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Old 11-05-2012, 02:46 AM
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off topic question

okay so, busses do it, utility trucks do it, garbage trucks do it, etc. I have been wondering why it is that these giant Diesel engines make that hissing sound. usually when idling. it sounds like a valve opening to let a buildup of pressure out of somewhere?
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Old 11-05-2012, 03:06 AM
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Air brakes?

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Old 11-05-2012, 03:39 AM
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The sound your hearing is most likely the air compressor cycling off. Just like an air compressor in a shop, when they cycle off, they relieve the pressure from the cylinder on the compressor so that the next time it cycles on, it doesn't have to fight the compressed air inside of it to turn over. The air is used for a variety of things on these big rigs, like air brakes, locking of axles, air horns, trailer brakes, air bags (suspension), seat height adjustment, window up and down, etc.


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Old 11-05-2012, 11:42 AM
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Thanks Badyota, I think you pretty much nailed my question head-on. Makes perfect sense now.
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Old 11-05-2012, 11:54 AM
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Quote:
Originally Posted by badyota82 View Post
The sound your hearing is most likely the air compressor cycling off. Just like an air compressor in a shop, when they cycle off, they relieve the pressure from the cylinder on the compressor so that the next time it cycles on, it doesn't have to fight the compressed air inside of it to turn over. The air is used for a variety of things on these big rigs, like air brakes, locking of axles, air horns, trailer brakes, air bags (suspension), seat height adjustment, window up and down, etc.


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The air compressor in a semi or anything with air brakes is gear driven and never stops running until the engine is shut down.

OP are you talking about the "sneezing" you hear every couple of minutes? If so that is when the air tanks reach their set pressure levels usually around 120 psi then it sneezes to bleed off enough pressure to keep from over pressurizing the system. If you hear a constant hissing than there is an air leak somewhere and that is not only illegal to take down the road but dangerous and needs to be fixed immediately.
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Old 11-05-2012, 12:36 PM
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I stand corrected on the compressor function and that it does stay running. However, not all are gear driven. A lot of the older trucks have belt driven compressors. I believe he is still talking about the quick hiss, which is the air governor relieving excess pressure.


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Old 11-05-2012, 12:39 PM
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That is correct but ALL newer ( I don't know what year it started but all that I have seen that are mid 90's to present ) trucks have gear driven power steering pumps and air compressors. They stopped the belt driven because if you loose a belt you loose your ability to steer or stop.

I think your right in thinking he is talking about the "sneezing" of the governor.
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Old 11-06-2012, 09:48 AM
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Yea.. "Sneezing" is a good analogy. I just never knew what to search up in Google to find my answer haha. I ask because I've always heard it, and I'm in my last year of high school, forced to ride the school bus on some days. One morning, we were picked up in an older replacement while the other bus was being serviced, and we were put on an older one where the "sneezing" was louder. Only within the past year, I've discovered my interest in mechanics and finally solved a self-mystery.
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