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Air con won't start - please help me diagnose!!

lube Jul 23, 2017

  1. lube

    lube New Member

    Hi All,
    First post here, got quite a strange air-con problem that I'd appreciate any help diagnosing. Car is a 2003 A4 B6 1.9tdi (engine code AVF). Have just replaced the G65 pressure sensor to no avail. Here are the symptoms:
    • Air con worked last year, but this year was blowing warm.
    • Last year the ECON light stuck on for a while under a wee
    • Had not been re-gassed since I owned the vehicle (5 years) so had system professionally re-gassed using an automatic machine that flushed, tested, lubricated and re-filled.
    • Immediately after the re-gas the air con was still blowing warm air, both when vehicle stationary and under motorway driving.
    • Fuses on box by drivers side door all intact.
    • ECON light off (but goes on if I disconnect the G65 pressure sensor or corresponding 5A fuse).
    • VCDS lite not reporting any relevant fault codes (codes re engine mount and steering angle sensor assumed irrelevant). The first time I checked this a few days ago I got no faults. I thin re-checked and got an "00819 high pressure sensor (G65) 29-10 short to ground - intermittent", so I replaced the sensor with a brand new hella-behr part. Thinking about it I was pulling the fuses to check if the econ light went on/off, this may have been the cause of the fault code.
    • Auxiliary fan not coming on, voltages at the 4-pin fan connector next to the PS reservoir don't change when I switch ECON on/off.
    • No drop of engine revs when I switch ECON mode on/off.
    • Air con compressor pulley seems fine, can't see or hear any problems.
    • VCDS reports a Refrigerant Circuit Pressure of 5.0 bar and a High Pressure Signal (G65) of 20.5% at about 18C ambient temp, which according to VCDS is about right.
    • G65 pressure sensor replaced, fault codes cleared, air con still not working, no aux fan etc.
    • VCDS compressor shut-off criteria: all 0.0
    • VCDS Compressor torque status: 0, Required Engine Torque: 2.0 Nm
    • A/C valve (N280) current: 0.648 A
    • A/C valve (N280) duty cycle: 62.4%

    Based on the above I think this means:

    • The aux fan is not the problem. If it was, the air con would still blow cold under motorway driving (as in this thread)
    • The G65 sensor may have been faulty, but now it certainly isn't.
    • The compressor is not starting.
    • According to the shut-off criteria being all zeros, the compressor should be running.

    A few things I was wondering:
    • If the compressor was faulty, would the aux fan run anyway? According to this thread I suspect the fan only starts if the pressure is above a certain point, i.e. if the compressor is running.
    • How to work out if the compressor is faulty?
    • What other diagnosis checks can I do?!!
    I'm starting to suspect that it might be the compressor, although I did see another thread about the N280 valve sticking without throwing a fault code.

    As you can see I have done a fair bit of diagnosis and probably replaced the G65 sensor for no reason, so any help would be appreciated!!!
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  3. zoney

    zoney Member

    Im having the same problem as above tested with vcds and all seems fine although i think i still have a leak cause after 1 week the pressure in the system has gone from 5bars to 1 bar and econ light is now back on :( although i dont think the system can be fully regassed if the compressor isnt kicking in so thatt could exspplian my low pressure reading .
    From what ive read if you remove the belt from the compressor and the pully spins freely then its a new compressor required as part of it has failed and are prone to it . I have yet to do this test on mine and have been quoted 185 quid for aa new compressor at a local car parts place
  4. lube

    lube New Member

    If your econ light is on then you do not have the same problem as me. If you are reading 5 bar to 1 bar in one week it sounds quite likely this leak is your problem. Once the pressure drops too low the pressure switch would cause the econ light to come on and would stop the compressor from running. Suggest you get your system leak tested, fixed and refilled before you start tearing out the compressor!!
    In the mean time I re-posted this problem here due to no response on this thread. Hope this helps someone.
  5. zoney

    zoney Member

    The thing is i have had the system tested for leaaks and pressure tested which reports back as all been fine . But as i said above if the compressor isnt kicking in when the system is filled is it completely filling ????? hence after a period of time the pressure level in the system dropping ?
    I will keep an eye on your other from post that seems to at least be getting some helpful replys unlike the lack of replys on here
  6. lube

    lube New Member

    With the engine off your system pressure should be constant at around 5 bar. The system can be filled irrespective of whether the compressor is working or not (I had mine filled even though the compressor was disengaged due to missing front hub). Like I said if your system pressure went from 5 bar to 1 bar you either have 1) a leak or 2) a faulty pressure sensor. If the pressure tester is saying you don't have a leak then either 1) you need to replace your tester or 2) you have a faulty pressure sensor!

    From what you are saying it sounds quite likely you have a faulty pressure sensor (G65 thrust sensor). This is easy to replace by removing the passenger headlight and unscrewing the sensor (at least it was on my B6 A4 but could be different depending on what car you have). In trying to fix my problem I ended up replacing the pressure sensor with a new Hella-Behr part for £32 off ebay.
  7. zoney

    zoney Member

    The g65 sensor has been replace due to it been faulty at the start of this i also had to replace the condensor so i no those to parts are fine although the compressor is not kicking in so im guessing theres a problem there as well . The last time i had it filled they put some dye in the system and they checked it and cant see any marks from the dye .

    On another not its good to see you have yours now fixed i will have to look at my compressor to see if i have any bits missing :D
    I will be trying another place that deals with air con to see if they can find the fault

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