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Thread: Air con blows cold only on freeway.

  1. #1
    Join Date
    Dec 2003
    Location
    Minneapolis, MN
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    577

    Default Air con blows cold only on freeway.

    Last summer, I put a new Aux fan in my car and had my mechanic recharge the AC. Everything checked out. Pressure good, no leaks. But, the mechanic told me the air coming though the vents never got as cold as it should. Sure enough, it only gets slightly cool when running below freeway speeds (about 50mph). If I take an extended freeway trip, it's gets much colder, more like how it did when I first bought the car.

    He suggested the heater core area might be clogged and wanted to rip into the dash to find the problem. I said "no." Another mechanic friend suggested that the radiator might be really dirty/clogged, but wouldn't the engine be overheating if that were the case? Any other suggestions as to what I should look for. Or how best to tell if that mysterious area between the blower motor, the pollen filter, and the heater core is the culprit?

    thanks in advance!

  2. #2
    Join Date
    Nov 2006
    Location
    Germany
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    Quote Originally Posted by BennyM
    Last summer, I put a new Aux fan in my car and had my mechanic recharge the AC. Everything checked out. Pressure good, no leaks. But, the mechanic told me the air coming though the vents never got as cold as it should. Sure enough, it only gets slightly cool when running below freeway speeds (about 50mph). If I take an extended freeway trip, it's gets much colder, more like how it did when I first bought the car.

    He suggested the heater core area might be clogged and wanted to rip into the dash to find the problem. I said "no." Another mechanic friend suggested that the radiator might be really dirty/clogged, but wouldn't the engine be overheating if that were the case? Any other suggestions as to what I should look for. Or how best to tell if that mysterious area between the blower motor, the pollen filter, and the heater core is the culprit?

    thanks in advance!
    Pressures were good and no leaks but the system needed recharging?? I wonder if the freon dissapeared just like last time?

    Your symptoms sound like a lack of capacity problem or hot water getting into the air box through the heater system.

    Things that can stop up the flow of freon are your receiver/drier (clogged filter), junk in the expansion valve/orifice tube (don't know which one you have) from a receiver/drier falling apart, or a weak compressor that doesn't have the flow no mo'.

    Do you have the auto climate jobbie or is a manual rig? Either way, check that the heater valves are not stuck open allowing hot water into the heater core (and the air box) when you want cold air. If the heater valves are behaving, then maybe your air temp doors in the air box are not behaving and allowing warmish air to mix with your cold air.

    I agree that if you had a clogged radiator, you would know that by now.

    Brian

  3. #3
    Join Date
    Feb 2004
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    4,894

    Default

    All you need is a good set of manifold to find out what is the problem... like for example... on R134a system... with AC compressor running and aux fan running,.. low pressure is about 25 to 30 PSI...and high side should be around 240 PSI...

    If the High side pressure is above 300, then your aux fan is not running...

    If the high side pressure is below 200, then your expansion valve is bad.

    You need to check if your heater monovalve is working too... because the heat it put out... will overcome the AC that it will never cool down the car.

  4. #4
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    Mar 2005
    Location
    Houston, TX
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    505

    Default

    Good advice - sounds like a heater valve could be the culprit - and, be sure that the heat thumbwheel in the center of the dash vents is on full cold.

  5. #5
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    Minneapolis, MN
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    Default Interesting.....

    My aux fan does run when necessary. The expansion valve is the newest component and has lived a pretty low key life as I hardly ran the AC for a couple years. I really don't think it's bad.

    Also, if the heater were interfering, wouldn't that mean that the system would never blow cold?....even on the freeway where it presently still gets cold?

    What's the easiest way to check the heater valve?

    Thanks.

  6. #6
    Join Date
    Dec 2003
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    Minneapolis, MN
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    Default Thanks, Brian!

    Thanks for your insight. I have a few thoughts for you in response.

    I should have given you more back story. A few years ago, the aux fan seized, causing the AC system to over-pressurize and blow the expansion valve. All the refrigerant (R-134) leaked out then. I had the valve replaced, and didn't get it fully recharged until I could put an aux fan in.

    Interesting...I never thought of the heater system interfering. How does this all work with the higher speed correlating with colder temp?

    I believe all of these primary AC components are 5 years old at most. For the sake of my wallet and experience level, I'm going to assume that the receiver/dryer, compressor, and expansion valve are all still good and I'm going to look for causes elsewhere.

    I have the manual rig and yes I have the thumb wheel at cold. I'm up for checking the heater valves and temp doors. I wouldn't be surprised if it was a stuck door. Any suggestions on checking these would be much appreciated. I assume I'd have to take apart most of the upper center console?

  7. #7
    Join Date
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    Germany
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    Default

    Quote Originally Posted by BennyM
    My aux fan does run when necessary. The expansion valve is the newest component and has lived a pretty low key life as I hardly ran the AC for a couple years. I really don't think it's bad.

    Also, if the heater were interfering, wouldn't that mean that the system would never blow cold?....even on the freeway where it presently still gets cold?

    What's the easiest way to check the heater valve?

    Thanks.
    Please define 'cold' for us. As in get an instant read or 'a/c' temp gauge to fit in the center vent. Compare temps at idle and at highway speeds. You should be getting around a 40 degree F drop over ambient. You might be surprised how warm 'cold' can be when using your skin as a temp gauge.

    With the car warming up, take a feel of the heater hoses at the valve and going into the firewall. If hot water is making it past the valve and into the firewall with everything set to full cold, you have leaky valves.

    Brian

  8. #8
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    Germany
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    Default

    Quote Originally Posted by BennyM
    Thanks for your insight. I have a few thoughts for you in response.

    I should have given you more back story. A few years ago, the aux fan seized, causing the AC system to over-pressurize and blow the expansion valve. All the refrigerant (R-134) leaked out then. I had the valve replaced, and didn't get it fully recharged until I could put an aux fan in.

    Interesting...I never thought of the heater system interfering. How does this all work with the higher speed correlating with colder temp?

    I believe all of these primary AC components are 5 years old at most. For the sake of my wallet and experience level, I'm going to assume that the receiver/dryer, compressor, and expansion valve are all still good and I'm going to look for causes elsewhere.

    I have the manual rig and yes I have the thumb wheel at cold. I'm up for checking the heater valves and temp doors. I wouldn't be surprised if it was a stuck door. Any suggestions on checking these would be much appreciated. I assume I'd have to take apart most of the upper center console?
    With the system overpressure event now known, I wonder the condition of your compressor and clutch. Is it possible your clutch got cooked turning the compressor hard enough to blow up your expansion valve and now it slips enough to under-drive your compressor at low engine speeds? This is kind of a stretch as you would figure the weak/burned clutch couldn't hang at high engine speeds...

    Or is the compressor worn and weak from the years and the overpressure event, causing poor low speed performance?

    It would be interesting to see the gauges during stationary and highway operations to see what is going on. I assume you have a clear sight glass, as in plenty of freon to get a steady supply of liquified freon at the expansion valve?

    If you can do the temp gauge thing I wrote about above and let us know the results. The temp gauge shouldn't cost too much (<$20 for sure).

    Brian

  9. #9
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    Jan 2004
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    Melbourne, Australia
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    Make sure you have the aux fan wired up correctly and that it is not spinning backwards and blowing in the wrong direction.

    Bill G

  10. #10
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    Mar 2006
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    Maryland, Virginia, D.C.
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    Default

    The heater valve does get stuck open. Also the aux fan resister may need a replacement, but I'd bet on the heater valve.

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