I'm talking about the pressure relief noise you hear every few minutes, not when letting off the brake pedal. I'm curious because I would like to add onboard-air for air horns and tools and would love to be able to do this (very high cool factor). Is it just a popoff valve set to a certain pressure, or is this a valve to remove moisture from the tanks every few minutes, or am I completely wrong? How is this done?

 

Both.

There are automatic moisture valves that periodically open to let water out of the bottom of the air tanks.

Usually, though, you are hearing the the brakes-- when the air system is back up to pressure, the on board compressor goes into unloader mode, and this transition can cause a loud release of air.

Many times, it's better to keep the compressor running all the time and just use an unloader to make it "freewheel", rather than engage and disengage the clutch.

After all, with something as critical as the air compressor, you want max reliability...

 

It must be the unloader I'm hearing then. The one I'm talking about occurrs even when no one is in the truck and it's idling.

 

Its just releasing the pressure, it gets to a setpoint, then dumps out the extra pressure, then the pump builds it back up and then it dumps it out again. LIke Hohn has said, max reliability for big rigs so Im pretty sure they have that air compressor pump running all the time.

 

That's the air drier your hearing. It takes the moisture/condensation out of the air tanks.

 

So it is electronic that it seems as if it's timed?

 

Nope! Its ran off of air pressure.

 

It's not electronic but, if the truck is idling for a while it takes the same amount of time to build preasure and dump it out the air dryer. If you ever ride in one while going down the interstate it will dump more often because at higher RPM's the pump builds preasure faster.

 

Most big rig air compressors are gear driven with a govener to control pressure. As stated the release you are hearing is the air dryers. To put this system on a pickup is kinda overkill. Put a drain valve in your tank and pull it open every now and again and you will be fine.

 

Yes, it is most likely the air dryers you are hearing. Most big rigs have 2 line dryers. They operate similar to the line dryers the Coast Guard uses on high power transmitters that use pressurized transmission lines. The pressurized air leaves the compressor and goes through the dryer. In the dryer is desicant crystals that absorb moisture. To purge the desicant chamber of the absorbed moisture the chamber is relieved of its pressure and returns to atmospheric pressure. That action is whats makes the PSSSHHHH sound. The reason most big rigs use two drier chambers is to always keep one in service and keeping dry air in the lines, while the other is purged of moisture. A solenoid valve switches between the two periodicly.

 

Interestig...learn something new every day. Your right though, as cool as that would sound...it's a bit over kill. Oh well, the air horns will sound sweet, though!

 

Price one of those suckers and then find a place for it, you will be convienced that it is over kill.

 

Bindix Air Dryer OUCH!

 

How about this idea, though?

Air canister actuator to pull the cable for the parking brake...controlled by the air parking brake valve up on the dash.

I can see it now...pull into a parking spot, put the truck in park, pull the valve..."PPSHHHHHHH!!!"

Now how cool is that???

 

I'm sorry, I just can't resist, it's too easy....


I PASS GAS asking about How do big rigs make that "PSHHHHHH" sound?
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I know this is going to cause a "stink".