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    Ross-Tech Employee DrPeter's Avatar
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    Tis’ the HVAC Season

    Tis’ the HVAC Season

    By: DrPeter

    In this Tech Tip I will be explaining some quick and easy diagnostic tips for the Volkswagen/Audi HVAC systems. We will be focusing on the systems that are diagnosable with the VCDS scan-tool. Even though the system designs maybe slightly different between brands and models, they are generally similar enough that we can diagnose them using the information contain herein. It is always advisable to use the current repair information and training materials available and understand the system that you are working on, a little time spent learning will save you a lot of wasted time fixing your mistakes when you could have been fixing the vehicle.

    Yep, got to say this stuff because frostbite hurts more when the flesh thaws and blindness caused by refrigerant gases blasting out at 400psi can really suck when trying to work on cars, so... As with any potentially hazardous system of the automobile, we must remember to follow all of the manufacture’s safety, caution and warning notices found in the Factory Repair information. Also pay attention to any local, state or government laws regarding properly trained and/or licensed personal when working with regulated materials, such as refrigerants. If you do NOT know what you are doing or have not been properly trained, STOP! HALT! ARRÊT ¡ALTO! 停止

    I have been trained in many aspects of the automotive and commercial HVAC systems (and living in SW Florida helps, AC season 365). I suggest anyone who is working with the HVAC systems in the Automotive world to get the right training and certifications, in 1993 I became a member of , Mobile Air Conditioning Society (MACS) Worldwide, went to the class and became certified, so I suggest to cover your butt!

    Diagnose first, then fix:
    Diagnosing the modern Volkswagen/Audi HAVC system can be a bit challenging at times, certainly not impossible and troubleshooting is now easier with newer HVAC Controllers having more diagnostic capabilities compared to older generations. Yes, I will try to do a small tech tip on the older MK4 systems, though these systems are not completely diagnosable with the VCDS tool.

    Here are the quick and easy checks you can use to help identify the cause of the concern within the HVAC system.

    1) Check the basics in the [08-Auto HVAC] controller with VCDS.
    • Address any fault codes accordingly, use the Ross-Tech wiki and or repair manual
    • Make sure Basic Setting are complete
    • Verify coding is correct
    • Check for correct part numbers or controller updates

    2) Check the Measuring Value Blocks(MVBs) for the following (most MVBs will be labeled and you will be able to identify the data in the field):
    • Compressor shut off code
    • Refrigerant Pressure from the Sensor G65
    • Outside Temperature (filtered and unfiltered) (adjusted vs. not adjusted)
    • Evaporator Temperature
    • Vent Temperatures

    3) Check actual Refrigerant pressure
    • The best way to check AC system pressure is to use a good quality AC Gauge set. Verify the system has pressure, because the Refrigerant Pressure Sensor G65 maybe incorrect and the MVB data could show a different pressure. Also remember what Jef says, check the charge, pressure will not indicate a full charge.
    • Learn how to read the gauges, it is like reading between the lines, it will tell you a lot!

    4) Check for proper operation of cooling fans
    • In many vehicles an [01-Engine] [Output Test] will activate BOTH fans, if equipped
    • The HVAC system in many cases will not function when the fans have failed. Check ECM for codes too.

    5) Check for damage:
    • Compressor shaft is spinning (shear hub is not damaged)
    • Wiring, connections’
    • Hoses, fittings and missing valve caps(primary seal)

    Addition notes, tips:

    On some 2006 to 2009 vehicles it may be necessary to reset the Temperature Sensor, this can occur when the AC controller or ambient temperature sensor were replaced. To reset the Temperature Sensor, drive the vehicle above 30mph or let the vehicle sit, engine idling, for about 20 minutes.

    2012 and newer Jetta sedans may have a customer concern of A/C shutting off when coming to a stop. When checking the HVAC controller for the Compressor Shut off code, [12- Shut-Off requested by Engine Control] maybe found, this is usually considered normal, the ECM is programmed to command the HVAC controller to shut off the compressor to allow increased engine rpm/vacuum when braking, mostly when the engine is cold, pre-catalyst operating temperature.

    If your compressor shut off code is [0], check the N280-Refrigerant Regulating Valve** valve, this is what regulates the amount of refrigerant that the compressor ‘pumps’ through the system. The N280-Refrigerant Regulating Valve activation signal can be checked with an incandescent light bulb in its place. Unplug the N280-Refrigerant Regulating Valve, connect a test bulb to the connector from the HVAC controller and start the vehicle, turn on the AC and watch for the activation command from the HVAC controller. If the bulb lights and the compressor had failed to operate prior, then the N280-Refrigerant Regulating Valve and/or compressor has most likely failed.

