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New Sound: compressed air relief

On our Thanksgiving trip I started hearing what sounded like the 12 volt compressor high pressure pop off activating. While accelerating through the gears I could hear what sounded like an old train: cho, cho, cho.
It seemed to be in time with the rpm's; higher the rpm, the faster the sound was, until I got the 4th or 5th gear. After that no sound.
It is not coming from the air dryer (as it is a new unit). My normal pressure is between 115 and 125 psi.
Could it be the engine air compressor relieving itself? or could it be from the pop-off?
I am thinking the compressed air gets too high with the higher rpm's and relieves itself.
Could it be the main regulator for the engine compressor? Any ideas?

Re: New Sound: compressed air relief

Reply #1 –
Morning,
Will it do it parked on high idle?
If it does after the air pressure is up and you can witness it that would help!
I wonder it the air governor can do that?
JD

Re: New Sound: compressed air relief

Reply #2 –
Normal sequence:  Engine driven air compressor loads, pressure in Wet tank increases until D2 sends "Unload" signal to dryer, dryer "pops off", air pressure in Wet tank gradually decreases (from normal operation of brakes/suspension) until D2 sends "Load" signal to compressor, sequence repeats.

This sequence happens faster as RPMs increase, but it is still the same sequence.

If D2 fails, the "unload" signal is never sent, compressor keeps pumping until Wet tank reaches approx. 150 psig, at which time the pressure relief valve on Wet tank opens.  Compressor keeps pumping, pressure in Wet tank stays around 150 psig (or whatever the relief valve will maintain) until the engine is shut off.  I have had this experience...

I don't think you could hear any of these things happening when accelerating through the gears.  Your dash air pressure gauge would indicate the "over-pressure" situation in case of D2 failure.  I would suspect the source of the mysterious sound is something closer to the driver compartment, but I can't guess what it might be.

Re: New Sound: compressed air relief

Reply #3 –
The air sound does not happen at fast idle (I checked) and the expected relief from the dryer works normally.
The dash air gauges never get above the 125 mark.
It really sounds like it is coming from the small air compressor pop off as that one is the only one I might be able to hear while driving.
Is there a check valve in that system that keeps the small air compressor from "sensing" the coach air pressure? I know the two systems are connected somewhere as it keeps the air pressure up in the coach while parked for a while.

Re: New Sound: compressed air relief

Reply #4 –
If you suspect the 12v compressor, try pulling the fuse.

Re: New Sound: compressed air relief

Reply #5 –
Quote from: Old Hippie –
Is there a check valve in that system that keeps the small air compressor from "sensing" the coach air pressure? I know the two systems are connected somewhere as it keeps the air pressure up in the coach while parked for a while.

(Note: Your coach is quite a bit newer than mine, so the air systems may differ)

On my air system diagram, the 12V aux compressor output is connected to both the "Front" and the "Rear" air tanks.  On my diagram, there is a one-way check valve adjacent to the compressor output, and another check valve near the inlet to both tanks.  SO, if 2 check valves (one on either tank + the one at the compressor) were to simultaneosly malfunction, then the 12V compressor would "see" the pressure in the main tanks.  If there is a pressure relief device (pop-off) at the 12V compressor (doesn't show one on my diagram), and if it is set at something below your normal coach air pressure range (roughly 90-125 psi), then it should activate.  That could be the sound you are hearing, but I would think this scenario would occur continuously anytime the engine is running - not just when you are driving.  So I am still puzzled...

Re: New Sound: compressed air relief

Reply #6 –
Thanks for the ideas.
I think I will completely turn off the small compressor before my next drive and see what that does. I know I have had to trun off the HWH system so the small compressor would not run while the coach air was pumping up before. That occasion caused the pump to run ( and get very hot) and it did lift the pop off a time or two.

Re: New Sound: compressed air relief

Reply #7 –
Quote from: Old Hippie –

It really sounds like it is coming from the small air compressor pop off as that one is the only one I might be able to hear while driving.
Is there a check valve in that system that keeps the small air compressor from "sensing" the coach air pressure? I know the two systems are connected somewhere as it keeps the air pressure up in the coach while parked for a while.

If your '02 is like our '03, there is a solenoid valve that's operated by the "air tank" switch on the panel to the left of the driver.  This valve connects the aux compressor to the rest of the coach system - useful if you want to start airing up without running the big engine before you pull out of a campground (being quieter for your neighbors), but it shouldn't be left on otherwise.  I know we had our solenoid valve fail in the "open" position - easy to check.  If the "air tank" switch is off but you hear the aux compressor running, watch the A and B tank pressures on your Driver Info Center (ignition to accessory, don't start the coach).  If the pressures rise, the valve is leaking through.

Re: New Sound: compressed air relief

Reply #8 –
Revisiting this with another thought (sure wish we could hear the sound)...  thinking about the "choo.... choo.... choo" and what I've heard outside the coach while Steve was driving (in fact, accelerating).  Our aux compressor relief valve sounds like a long "pssshhhhheeeeeewwwww" with almost a whistle-like end to it.

The other possibility of what you could be hearing is the front ride height valve exhausting, and in fact likely as it's designed to.  There is a "muffler" cap on the end of the ride height valve that, well, muffles the sound of the exhaust.  If the cap has degraded or disappeared, the sound of the ride height valve would be much louder and you could hear it repeat with greater frequency than the aux compressor relief valve typically would.

Links to the muffler and also to someone who described what I'm wondering if you're hearing

Leveling Valve Muffler

Part Number Collection

What's that sound?

Re: New Sound: compressed air relief

Reply #9 –
All of the mufflers are in place. I will check the valve to see if it is failed open.
Where is the D2 located?