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Thread: Replacing the blend and mode door actuators, Including ATC & MTC Trouble Codes

  1. #1
    LH Electronics Specialist
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    Arrow Replacing the blend and mode door actuators, Including ATC & MTC Trouble Codes

    Description of the devices:

    The blend door actuator is a servo responsible for controlling the mix of warm air from the heater core and cool air from the A/C and outside. If it malfunctions, your climate control system may not respond properly when you change the temperature on the climate control system.

    Both the manual and the automatic systems use this servo to control the blend door. If your ATC system fails to automatically change temp, this is also an indicator that the servo has failed.

    The Mode Door Actuator controls the direction of airflow. It selects which way air flows through the different ducts. If your climate control system has lost the ability to control which vents the air comes out of, the MDA has probably failed. Both the ATC and MTC systems use this device.

    Both failures can be seen by the climate control system's integrated diagnostics system.

    Accessing the Integrated Diagnostic System:

    Procedure for Automatic Temp Control (ATC) equipped vehicles:


    With the engine ON and the vehicle stopped set the temp to 75*F. Next hold down the Floor, Mix and Defrost buttons until the ATC display begins to flash. While the display is flashing, the system will run it's tests and calibrations.

    When the display stops flashing, the tests and calibrations are complete.

    The ATC display can only display one error code at a time. Any stored codes can be scrolled through by pressing the Panel mode button once for each code. DO NOT press any other button or you will have to start the test over. Note the codes and refer to the list below to determine what system is malfunctioning.

    Procedure for Manual Temp Control (MTC) equipped vehicles:

    Set the FAN SPEED knob to anything except OFF.
    Set the TEMPERATURE knob all the way into the COLD position.
    The MODE knob must be in DEFROST.

    The A/C button can be on or off.

    Press and hold the EBL (electric back light, a.k.a rear windshield defrost) button until AC00 appears in the odometer display. You will hear a chime and the A/C light will blink.

    Release the EBL button and wait for the A/C light to stop blinking.
    Codes will display in the odometer in the form ACxx where xx is the actual trouble code. Only one code can be displayed at a time, to scroll through the codes press the A/C button. You will hear a beep every time the button is pushed. When all codes have been displayed, the odometer will return to normal. DO NOT push other buttons or turn any knobs while doing this or the test will be aborted and you will have to start over.

    List of Climate Control DTCs:
    23: Blend door actuator feedback failure (bad servo)
    24: Mode door actuator feedback failure (bad servo)
    25: Ambient temp sensor failure
    26: ATC In Car thermister failure (ATC only, bad ATC head or ATC head disconnected while battery is connected)
    27: ATC in car sensor failure (ATC only)
    31: Recirculation door actuator stalled (stuck door or bad servo)
    32: Blend door actuator stall failure (stuck door or bad servo)
    33: Mode door actuator stall failure (stuck door or bad servo)
    34: Engine temperature message not received (ATC only, bad ATC head or bus failure)
    35: Evaporator sensor failure
    36: ATC control communication failure (bad ATC head or bus failure)
    37: Blend door actuator output shorted to battery (bad servo or wiring fault)
    38: Blend door actuator output shorted to ground (bad servo or wiring fault)
    39: Mode door actuator output shorted to battery (bad servo or wiring fault)
    40: Mode door actuator output shorted to ground (bad servo or wiring fault)
    41: Recirculation door actuator output shorted to battery (bad servo or wiring fault)
    42: Recirculation door actuator output shorted to ground (bad servo or wiring fault)
    43: Common door shorted to battery (probable wiring fault, could be a servo that failed in an odd manner)
    44: Common door shorted to ground (probable wiring fault, could be a servo that failed in an odd manner)
    45: A/C control blend door input open or shorted with respect to ground (MTC only, wiring fault or bad servo)
    46: A/C control blend door shorted to battery (MTC only, wiring fault or bad servo)
    47: A/C switch failure (MTC only)
    48: A/C control mode door input shorted to ground (MTC only, wiring fault or bad servo)
    49: A/C control mode door input shorted to battery (MTC only, wiring fault or bad servo)
    50: A/C control electric backlite (EBL) switch fails (MTC only)

    51: System voltage too low for calibration!!! (check battery cables, battery and alternator!)


