Humming when ac kicks on engine fan

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Blue2547, May 18, 2021.

  1. Blue2547

    Blue2547 Bobtail Member

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    So I have a 389 with a 6nz in it recently it started to make a noise right before the ac kicks the engine fan on and it only does it with the ac on if you hit the fan switch it does not do it best way to describe noise it sounds like a train horn that barely has the valve open what could this be Thanks in advance
     
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  3. Blue2547

    Blue2547 Bobtail Member

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  4. BoxCarKidd

    BoxCarKidd Road Train Member

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    Sounds strange. Does it make that noise when the fan engages for engine temperature?
     
  5. FLHT

    FLHT Medium Load Member

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    Hose to fan hub leaking ?
     
  6. Blue2547

    Blue2547 Bobtail Member

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    No sir not at all it almost sounds like it's coming from under the cab and whats really weird is you can't hear it from outside
     
  7. God prefers Diesels

    God prefers Diesels Road Train Member

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    If the A/C is kicking the fan on, and that's the only time you hear the noise, could it be your expansion valve?
     
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  8. skallagrime

    skallagrime Heavy Load Member

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    And if thats the case, the conclusion would be its on a slightly low charge because the compressor never gets all the gas compressed to liquid ONLY WHEN FAN IS ON and the hum is gas burbling through as well as liquid.

    (Sorry gpd, just wanted to finish that line of thinking so people know WHY that would happen)
     
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  9. God prefers Diesels

    God prefers Diesels Road Train Member

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    No, that's good. I didn't know why they made noise, I just remember they did. It's good to have the other half of the story.
     
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  10. Inderjit

    Inderjit Light Load Member

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    The fan normally kicks on when the refrigerant pressure reaches 250 PSI.
    The pump should shut off at 300 PSI to protect the system. I am wondering
    if the switch is failing and turning the fan on when the pressure is too high
     
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  11. skallagrime

    skallagrime Heavy Load Member

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    Technically its possible the valve is stuck too open, never really allowing liquid to build, but since its intermittent with fan, less likely.

    Gas to liquid and back is weird, especially when we start tampering with it in applied engineering.
     
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