once for investigation. Once to have the motorised flap replaced (made no difference), again to have the dash mounted control unit replaced (still made no difference). The dealer is running out of ideas. I did some experiments blowing cold air on the small grill in the courtesy lamp housing and on the small grill in the dash mounted control unit and proved there's not a working temperature sensor in either of those locations (the heater didn't come on). So where is the sensor? It must be somewhere as it does seem to eventually respond, albeit slowly. I suspect it's somewhere inside the ducting that draws air from the footwell, but can't spot it. Any help appreciated.
     
  14. The Pan Man

    The Pan Man Senior Member

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    Warwickshire
    Your Mercedes:
    2008 E 500 A/G Classic colour combination.
    Just my 2 pennuth, Get firm with the dealer there is a fault on the car it's up to them to fix it. Book it in get a courtesy car from them and point blank refuse to take the car back until it's sorted. Any negative response ask to speak to the dealer principle there and then make a fuss, don't be fobbed off.
     
  15. OP
    aclassdriver

    aclassdriver New Member

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    Your Mercedes:
    A Class / 2011 / 180CDI
    I think I'm finally getting to the bottom of this. My local Mercedes garage haven't got a clue, and Mercedes Customer Service refuse to answer technical questions and just refer me back to the dealer. Hopeless!

    So, I purchased a DVD copy of the Mercedes Workshop Information System (WIS). Using this, together with a bit of internet trawling I have reached some conclusions of my own.

    If any other forum members can confirm or contradict any of this it would be great to hear from you.

    The diesel engine version of W169 (with engine type 640) is fitted with an electric PTC heater booster (R22/3). The "theory" appears to be that the diesel engine is so efficient that in cold weather the conventional heater (which relies on the heat from the engine coolant) may need some help. So the control unit is programmed to switch the electric booster heater on if the outside temperature (as indicated on the dash display) shows a temperature of 8 deg C or less.

    The cabin air temperature sensor (N22) for the Air-Conditioning System is recessed deep behind a small grill on the centre console - immediately below the air recirculate button and in between the temperature setting knob (16 - 28C) and the fan speed knob. Because the cabin air temperature sensor is recessed and also doesn't appear to draw cabin air across it (no little fan?) it takes ages to respond.

    The root of the problem appears to be that the electric heater is very powerful (it's linked to a 150A fuse!) and the cabin temperature sensor responds much too slowly. The end result is that on cold mornings the cabin temperature overshoots massively (not helped by the fact that warm air rises and the sensor is low down).

    The situation gets worse as the engine starts to warm up, as the conventional heater adds to the heat input. The air coming out of the vents makes the direction adjusters too hot to touch.

    Finally the cabin temperature sensor catches up and detects that it's far too hot and so switches the flap to route cold air into the car. So suddenly the vents blow cold air.

    Eventually (after 20 mins) the system settles down.

    The problem is worst when the outside temperature is just below 8 deg C, since the incoming air is not that cold an so gets even hotter from the heater.

    Pretty crap system really!

    If anyone can comment on the following (or the above) it would be great.

    1) Should the cabin temperature sensor have a fan that draws air over it? Mine doesn't appear to suck air from the cabin.

    2) Does anyone know a reason why I shouldn't ask for the PTC heater booster to be disconnected? We don't get really cold weather where I live in the UK, so I don't think this extra heater is really necessary.

    THANKS!
     
  16. mattkh

    mattkh Senior Member

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    Location:
    England
    Your Mercedes:
    W168 1999 1.6
    Hi
    It may just be a simple case of removing a fuse.
    Does your WIS dvd mention the location of the fuse used for the PTC heater ?If it does, then just remove the fuse and wait for the cooler days to test the theory.
     
  17. turbopete

    turbopete Senior Member

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    Location:
    Spennymoor
    Your Mercedes:
    2010 '60' Ford Mondeo 2.0TDCi Zetec (sorry)
    The heater booster can be turned off via the cluster menu, id imagine. I know mine on the 203I can be
     
  18. OP
    aclassdriver

    aclassdriver New Member

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    Leicestershire, UK
    Your Mercedes:
    A Class / 2011 / 180CDI
    I've trawled through the WIS and it's not a "plugin" fuse that I can just pull out. It's a bolted in fuse link that's set low down in a separate compartment on the side of the main fuse box. The battery has to be removed to get at it, and on the compartment there's a "tamper proof" seal. As the vehicle's still under warranty, if I do decide to try heater disconnection, I'll ask the dealer to do it.

    The other thing I found in the WIS is that the "in car temperature sensor" (that is behind the grill in between the knobs on the heater control panel) should be "equipped with an aspirator blower which draws air from the passenger compartment required for the temperature measurement". I can't detect any airflow in or out of the grill in front of my car's sensor...

    However, the dealer claimed to have replaced this unit, so surely 2 units wouldn't have the same fault? But then they also told me my car has no cabin temperature sensor...

    Think I need to pay them another visit :-(
     

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