    Here is a list of typical Compressor Shut off codes:
    - 0 = Compressor ON
    - 1 = Compressor OFF: Refrigerant Pressure too high (> 32 bar)
    - 2 = compressor OFF: Basic setting not performed
    - 3 = Compressor OFF: Refrigerant Pressure too low (< 2.0 bar)
    - 5 = Compressor OFF: Engine Start Detection
    - 6 = Compressor OFF: ECON Mode active
    - 7 = Compressor OFF: A/C System / FAN OFF
    - 8 = Compressor OFF: Outside Temperature too low (< 3.0 °C)
    - 10 = Compressor OFF: Vehicle System Voltage too low (< 10.1 V)
    - 11 = Compressor OFF: Engine Temperature too high (> 118 °C)
    - 12 = Compressor OFF: Shut-Off requested by Engine Control
    - 13 = Compressor OFF: Vehicle System Voltage too high (> 17 V)
    - 14 = Compressor OFF: Evaporator Temperature too low
    - 16 = Compressor OFF: Compressor Activation faulty
    - 17 = Compressor OFF: No or implausible Signal Pressure Sensor
    - 18 = Compressor OFF: Vehicle Speed Shut-Off
    - 19 = Compressor OFF: Shut-Off requested by Central Electronics Control (Load Management)

    (This list may vary form vehicle to vehicle, Example: The Audi A6 (4F) is different, so check your repair manual or Measuring Value Block pop-up)

    The newer HVAC systems use a pressure sensor – G65, not a pressure switch. The G65 can send information about the refrigerant pressure to the HVAC controller, a pressure switch, like the F129 cannot.

    • Located in the same place as the F129, the G65 is different, yet does the same functions as the F129.
    • It uses a pressure sensor and a microprocessor to create a PWM (Pulsed-width Modulated) signal
    • Pin 1 & 3 provide Ground and Power, PWM signal is on Pin2.

    Update: August 31, 2015

    The following Information has been deemed useful:

    - 2006+ Volkswagen New Beetle/Convertible, the N280-Refrigerant Regulating Valve is controlled by the ECM.
    Use Measuring Value Block 135, 136 and 137 for information about,
    ... the Duty cycle of the coolant fan
    ... the Relay 1 for the coolant fan
    ... the coolant fan after-run
    ... Condition of the AC
    ... Condition of the AC Compressor
    ... A/C-Pressure
    ... Fan request from A/C-System

    -2006 to 2010 Volkswagen New Beetle/Convertible.
    Fault codes P2612 and P2613 - may caused by wiring connections and or wiring harness routing/damage.
    see the Ross-Tech wiki page for more information.

    19044/P2612/009746 - A/C Refrigerant Distribution Valve: Short to Ground

    -2011 to 2015? Jetta and 2012 to 2015? Beetle - A/C Controller Basic Settings. (just in case you did not see the wiki page)

    - 00898 - Control Circuit A/C Compressor: No or incorrect Basic Settings

    Added Information (12 July 2016):

    G65 Pressure vs. Temperature chart
    (When key is on, engine is off, static system pressure)

    15C - 59F - 4.0 bar
    20C - 68F - 5.0 bar
    25C - 77F - 6.0 bar
    30C - 86F - 7.0 bar
    35C - 95F - 8.0 bar
    40C - 104F - 9.0 bar

    Important to note about the G65 sensor:
    - A reading of 0.0 bar would indicate an absolute vacuum
    - At Sea Level (atmospheric pressure) the typical reading is 1.0 bar

    Typical N280-Refrigerant Regulating Valve resistance (on working vehicle) = 10-12 Ω

    Added Information (1 June 2018):

    We strongly recommend NOT to plug the 'V51 - Coolant after run pump' connector to the N280-Refrigerant Regulating Valve.
    -If this is done, the HVAC system will not cool the vehicle.

    - This can happen, usually when there was repair work to the front of the vehicle.
    Someone accidentally swaps the two connectors.

    - There should be a fault code for the N280, see below example, mostly this happens with the 2.0L Volkswagen, where the connectors are close enough to each other so that they can be swapped.

    Example from a 2011 Passat CC 2.0L(CBFA-engine)
    This vehicle had a fault code in the [08-Auto HVAC] controller: B10A9-Control Circuit for A/C Compressor
    N280 connector
    Pin1 = br/sw
    Pin2 = gn/sw

    V51 connector
    Pin1 = vi/rt
    Pin2 = br

    Well, hope this helps and remember to check your basics, that includes any technical service bulletins TSBs that may apply,

    Last edited by DrPeter; 06-01-2018 at 02:40 PM. Reason: Added Information (12 July 2016)

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