    Both the blend door and mode door servos are in the same general area in the dash and while they are similar in construction, they are not interchangeable.

    The blend door actuator is the hardest to access, it is located at the bottom center of the dash directly above the center hump for the exhaust. The mode door actuator is located higher up in the center portion of the dash and is visible when the radio is removed.

    Tools required for the procedure:
    5/16" wrench (ratcheting wrench STRONGLY recommended!)
    A 5/16" socket on a 1/4" ratchet may come in handy as well but may not be usable on the blend door actuator due to the tight fit.
    #2 Phillips screwdriver
    T15 Torx screwdriver
    Needle nose pliers
    Other technician aids such as good tunes, a fan, insect repellent and maybe some beer! Plan on this taking an entire afternoon.

    To gain access to the blend and mode door actuators remove the following:
    Glove box
    Center console
    It is easier if you remove the radio as well, especially when servicing the mode door.
    Some of the lower ducting will need to be removed as needed. Use the pliers to help with any clips you may need to remove

    Some screws will need to be accessed from one side of the car while others will be needed to be accessed from the other side. Clear both footwells of junk before proceeding.

    Click the images for a bigger version
    Removing the glove box and center console


    The first screw for the blend door actuator must be removed from the driver's side, the other two are removed from the passenger side. Note the screw I have started to remove and the insanely close working space. The screw closest to the firewall is a royal pain!


    Once you get it out, note the position of the shaft. The shaft is keyed and it must be in the same position on the new one to fit in directly. The problem is, due to a worm gear in the servo mechanism, you cannot turn the shaft on the actuator itself. What you must do is turn the door somehow until it lines up with the new actuator. I did this by placing the new actuator up against the door actuator shaft socket and rotated it until it lined up.... this is a LOT easer said than done.

    The job is not easy, especially if it's a blend door actuator that has failed. The mode door is considerably easier to repair. Good luck and enjoy your properly working A/C system.
    Last edited by hardwareguy; 12-27-2009 at 07:22 PM. Reason: What's this EBL button you speak of? Its the rear defrost!

  2. #2
    Silver Es
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    Can a mod please ADD to the Title ATC TROUBLE CODES at the end..Makes it alot easier for people to search THX

  3. #3
    Imported. From Detroit. Sneke_Eyez's Avatar
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    Sure thing.
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  4. #4
    Senior Member LTrain01RT's Avatar
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    I need to change my mode door actuator sometime
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  5. #5
    Resident F1 LH driver LOUD02Special's Avatar
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    Beware, the MDA may not be the problem. I am on my third and still throw a code 24. Updates to come.
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  6. #6
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    Quote Originally Posted by LOUD02Special View Post
    Beware, the MDA may not be the problem. I am on my third and still throw a code 24. Updates to come.
    Check continuity between all the MDA wires and the ATC head and verify that the mode door is not jammed. Perhaps the ATC control head is faulty as well.
    2004 300M Special...175,000 miles.
    2002 PCM installed...now to fix that airbag light due to the VIN code mismatch. :|

  7. #7
    Resident F1 LH driver LOUD02Special's Avatar
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    Quote Originally Posted by hardwareguy View Post
    Check continuity between all the MDA wires and the ATC head and verify that the mode door is not jammed. Perhaps the ATC control head is faulty as well.

    ATC head has been replaced, mode door movement has been verified. There is some kind of TSB that IS300M found that he's gonna get for me.
    Carlisle 2007 "Most Unique Modification"
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  8. #8
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    Quote Originally Posted by LOUD02Special View Post
    ATC head has been replaced, mode door movement has been verified. There is some kind of TSB that IS300M found that he's gonna get for me.
    Let us know what you find and I'll post the TSB up in the FAQ so it doesn't get lost in a sea of posts.
    2004 300M Special...175,000 miles.
    2002 PCM installed...now to fix that airbag light due to the VIN code mismatch. :|

  9. #9
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    Default done deal

    Is it a done deal that you have a bad servo when you get an 23 error code?

  10. #10
    Resident F1 LH driver LOUD02Special's Avatar
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    Not a done deal but certainly the first place to look/part to replace.
    Carlisle 2007 "Most Unique Modification"
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    Past - 2002 300M Special
    Present - 2007 Chrysler Aspen Limited

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