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  1. #1
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    air compressor question

    I think whats called the unloader valve for my air compressor is very noisy when it releases air, kind of like a flapping sound similar to letting air out of a large balloon. Is this normal? Truck is a 19 freightliner, cummins n-14. The noise is so loud my driver opens the draincock so he can sleep.


  2. #2
    Bobtail Member
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    unloader vavle is in the compressor which lets the aircompressor bypass the air. The purge valve at the air dryer, released the air from the dryer to the compressor. Not sure what you are meaning he has to open the draincock..

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    Quote Originally Posted by adamm View Post
    unloader vavle is in the compressor which lets the aircompressor bypass the air. The purge valve at the air dryer, released the air from the dryer to the compressor. Not sure what you are meaning he has to open the draincock..
    maybe so the air compressor runs all thte time??

  4. #4
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    This might help
    Attached Files Attached Files

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    I'm not real sure on this because it's been several years since I had to work on the air system, so I've forgotten some of the componants and their names, however I've heard that noise your describing on other trucks before. I'm not sure what causes that. I guess the first thing I would want to know is the air purgeing out the air dryer or safety valve near the governor? If it's coming out of the safety valve on the governor/next to the compressor then you have blockage somewhere. If it's purging out of the air dryer and still making that noise then maybe the spring that closes the valve is getting worn? Sorry for the lame response. Like I said it's been awhile since I've had to work on the air system (knock on wood). I'll be curious to find the answer myself.

  6. #6
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    by the replies so far it seems there is a governor in the air compressor system which shuts off the compressor when it reaches the set pressure. If that is the case, then my governor must be defective as the unloader valve releases pressure every few minutes while engine is running. This can't be good for the compressor. Any feedback on this is very much appriciated.
    I think my driver is opening a drain valve on the air dryer to keep the unloader valve from making the noise.
    I may be referring to the unloader valve wrong, but it what releases excessive air pressure in the system.
    As a side note I do have a minor air leak somewhere behind the dashboard, I looked for ever for it, no luck so far.
    Last edited by seapup; 09.16.2010 at 08.51 AM. Reason:: additional info

  7. #7
    Road Train Member canuck in da truck's Avatar
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    might be an idea to see if maybe you have a new leak that is dropping the air pressure--causing the govenor to to start the aircompressor up?
    should be no drain valve at the air dryer--he must be opening srainvalves on the air tanks--and that will make the compressor run all of the time as well--because it isnt getting to cut off pressure
    if your air dryer is purging all of the time--could be that it needs a kit

  8. #8
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    Quote Originally Posted by seapup View Post
    by the replies so far it seems there is a governor in the air compressor system which shuts off the compressor when it reaches the set pressure. If that is the case, then my governor must be defective as the unloader valve releases pressure every few minutes while engine is running. This can't be good for the compressor. Any feedback on this is very much appriciated.
    I think my driver is opening a drain valve on the air dryer to keep the unloader valve from making the noise.
    I may be referring to the unloader valve wrong, but it what releases excessive air pressure in the system.
    As a side note I do have a minor air leak somewhere behind the dashboard, I looked for ever for it, no luck so far.
    Someone correct me if I'm wrong, but the compressor can't shut off like a electric driven air compressor because the compressor on your truck is directly driven off the engine and I believe that is where the unloader valve come into play. If you don't know when the last time your air dryer element has been replaced, I would change that out first since if you can't remember it would probably be a good idea anyway. After that, if the air is purging off of the safety valve next to the compressor then I would buy a unloader rebuild kit. What the other driver is doing is allowing the air pressure to never build high enough for the air system to ever purge. That works, but it causes your air compressor to be working non-stop when in reality I think most compressor are designed to only be compressing air about 25% of the time. If it's working harder then that, you either have a bunch of leaks or the compressor was spec'd to small for your application. Also, it's getting close to winter so while your down there, I would make sure your heating element on your air dryer is good. Again, it's been awhile so I'm not 100% sure. Good luck.
    Last edited by lego1970; 09.16.2010 at 04.14 PM.

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    i read on a diesel repair site that the safety valve should open at 250 psi.
    Is it possible that the safety valve is bad as my gauges don't go above 125

  10. #10
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    I really don't know. Sorry.

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