A/C only blowing a whisper... sometimes

Discussion in 'Engine Performance + Maintenance' started by scottydl, Jul 8, 2020.

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  1. scottydl

    scottydl I'm Awesome

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    After my A/C was blowing hot and the rear evaporator recently found to be the slow-leaking culprit, the system was topped off by a shop with refrigerant, oil, and stop-leak. My hope is that the leak is slow enough that I can keep it functional each summer with occasional refills, and without replacing the entire system or removing the rear function (separate thread on that here).

    I just got back from a 500+ mile trip in the Suburban, towing my 9500# RV trailer. The engine was working hard, but I watched the temp closely to make sure it remained under control. Outside temperatures were 80's and low 90's so I was running the A/C. It was always cold, but sometimes the front air vents barely blew with any force, like a whisper. I could turn up the blower speed and the motor responded as expected, but airflow wouldn't increase. Then other times the system would suddenly be blowing fine. I couldn't tie the issue to anything I was doing with the driving or engine temp, if that affects A/C performance. I tried all sorts of things -- changing Upper/Lower airflow modes back and forth, fan speed, system turned off then back on... but could never seem to cause the low airflow OR fix it myself on the fly. It was very inconsistent.

    Does this sound like a RECIRC issue when on Max A/C? Blend doors malfunctioning? Sometime else? Or is this just typical with 95 or older trucks? FWIW I recently replaced the blower motor and cleaned the front evaporator, and the motor sounds/responds just like it should.
     
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  2. HotWheelsBurban

    HotWheelsBurban Burbs are great!

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    Could be the blower motor resistor
     
  3. 454cid

    454cid Sooper Pooper

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    I would think one of the doors, seals, or maybe the control panel having an issue, if the blower has been replaced, and sounds like it's working.
     
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  4. brian c lindsey

    brian c lindsey Newbie

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    does fan motor change speed when you change speeds or change vents , if not i would also say blend doors,when it happens check other vents and see if air is coming out when supposed to be closed. there is one door controller you have to remove dash to get to, other two , not so bad
     
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  5. scottydl

    scottydl I'm Awesome

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    Yeah the blower is working perfectly. When the air output is low, I can turn the fan speed up or down and the blower does just what it should... but the actual air blowing out of the front vents doesn't change regardless of how high the blower is running. I couldn't feel it coming out of any other vents either, checked defrost and lower/feet.

    Except sometimes when it works fine! Pretty confusing.
     
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  6. Schurkey

    Schurkey I'm Awesome

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    Resistors don't come-and-go. They pop, and they're done. Typically, it's the thermal limiter on the resistor pack. The resistors themselves are fine, the thermal fuse dies and then you have no blower except on "high".
    IF (big IF) this image posts, you can see the thermal limiter wired in there with the three resistor coils. The whole thing is rusty due to a plugged evaporator drain "drowning" the resistor pack.
    [​IMG]



    1. Debris in the ductwork, when it flips up and blocks flow, you have little air movement. When it falls back down, the air blows fine.

    2. Blower fan spinning on the blower motor shaft. When it grabs and turns, it blows air. When it slips, it blows poorly.
     
  7. alpinecrick

    alpinecrick I'm Awesome

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    I hadn't thought of that until you reminded me--a buddy had that happen with his square body truck a long time ago. The blower motor would run like a mutha' but the fan had stripped out on the shaft.
     
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  8. east302

    east302 I'm Awesome

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    Did it work better after the a/c had been off for a while?

    Could be evaporator freezing up and restricting air flow. Have the refrigerant pressures checked in that case.
     
  9. scottydl

    scottydl I'm Awesome

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    Hmm, this seems like a possibility... I can't say for sure if the problem was happening only after the system had been run for awhile and could have frozen, but it's something I'll pay attention to. The refrigerant pressures have been recently checked and are within spec, according to the shop I use. Mechanic gave me the exact numbers when he was troubleshooting the system, commenting that the system was holding pressure below its rated max, even though the rear evaporator indicated a leak after he ran dye through the system.

    Another possible culprit could be the blower motor (although it's fairly new) slipping on the shaft and not outputting airflow like it should. I'll have to see what the warranty is on that thing, might be able to swap it out and confirm/eliminate that.
     
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  10. brian c lindsey

    brian c lindsey Newbie

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    i would pull blower motor check it and inside the hole the motor sits in for debris
     

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