Why does my A/C get colder the slower I drive?

Discussion in 'Automotive Electronics' started by strantor, Jul 2, 2015.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    I've had my share of beater cars with lackluster A/Cs, and they all had one thing in common; if the A/C worked at all, it only worked on the freeway going 60+ mph. Sitting in traffic, when you needed it most, the A/C would settle in someplace between disappointing and unbearable.

    The '05 GMC pickup I'm driving now is just the opposite, and I've correlated it to relative wind speed, not engine RPM. Going 70mph down the highway with the hot/cold slider all the way on cold, and the fan speed on max, the vent temperature is barely acceptable. But if I move over and draft behind an 18-wheeler, or if I encounter slow-moving traffic, I have to move the hot/cold slider more toward mid-range, lest my nipples shatter. I tried driving 70mph for a while in 3rd gear @ 3000rpm to correlate it to RPM and there was no change in A/C performance.

    It's a manual control A/C, not a thermostat
    /"climate control" type, where you set the temperature and it maintains the setpoint.
    ac1573502_is.jpg
    If it were a thermostat/"climate control" type, I would suspect the interior temperature was being affected by the exterior temperature. But since it isn't, I don't know what to suspect. When I google it, I'm given a long list of people describing the opposite problem, except for one forum thread about '03 Lincoln Towncars, but those cars have climate control. In that thread, there is a bunch of people describing that same problem, and a wild array of suggested fixes, but no confirmation given by anyone who got it fixed, except for someone saying a low pressure switch.

    I suspect it might be electrical so a switch makes sense. But I want to have a stronger feeling about it before I go bleeding my refrigerant, since I have done more harm than good in the past by "fixing" automotive A/C systems.

    @#12 BATSIGNAL.
     
  2. Eric Binkerd

    New Member

    Jun 18, 2015
    19
    3
    A set of gauges would be best to determine if the freon is low. You can get a cheap set from Harbor freight. As for the thermostat, all a/c systems have them. Typically they are built into or near the evaporator. This tells the system if the evap core is too cold. It is to prevent freezing. This is not the case for you though.

    When you run your a/c is it on recirculate or fresh air? Recirculate will close the outside door and prevent air from coming in. If the a/c system acts the same way when on recirculate, I would suspect low freon.
     
    strantor likes this.
  3. Randy 7140

    Member

    Jun 17, 2015
    32
    6
    Hi Stantor,
    A few quick questions.....

    1. Have you checked and cleaned the condenser coils?
    2. Can you hear the compressor clutch kicking on and off at any time when its not working well?
    3. Do you have a manifold gauge to read hi/low pressure levels?
    4. Is your radiator fan working?

    I'm going to say first check and clean the condenser. They can get really easily clogged with road dirt, bugs etc...An overcharge can cause intermittent issues as well.
     
    strantor likes this.
  4. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    Oh! good to know, thanks!
    I'm not sure. I'll try it and see.
     
  5. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    no
    I'm not sure. I'll keep an ear open for it. When I first got the truck I remember hearing the compressor kick on and off all the time, but I can't remember hearing that anymore. Maybe I just got used to it, or maybe it stopped.
    yes
    I haven't checked it, but I assume it is; I don't have any problems with coolant temp ever.
    will do, thanks!
    And I'll post back here the unanswered questions.
     
  6. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Oh. Blocking the wind causes better performance? First reaction? Lack of high side pressure against the metering orifice. If the metering orifice is partially clogged, that still meets the definition of, "not enough pressure against the metering orifice (you have today)". A slight shortage of Freon fits this, too.

    Problem is, a lack of high side pressure causes a lack of low side pressure. At high RPM, the compressor has way too much capacity for the job. When the compressor tries to suck the low side pressure so low that the evaporator coil is in danger of freezing into an ice block, the low side pressure switch shuts the compressor clutch off until the pressure rises into the safe range. This causes a vicious cycle. Not enough high side pressure, diminished flow, not enough low side pressure, cycle the compressor off and on, > not enough high side pressure. The faster you go, the behinder you get. ps, your description eliminates the low side pressure switch as the problem. If the clutch is cycling too much, it's because the pressure switch is correctly interpreting the conditions present.

    Some models of Ford used a low side metering valve instead. It will not let the pressure in the evaporator coil go below 40 PSI and the compressor just sucks on a closed valve when pressures get too low. This valve makes high side pressure almost irrelevant. The evaporator coil is essentially part of the condensor. If there's any Freon to work with, most of the mass of liquid Freon sits in the evaporator coil waiting for the pressure to get low enough for it to boil. Too much air flow through the condensor will not cause a failure in that system.

