Question about Heating & Cooling

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For some time AC (92 Toyota Corona import) AC not working properly. Lately windows fog up rapidly in wet & humid weather - V dangerous for driving as AC needed to dehumidify B4 blow to Windscreen in this car. Took to repair shop yesterday. They confirmed gassing OK! but also confirmed - - AC not working properly when first tested. - After test attachments applied AC began working just fine - all within spec. They suggested fungus buildup problem in evaporator and vents and so I agreed to chemical treatment (~#70.00). This afternoon picked up car. AC working just great. But 15Km down the road (after admittedly fiddling a lot with dash controls), I am currently back to - not working! Vent Temperature that began as very cold (normal) on leaving shop is now only noticeably cooler than external temperature. When I took delivery of car, venting was selected Recirc. Shortly after switching from car-floor vents to demist and fresh, I encountered the same old problem. Switching back did not alter performance of AC. Having phoned service center straight back they agreed there was some kind of intermittent problem with my AC system and agreed to take another look. Meanwhile I am attempting to personally gather whatever helpful evidence I can to assist in tracking down this problem at the lowest cost possible as I am currently 160.= poorer for no-change. I cannot afford to throw money like this at an old car (currently unemployed). Can you - - suggest ways I can evaluate nature of problem in order to assist in this process as customer? - suggest most likely points of failure (corroded electrical terminations) etc. that I can suggest to service techs should be specifically inspected for intermittent issue described above? Yours sincerely, Jim Parr

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  • jim_parr Feb 05, 2010

    Thank you sir/madam.
    May I have some time to pass on the question to Service chap? Your answer makes a lot of sense to me in respect of the fogging - severe.

    But does this solution (point of failure) also cover the simple fact that AC delivered normal performance immediately after leaving shop but then failed to deliver adequate cooling shortly after?

    Yours sincerely,
    Jim Parr



  • jim_parr Feb 05, 2010

    I think previous suggestion has merit and it will be checked out. However, I am primarily concerned that AC system literally goes on/offline so-to-speak -- mostly offline these days.

    I should also mention previously described trip from repair shop is middle summer here in Auckland NZ so days are usually hot & steamy.

    Yours sincerely,
    Jim Parr

  • jim_parr Feb 05, 2010

    Thank you for response.
    Yes - as electronics engineer I can certainly understand all that you say as I deal with this sort of problem on a rather different physical scale to yours daily (when I am working). Nonetheless, because it is clear that my local service shop is stumped for options about where to look I was hoping for some direction in respect of where most probable cause/s for problem exists.

    You cannot solve my problem by email obviously - but you may be able to offer further suggestions as to what is going on. That is my hope.

    I think the service center I mention has competent staff but they are not very ('core of problem' oriented), and this is why I am talking to you.

    I do hope you can assist further - in spite of your accepted and perfectly understood comments in regards to intermittent problems of this nature.

    For example, there is probably a very straightforward decision/checking logic tree that can be followed for my problem (if someone has written it up).

    I was also hoping I could provide evidence of some solid kind to my service center by conducting simple experiments (that may take some time) over next few days in order to save myself money and them chargeable time.

    Yours sincerely,
    Jim Parr



  • jim_parr Feb 05, 2010

    Seems to me you are saying in your reply that -
    - the problem is actually control (electrical).
    - the problem is not feedback but command based.
    - the problem may be readily discovered as failed solid-state or mechanical dashboard 'AC mode' control issue as delivered from dashboard. Is this a worthwhile option to explore? I have no flickering lights on command buttons or other disturbing symptoms but I am intrigued that AC (delivery) just switches OFF when it pleases when commanded to be ON.

    I believe as long-term user of this vehicle that some form of 'control' failure is a highly probable cause for AC performance changes and intermittent behavior. I think this might be dashboard command delivery or (more likely) some form of connector failure at AC unit.

    What say you?

    Yours sincerely,
    Jim Parr



  • jim_parr Feb 05, 2010

    Not so much that previous contributions are incorrect or irrelevant - it is just that I believe the issue to be ON/OFF (and therefore electrical only) and FixYa resource data has not yet offered a schematic or otherwise solid signal name in 92 Toyota Corona wiring loom to inspect for flaky or intermittent signals.

    Yours sincerely,
    Jim Parr

  • jim_parr Feb 05, 2010


    What? Is there a heater core in a compressor based system?

  • jim_parr Feb 05, 2010

    Or are you specifically separating AC from basic heating mechanism. Because that would make sense. Can you clarify please?

    Jim Parr



  • jim_parr Feb 07, 2010

    Hi Dougie,
    Thanks for comments. Your last - host of basics to check out - plus several relevant suggestions from others seems to offer the comprehensive check-it-out suite. It HAS been in repair shop (AC specialists amongst other services).

    From the several contributions (thanks to all) I surmise -
    1)
    From dry interior (checked) the fogging is very possibly the first suggestion - perished or broken seal at firewall allowing moisture to re-enter the venting system. This is most consistent with facts as personally experienced.
    2)
    That intermittent problem is most likely due to connector corrosion/oxidation at the climate control module... OR ...because I know I have a slightly sticky 'blend-door' (not used much) and often hear a loud 'thunk' when switching from demist to cool/floor-vent settings. Subsequent operations are usually audible but far less so.

