A step by step guide to troubleshooting and understanding AC in our SUVs

Discussion in 'Tech Info' started by UmmScott, Sep 1, 2019.

Car Parts
  1. UmmScott

    UmmScott Member

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    As a professional HVAC and refrigeration tech. I am happy to share how to go about properly troubleshooting the single and DUAL AC systems. :)

    First off, dealing with the refrigeration system in anyway can be dangerous if you dont know what you're doing. I am not liable for anyone's actions. This is meant as a guide :)

    As with starting with ANY AC issue, the very very first thing to check is AIRFLOW. you should have good strong airflow through the panel vents as well as the rear unit as well. In addition, ALL mode doors must work properly and most importantly the Blend and recirculation doors. On full cold they must COMPLETELY block the path to the heater core. This is a 100% must and you WILL chase your tail on a problem if you cant verify this.
    Once you have confirmed proper operation of the system electrical wise, and you still have lack of cooling,....NOW...you may move to the actual refrigeration system in the car. I will be explaining how to trouble shoot lack of cooling, and basic issues you encounter. Description of the auto and dual climate systems I will do separately (not in this thread).

    To begin. I do NOT condone using those stupid cans with cheap/lame charging hoses that only connect to the low side. Using this WILL get air in your system. You will cause more problems and...again..chase your tail.
    Either educate yourself and do the job properly, or have someone who knows how to help u :).

    The best thing about ALL vehicles is they are a WEIGH in charge. Right on the hood they say.. "takes x.x pounds of 134a"
    Our SUVs with rear AC take roughly 3lbs of refrigerant. They need that EXACT amount...not 5 oz more...not 5oz less, but that charge.
    Begin with making sure that that amount is charged into the vehicle and that will help you determine what the issue is.
    Next... make sure that clutch radiator fan is moving air!!! That dumb thing better be Mooooving air when its hotter than blazes outside. If... its screaming hot outside and your engine is hot and the AC is on...its gotta engage on high speed otherwise you'll have crappy cooling. A symptom of this would be poor cooling in stop/go traffic but getting better on the highway. Replace clutch with the GM clutch, not aftermarket or better yet do the electric fan conversion and you'll kick ass cooling while at idle.

    If you have weighed in the charge (which includes a proper pressure test and evacuation... get someone to do this for you!!!), you can then narrow down on what the problem was.. IE: low on charge, overcharged, failing condenser coil, weak compressor or failed metering devices.

    My next post will go over some general troubleshooting tests that cover specific symptoms.

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  2. Doubeleive

    Doubeleive Supporting Member

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    i'm going to just chime in here and state a "Tech2" diagnostics scanner (the one designed to troubleshoot these vehicles and other similar GM) will go a long ways in helping to determine the cause of HVAC faults on these. It will save you a lot of guesswork. carry-on.
     
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  3. UmmScott

    UmmScott Member

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    I'm going to cover troubleshooting the system now that you have verified the proper charge is in the system.

    Do not go further until you have the proper charge. If getting the proper charge fixed your issue, then you most likely were low and have a leak somewhere and that...could need fixed.

    Symptom : poor cooling in front and rear AC systems..
    Could be low on charge, overcharged, air in system, blockage or poor heat transfer from condenser coil, and weak compressor. Many things can cause this. My performance test (later post) can help verify the weak compressor.
    You'll need to check pressures. If they're both getting very high especially above 250 on the high side, that condenser isn't getting rid of its heat and you'll have to look into your clutch fan possibly.
    Again..theres many possibilities here.

    Symptom: poor front cooling but normal to good cooling at rear ac.
    Actually...this is usually low on charge. Our systems use an Expansion valve in the rear unit and orifice in the front unit. Expansion valves can adjust to allow more refrigerant into the evaporator if needed.
    So when a system is partially low, the rear unit will hog the refrigerant ergo starving the front coil making it not produce as much cold air.

    Symptom: normal/good cooling in front but none in rear unit.
    Check blend doors first. If ok, feel piping underneath vehicle where they enter the cab. Larger line should be very cold. Small line should be warm to hot. If they're both warm, likely the expansion valve is defective. (That sucks )

    My next/maybe last post to my guide will be a performance test.

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  4. UmmScott

    UmmScott Member

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    Absolutely...especially with the dual/auto climate systems.
    Really helps there

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  5. UmmScott

    UmmScott Member

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    Ultimate performance test..

    Do this test after verifying charge is correct, airflow is correct, Condenser is clean and the fan clutch or fan motors operate at full capacity. It should be a mild to hot day to do this..

    Start car. Set all HVAC controls to high speed, coldest temp, recirculation on. If you have gauges tap on to both high and low sides. Let the engine get fully warmed up and allow the cabin to come down from insanely hot to below 80F to 70F roughly.
    If....your system is working correctly, then it should pull down the cab decently. Low side could start out anywhere from 50 to 70 but should quickly start pulling wayy below 50 and be proportional to the cabin temp.
    High side psi will climb above 250 but if it starts getting higher than 350, there is definitely a heat transfer issue in the condenser. Either internally or airflow with that stupid fan clutch LOL. You'll have to stop this test here and correct this problem.
    After 10 to 15 mins proceed...whether it has cooled down fully inside or not.

    Next, turn the rear system completely off. Reduce the front systems blower to the SLOWest setting, recirculation still on and still coldest setting.
    Your pressure should drop dramatically. The rear system Expansion valve should close down which further lowers the low side pressure.
    After 5 mins your low side pressure should be getting below 40...then 35...then 30. You may need to slightly elevate idle to 1000 or 1200.
    Basically...it should pull to the cutout point of the clutch cycling switch which is 27 psi roughly. If your system achieves this, then it is working perfectly and great!.
    If you cant get the compressor to pull down past 40psi, chances are it is weak internally or the rear system exp valve is stuck open. I highly doubt the latter. Exp valves dont fail open generally.

    Another thing you could do is disconnect the blower motor in the front so the system is on but theres no air flow.
    Again..that compressor should pull down below 35psi in no time. If the pressure stays high, that comps life is near the end and likely needs replaced.

    I hope this long ass post helps anyone in need. Might be more boring info than you could ever want.
    Feel free to message me for any questions.

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