    Don't like the first two answers. :(

    If there is an interior thermostat, which you said there isn't, it might modulate temperature by changing the percentage of air that goes through the heater core during the interior air flow loop, and it might modulate temperature by changing the duty cycle of the compressor clutch. I am familiar with the first option. There is also the option of a leaky diaphragm on the vacuum motor that alters the flapper position in the interior loop. If the engine vacuum declines at high speed, a slightly leaky vacuum motor might fail toward the position that sends a higher percentage of interior air through the heater core.

    Edit: Cancel that. If the vacuum motor was diverting too much air to the heater core, it would still be bad (at the same RPM) after you adjust the controls.

    Getcher gauges out and measure the Freon pressures. Put a brick (or a wife) on the gas pedal to hold at 2000 RPM during measurements. Doors open, windows open, fan on HIGH, adjustments to COLD. If it's low on Freon, it isn't going to be off by an awful lot. If adding Freon seems insanely ineffective, look for a clogged metering orifice. With a clog in the system, the Freon just sits in the condensor as a liquid, refusing to move. Adding more Freon just makes a bigger puddle of liquid without enough pressure to push it through.

    Then again, it might just be a design compromise based on the idea that low RPM performance is what most people need. If you have to change the settings at 70 MPH, you're probably going to cruise at 70 for a long time.

    As always, read my signature. Every year, a new batch of (beginner) engineers pees in the soup.
     
    Last edited: Jul 2, 2015
    strantor likes this.
  7. tom_s

    Member

    Jun 27, 2014
    285
    333
    former is the safer option :)
     
  8. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Sad, but true. I used to have a wife that said it was abusive of me to ask her to hand me an empty coffee can while I was under a truck. I would not want her operating the controls while I try to look through the fan blade to see if the compressor is turning. :rolleyes:
     
    debe likes this.
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,046
    3,813
    Those trucks have silicone fluid slipper clutches on the cooling fan and it may look like it is turning but no real power. It is supposed to optimize fuel/power by keeping engine temp in ideal range. They fail - check (or have it checked).
     
    #12 likes this.
  10. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Method for testing viscous clutch as told to me by a mechanic at the Ford Dealership:
    If you can stop the fan blade at engine idle speed with a broom stick, it's broke.

    I have replaced those clutches with a fiberglass fan that flattens out at high speed.
    Less fan blade pitch = less air flow at high speeds.
    It worked for me. ;)

    ps, the fiberglass fans are noisier than the steel blade fans.
     
  11. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    Are you talking about the radiator fan? My truck has dual electric radiator fans.
     
  12. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    Sorry I haven't gotten back to this thread. I haven't done any further troubleshooting. I read a bit more online and drove around for a few days with a thermocouple in my air vent and I think I might just be expecting too much from my A/C. It blows between 39 and 54 degrees depending on driving conditions, at >100 degree ambient. I think that's about as good as it gets.
     
    #12 likes this.
  13. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    You are correct. If the delivered air gets down to 39F you are on the edge of turning the evaporator into a block of ice. The only consideration left is fan quality. Cooling Effect is the integral of Dt and mass flow.

    For instance, I once found a car evaporator so clogged with leaves etc. that it was growing mushrooms!

    ps, I have worn out more than one fan motor here in Florida.
     
  14. Silvio

    New Member

    Jul 2, 2015
    14
    0
    I would like to say something here if all the airflow passages in the cabin are clear and the condenser and fans under the hood are all functioning, it could be over charge of refrigerant and the system cuts off on the high pressure switch on high revs.
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Another obvious issue is ram air. The faster you drive, the greater the airflow into the cabin A/C intake, which means that the air flow past the heat exchanger is greater, which means that the air has less time to be cooled while it passes through the heat exchanger.
     
  16. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,145
    204
    A good test of AC performance is the doors open and look at outside temp and humidity and inside temp. There should be a 15-20 degree difference depending on RH.

    So, that's the Ac side of things,

    The recirculate damper is a big killer of inside air temp. If it's open, it will never feel cold.

    One vehicle I had, I put the AC in from a box and I am now EPA certified for R12 in automotive and small appliance.

    The car had a vacuum damper that would open 1/2 the exterior damper if the evaporator started to freeze up.

    The exterior fan has some effect. If the AC is not in recirculate, more exterior air will be forced into the cabin at high speeds which will result in a higher interior temperature.

    FWIW: Defog uses the AC, Defrost uses heat. The interior will also be more comfortable if you run the AC and heater at the same time when it's raining. The Ac will remove the moisture and the heater will heat air with less humidity and that heats more efficiently. The fog will also dissapear from the inside of the windows.
     
Loading...