    All in all, I now believe that TWO discrete problems exist - ingress of moisture downstream of AC rather than failure of AC unit to extract it effectively, and also an intermittent fault that may be little more than electrical connection or sticky blend-door.

    Yours sincerely,
    Jim Parr

    BTW, as I am planning to shop return with collected suggestions from Fixya, for further inspection Mon/Tues, and they are perfectly 'cool' with that, I am wondering what the normal process as Fixya client might now be. This is because I suspect more than one suggestion from different gurus will prove valuable - if only as specific points of weakness to quickly check.



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Hi Jim and Welcome to Fixya

First of all it sounds like all the Air dampers are intact as it WAS working fine and stopped. Here is most likely what happened.

The damper control cable that switches your air flow from recirculation to fresh air most likely has a bent cable or a loose cable retaining clamp that prevents cable movement when you move the selctor from recirc to fresh therefore not allowing the cable to travel full length.

What you can do is dive under the dash and watch the cable movement as a 2nd person moves the recirc to fresh air selector. When it is moved in one direction see of you can physically move the damper control drive arm further in the same direction after it has been moved with the selector. If you can still move the control arm then the control cable is not performing as it should and they will have to determine why.

Also the heater control cable that controls the position of the heater control water valve can cause similar problems for the same reasons above. Once again watch the travlel of the cable and try and move the arm on the water control vavle in the same direction. (Temp control selector movement Perfromed by a 2nd person) If it moves the cable and clamps will need attention.

Some vehicles have a 3rd cable that will mix hot air from the heater core with cold air from the AC evaporator coils. Depending on how the cabling is configured it usually is moved with the heat control. (Procedure same as above.)

It is never fun to dive under a dashboard........

If the AC Freon and Engine Coolant levels are fine you just have a cable issue that they need to correct. It should not be a major expense to get it fixed.

If they are a reputable shop the only thing you might be liable for is the cost of the new cable or clamps if required. They should wave the labor charges on a callback!

Thanks for using Fixya. Kelly

Posted on Feb 07, 2010

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Well for best defrosting / demisting action the air should be set to fresh air, as moisture inside the cabin (due exhaling hot, humid air, wet floor mats, etc) can cause a high moisture content in the cabin and poor general clearing of windshield especially on an older vehicle where cooling capacity is going to be less then a newer one, due to wear/ system design, refrigerant used, etc. Also check that the condenser (in front of the radiator in engine compartment) is not blocked with debris which would cause insufficient cooling of hot refrigerant and therefore poor cooling inside the cabin. As well the blend door should be checked for closing off air from the heater core when the a/c and cool air is selected. Check for frost on the a/c lines that could indicate a restriction in the system like excessive moisture in the receiver / drier which could freeze as the refrigerant circulates and create a slow down in the flow of refrigerant and poor cooling. Also you could check to see that the a/c clutch is engaging on the a/c compressor visually by looking at it as the system is running (engine running) it should be cycling. Hope this will help you. But with a/c problems there are a number of things that can cause poor circulation that would require a/c test equipment to check it with. bring it to a repair shop that specializes in a/c service. cheers

Posted on Feb 05, 2010

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I'm not sure if any of it's related but the first symptom points to a leak in your heater core. It will fog up till it's replaced.

Posted on Feb 05, 2010

  • Charles T Nevin
    Charles T Nevin Feb 05, 2010

    I can see your responses to other suggestions but I can't see what they say. So I'll say that I have seen a problem like yours with the a/c cutting off and on . It was in a climate control module. All I had to do was to unplug the module and clean the contacts on the male and female side of the plug. I discovered that it was a connection problem because I could usually get it to work by continuously pushing on different pads on the control.
    The fogging is probably no related unless the moisture has gotten on the wiring and rusted it.


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Jim, if this is indeed an intermitten problem, then I am sorry to say that no one will be able to give you a good, clear, and concise way to track down the problem.
Being a 14 year automotive technician, and an automotive diagnostician, i am constantly faced with intermitten, and also hard problems to diagnose, and always always always the hard problems are the ones that get diagnosed easily.
The problem with intermittens are that there is so many variables that you cannot possibly see. So it takes a strong diagnostic backround, instinctual nature and basic electrical thinking to track these things down talents most mechanics dont have. You are going to be bombarded after I post this with people who say that they can solve your problem, but many if not all will be bad guesses at best.

Here are somethings to try:

Have shop connect gauges and observe, with ac system on low side should be between 30-45PSI high side 200-300psi, gauges should both be steady and non fluctuating. cycling of the compressor should not be evident.
If proper pressure is present and the system is not turning on, make sure the A/C button on the HVAC controls are on.
If button is on and still no compressor action then testing of the high and low pressure switches is needed, current should pass through both, sensors and end up back at the compressor to power the clutch.
Hope this helps and as always happy motoring

Posted on Feb 05, 2010

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Have the guys check for a broken or rotten seal between the evaporater core housing and the firewall. If the foam has deteriorated the moister will get in and fog your windows up

Posted on Feb 05, 2010

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