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Thread: How does automatic HVAC work on our cars

  1. #1
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    How does automatic HVAC work on our cars

    Hi guys did search before lol
    how does our auto system work on our car
    when I press mine the fan goes on high
    when I put a small paper it gets sucked by the dome ligh
    thanks
    Erick

  2. #2
    E500E Guru nocfn's Avatar
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    Re: How does automatic work on our cars

    On auto, the air is sucked thru that vent and over a temp sensor, sending a signal to lower the fan speed. It takes a while since the hot air rises and the cold air sinks, but once the car is cooled off the auto does a very good job of keeping an ambient temp in my view. More technical than this obviously, but temp and speed are regulated pretty well, but full blast at the beginning has been my experience.
    1994 E500
    249/275 - 8F19 or 8F32 or 8320

    1991 560 SEC
    199/268
    2014 E350 Cab 799/264

  3. #3
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    Re: How does automatic work on our cars

    There are several inputs to the ACC system as found in the E500E and other US-spec W124 models:

    1) the vehicle interior temperature setting -- specified by the thumbwheel on the dashboard above the radio
    2) the ambient (outside) air temperature -- measured by an intake air temp sensor at the blower fan/resistor unit in the engine compartment
    3) the air temperature inside the vehicle -- measured by the passenger dash ambient air circulator motor & sensor located in the overhead console
    4) the ACC mode setting -- specified at the push-button unit (PBU)
    5) the ACC fan setting -- specified at the PBU
    7) the ACC system air temp setting -- measured by the sensor next to the evaporator

    The ACC system logic, located inside the PBU in the dashboard, is pre-programmed to use the variables identified by the above sensors and settings listed above to determine the optimal fan speed setting and temperature setting, and whether the heating system or air-conditioning system are activated. The "ACC mode setting" also controls which of the flaps are actuated inside the dashboard to direct warm and/or cold air to the correct dashboard & base-of-windshield ducts. The flaps are mechanically controlled by vacuum applied to six pods that comprise the system, which in turn are actuated by an electro-vacuum control ("Christmas Tree") located inside the dashboard just to the left of and slightly down/behind the passenger airbag.

    The interior heating system (control of the coolant through the heat exchanger) is controlled by the monovalve in the passenger side of the rear of the engine compartment. There are two monovalve designs that exist, depending on whether one owns an early or later E500E.

    The interior cooling system (air conditioning) is controlled by the "ACC mode setting", and is activated and operational when any of the left-hand three buttons of the five-button mode row are pressed. The air conditioning system is a whole nother beast, outside of the scope of this post. It is comprised of a number of individual elements, including the compressor (driven by serpentine belt), condenser (mounted in front of radiator), expansion valve, receiver/drier, and the evaporator deep inside the dashboard). The only modes where A/C is NOT activated is the "O" (off) and second-from-right button (economy/heating mode).

    There is extensive information that has been published by MB in their service manuals about the proper operation, testing and problem-diagnosis of the ACC system as found in the W124 chassis cars. In addition, there are various HOW-TO articles posted on this forum that detail how to replace the ACC pods (which as said control the mode flaps to direct airflow inside the motorcar), the ambient air temp motor, and other items.

    I highly recommend you read the following documents:



    Cheers,
    Gerry

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  5. #4
    E500E Guru nocfn's Avatar
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    Re: How does automatic HVAC work on our cars

    Thanks GVZ for the ....More technical than this... text I wrote
    1994 E500
    249/275 - 8F19 or 8F32 or 8320

    1991 560 SEC
    199/268
    2014 E350 Cab 799/264

  6. #5
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    Re: How does automatic HVAC work on our cars

    Also - when you press the "auto" button, if the temps have not yet stabilized in the vehicle interior, you may get high fan speed. This is normal.

    When using the air conditioning, if you want a "medium" speed, you'll generally need to set the temp wheel to a higher setting than seems logical, AND may also need to direct one of the center vents to blow up towards the temp sensor on the roof.

    All of which is why the automatic system really should have had a 'medium' fan speed button, or function. Most of the time high & low aren't what I want, and it's a nuisance to try and get the medium fan speed. Worse yet, it's a nightmare to convert to manual climate control, as nearly every part of the system is different. You'd basically need a complete Euro-spec donor car to extract everything required (including swapping out the entire heater box). Probably 100 hours labor total on the two vehicles. Which is why there's no "HOW-TO" thread on this yet.



  7. #6
    E500E Guru nocfn's Avatar
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    Re: How does automatic HVAC work on our cars

    truer words not ever spoken - a mid speed is definitely needed on the fan....
    1994 E500
    249/275 - 8F19 or 8F32 or 8320

    1991 560 SEC
    199/268
    2014 E350 Cab 799/264

  8. #7
    Member northNH's Avatar
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    Re: How does automatic HVAC work on our cars

    Regarding the lack of fan speed adjustability...

    A quick look at the wiring diagram/schematic for the blower motor, resistor pack and pushbutton blower speed switch shows it would be straightforward to intersperse an infinitely variable speed ("volume") control to allow ANY fan speed as long as pushbutton set to Max.

    And you'll also want to cleverly integrate your new chosen switch in the cabin...Rotary switch or thumbwheel in an upper switch slot?

    No more electrically challenging than replacing the single relay resistor that determines the fixed 12 second interval of the intermittant wiper setting with a common variable resistance control to allow infinitely adjustable interval.

    Much much less challenging than figuring out how to replace that god-awful lights/wiper switch with a setup with the wiper functions, including spring-loaded single wipe, on a separate stalk on the right, where it's belonged since the 70's.
    '92 300CE Black/burgundy Sportlined
    '94 320TE Silver/gray
    '86 BMW 635CSi Black/black 5spd
    '67 Volvo Amazon wagon Green/green 75Kmi time capsule
    '62 Volvo Amazon Black/red 4Dr 65Kmi time capsule

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  10. #8
    Mario albdentist's Avatar
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    Re: How does automatic HVAC work on our cars

    And you'll also need to cleverly integrate the new switch in the cabin....

    Maybe use the slide right below the air vent..?

  11. #9
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    Re: How does automatic HVAC work on our cars

    Perhaps a stock fader switch from an early car with the BE1432 could be adapted to the purpose.....

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