Air dryer purge sound.......but .............I don't have one?

[Freightrain]

Been going on for a couple years now. Just randomly hear a "pffst" sound while going down the road. Never any reason, don't see any fluctuation in air gauge. Doesn't seem to be during/after compressor kicks on/off because the gauge will be already at 130# and holding. Won't do it just sitting still in the driveway(wouldn't that be an easy way to find it?).

I replaced my relief valve on the tanks a couple years back thinking that might be it. Have a new regulator on the compressor, so that ain't it.

Girlfriend even gives me a look every time it goes happens. Can't really place where it is coming from, it is just "there". The only thing I can say is that it only came about after putting the air ride on(stretch job).

[41chevy]

Could the air ride compressing be causing the release of air when it travels on bumps? Paul

[Freightrain]

I've had that thought myself. Just the sound doesn't really sound like coming from behind. Kinda more from passenger side, front. sorta. Hard to discern when it happens so fast and only lasts a second. It doesn't always happen on bumpy or smooth road, no real rhyme or reason. Just not when parked.........which would lead me to being the suspension.

I can hit the dump button, but it really doesn't happen the same way.

[j hancock]

Depending on Neway system used, they are usually designed to run at 100 psi not 125-130 psi system pres.

How often is random?

http://www.huskidrive.com/files/downloads/neway_ard_manual.pdf

[wheeler73]

Some trucks have a spitter type valve on the air tank to purge moisture from them. They are square about 3-4 inches across and stick out from the tank. Just a possibility.

[Freightrain]

I know these tanks don't have spitters. Or it would do it just idling.

Could be hour, could be a few minutes apart. More prominent in town then on the highway. That would lead me to believe it is the bags purging off. Just sounds so drastic, like a dryer and not like I would think the levelers dropping off air. I don't think any truck would run at 100# psi. Mine only runs 130# now due to new govenor. The old one ran 110-120, this one is set a touch higher.

[j hancock]

The truck system pres is 125-130 psi. Neway wants their bags to run at 100 psi so a regulator should be in the supply line.

There are quick acting leveling valves and "slow" valves.

Maybe support the frame of the truck and disconnect the rod between the leveling valve and the axle and operate the valve and see what the valve exhaust note sounds like?

Might eliminate one of the variables.

[Freightrain]

I have replaced the valves(actually made it one instead of two), but think the noise came before that? I've got the manual for the suspension but never read the fine print, only to get part numbers for it. I never would have thought they would denote a regulator? The bags only see enough air to support the truck, so I doubt mine would ever see over 100# anyway.

I'll play with it and see what it sounds like. Or maybe I'll tell the g/f she has to ride on the back frame rails the next trip out LOL!!! There is a grate back there I can strap a chair to

[j hancock]

Might have to take her to a fancy dinner after that!

[41chevy]

Might have to take her to a fancy dinner after that!

Or at least buy her a fancy store bought hat like Granny Clampit has. . .

[rsb502]

if you are hearing a high pressure or high velocity air purge sound coming from under the hood your over pressure purge pop off valve may be going bad. There should be a small (1-1.5" round screw in pop off valve on the air compressor i think on that truck, it is the safety pop off for over pressure and should bleed air over 150. However they do go bad every now and then and bleed at a lower pressure, its just a spring in there under tension against air pressure, they can sound like a purge valve on a dryer, I have replaced several in the past so theres something to check out.

[Freightrain]

I replaced the relief valve on the tank(first thing I thought of), didn't realize there is one on the compressor itself? Strangely it never does it just idling at full charge. Only when on the road.

[rsb502]

I replaced the relief valve on the tank(first thing I thought of), didn't realize there is one on the compressor itself? Strangely it never does it just idling at full charge. Only when on the road.

yeah, some trucks have them on the compressor, some the tank, some both. I was going by experience with similar trucks on the placement so I may not be dead on.

[BC Mack]

on our transit buses, we fit a 150psi to the ping tank, and on the tu-flo 750 we have a 250psi relief valve... no-one knows why both but if the 150 fails to work you don't blow the head off the compressor..!!

BC Mack

[thomastractorsvc]

My $.02 cents

The air bags is pushing air back into the tank and popping the relief valve, there is supposed to be a check valve at the tank where you plumbed for the air ride to prevent air returning. If you have one perhaps it is weak, if you don't I would start there, the flexing of the air ride is more drastic at slow speed than at faster speed because you are already over the bump before it can react the same.

[Freightrain]

I have a new check valve inline to suspension.

[rsb502]

Thomas the air ride shouldn't run at high enough pressures to pop a relief valve, should only run 40-60psi loaded depending on which air ride and whether or not its set right of course. I would think if air ride built up anything over 100-120psi the hag would pop off the bellows before it could hit a 140psi relief.

[thomastractorsvc]

My thinking is that most air ride pressure is controlled by the height control valve and if the check valve at the tank was not working it would allow the air to flow back to the tank on a sudden impact vs releasing to atmosphere because the axel would be moving up opening the height control valve and with the added weight of the trailer pushing down would create a blast of air backwards into the tank like bellows blowing air.

Most air ride systems that I have seen and worked on don't have a separate pressure regulator except aftermarket add on front air bags and lift axels.