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Climate temperature offset on A4 B7 - diagnosing with VCDS

Discussion in 'VCDS (formerly VAG-COM) forum' started by S6, Feb 16, 2014.

  1. S6

    S6 New Member

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    Sorry - this is going to be long!

    I bought a 2006 A4 Cabriolet a few weeks ago, and became aware almost immediately that the interior temperature was too hot. Aircon seems to work (always difficult to tell in the winter, but it demists the windscreen pretty quickly, and the Econ light goes on and off with the switch - all good indications). There are no faults indicated using my new VCDS/Hex-USB+CAN (obtained from NHN yesterday - excellent service as expected!). I've been in touch with the garage that sold the car to me, but they claim that if the aircon doesn't show any fault codes it must be OK.

    I've spent several hours using VCDS to try to diagnose what might be wrong, but I don't really know how the climate control is supposed to work, particularly when it comes to which sensors are used to control the feedback. I tried resetting the basic settings using VCDS, and that seemed to cycle all the flap motors to work out min and max positions.

    Here are some tests I've done. Initially I wanted to prove to myself that there really was a temperature offset, so I set various temperatures (left and right sides linked together) and used VCDS to indicate the temperature read by the Dash thermistor, and the left and right flap positions bigger numbers indicate more heat let through).

    Ambient: 9.0 C
    Set Actual Flap
    16 19.5 127/135 (don't know why these are higher than for 18 C - might not have waited for readings to settle)
    18 21.5 123/131
    20 23.5 131/141
    22 25.0 154/162
    24 26.5 172/178
    26 28.0 182/192
    28 29.5 190/202

    So, it was good to see that the internal temperature sensor corroborated my feeling that the temperature is always a few degrees higher than indicated. There are dozens of forum postings on the web about exactly this problem - most owners just seem to have lived with it, and the A4/S4 has a poor reputation for temperature stability. Some found an easy fix in the form of a failed flap motor. All my flap motors are fine, as they track the specified value perfectly. Some suggestions have pointed at a Channel 11 adaptation setting which they thought might give a temperature offset, but I tried this, and it doesn't seem to do anything. I think it is required so the car knows whether it's in a cold/moderate/hot climate - no idea what it does with this information.

    We can cope at the moment by setting the temperature to 17 C as this gives a reasonably comfortable 20/21 C temperature inside the car, but the temperature does not have the pinpoint precision control that my old 1996 S6s used to have so I'm pretty sure something is wrong. I am worried that it might struggle to control the temperature in the summer when the ambient temperatures hit the high 20s.

    VCDS indicated that the fan that draws air past the Dash Temperature Sensor in the front of the Climate Control, was rotating at a sensible speed. Just to see what would happen, I blocked the vent for the Dash Sensor, and the indicated temperature went up to high 20s (it must be hot back there). I then checked to see if the flaps were moving to try to bring the temperature down, and they didn't budge over several minutes. So it looks as if the Dash Temperature Sensor is not part of the feedback loop. Very odd! Also, it appears that except for sensors in the outer dash and footwell vents, there is only the one temperature sensor in the interior of the car. I expected more than that - top of dash (with a little fan to suck air past the sensor) and headliner were pretty standard on most cars I thought. And I thought that dual temperature zone cars would have even more.

    So, what controls the temperature flap position? Could it be that there is no feedback - it just predicts a flap position based on some of the following:
    - temperature set on the front panel
    - coolant temperature
    - ambient temperature
    - sunlight sensors (where are they?)
    - evaporator temperature (is that important?)
    - speed of vehicle (might affect rate of heat loss?)

    Or do I just have a faulty climate control unit? I'm tempted to buy an ebay one to try swapping it out, but they're £100. Ouch!

    Here's another set of data from VCDS that I took later when I set the temperature controls to 16 C and left the car for 10 minutes to stabilise all the readings (engine at idle). The car never got below 20 C.

    [TABLE="width: 434"]
    [TR]
    [TD]A/C Valve (N280) - Current[/TD]
    [TD="align: right"]0.352[/TD]
    [TD] A[/TD]
    [/TR]
    [TR]
    [TD]A/C Valve (N280) - Duty Cycle[/TD]
    [TD="align: right"]36.3[/TD]
    [TD] %[/TD]
    [/TR]
    [TR]
    [TD]High-Pressure - Signal (G65)[/TD]
    [TD="align: right"]23.5[/TD]
    [TD] %[/TD]
    [/TR]
    [TR]
    [TD]Refrigerant Circuit - Pressure[/TD]
    [TD="align: right"]6[/TD]
    [TD] bar[/TD]
    [/TR]
    [TR]
    [TD]Current Value (Right Temp Flap)[/TD]
    [TD="align: right"]121[/TD]
    [TD][/TD]
    [/TR]
    [TR]
    [TD]Specified Value (Right Temp Flap)[/TD]
    [TD="align: right"]120[/TD]
    [TD][/TD]
    [/TR]
    [TR]
    [TD]Right Footwell Vent - Temp. (G262)[/TD]
    [TD="align: right"]39.6[/TD]
    [TD]°C[/TD]
    [/TR]
    [TR]
    [TD]Right Vent - Temp. (G151)[/TD]
    [TD="align: right"]28.2[/TD]
    [TD]°C[/TD]
    [/TR]
    [TR]
    [TD]Evaporator Outflow - Temp. (G263)[/TD]
    [TD="align: right"]4.5[/TD]
    [TD]°C[/TD]
    [/TR]
    [TR]
    [TD]Fresh Air Intake - Duct Temp. (G89)[/TD]
    [TD="align: right"]16.5[/TD]
    [TD]°C[/TD]
    [/TR]
    [TR]
    [TD]Measured Ambient - Temp. (G17)[/TD]
    [TD="align: right"]10.5[/TD]
    [TD]°C[/TD]
    [/TR]
    [TR]
    [TD]Dash Panel - Temp. (G65)[/TD]
    [TD="align: right"]20.5[/TD]
    [TD]°C[/TD]
    [/TR]
    [/TABLE]

    What I don't understand is why the flaps weren't shut down to their minimum positions (about 30) as the measured temperature (20.5 C) is way higher than the requested temperature (16 C).

    And one final thing - at the end of my session, I turned the controls to LO. Now, I believe this is supposed to give up on any kind of feedback and just try and run the aircon open loop to get the car to the lowest possible temperature. After ten minutes, the car was no lower than 15 C - and this with an external temperature of 9 C. I cycled the heater controls through various temperatures/vents/fan speeds and then tried again - this time it got much colder, but nowhere near as cold as I expected it to get. So there is an intermittent problem as well.

    Any thoughts/suggestions gratefully received.
     

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