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I am in Sedona and can't get under the bus but I hear a consistent noises between the drive axle while parked that sounds like a fog "beacon on the coast" is very low pitch and last about a second and goes off regularly around every 15 seconds. I have a compressor running to keep my aux air up so I don't dump the tags. I know I have one more 5 port Norgren to replace after this trip. Any ideas what this noises is from?
Hi Greg. You should dump your air bags to eliminate brake air as the problem. If you don't hear it after your bags are down and your aux compressor off then its not your air brakes.
Then raise the coach up and if you have left & right rear switches for your air bags , dump each of those seperately with out the aux air . In other words, try to isolate or elimate the noise one step at a time.
Turn off aux compressor and see if it stops. Most likely the aux compressor trying to pressurize the air chambers around the drive axel. I had the same thing happen to mine.
Gary I'll try your steps.
Charles the aux air compressor is only raising the pressure to 110 then goes off so do you still think it is a problem. One thing I haven't checked is to hear if it is making the noise when the compressor goes off.
I never found an answer to mine doing this. It sounds just like a fog horn as you say. I think it is air pressure trying to equalize between the canister that serve the drive axel brakes. It just stopped on its own I think or, maybe it stopped after I had all my air bags and Norgen's replaced. Not sure.
i had a similar problem on a bluebird and it was a check valve. just a thought.
Considering the location and age / assumed milage, I would definitely check the "unloader" sometimes also called the pop-off valve for the Prevost (or any chassis) suspension.
The unloader on your coach is located to the passenger side, directly between the drive and tag axles.
For this coach the brand will be Bendix and parts will be readily available.
The unloader is the device, about the size of a volleyball, that "spits" or pops off when the coach reaches max pressure at 120 psi + -. This is the psssst! sound you hear while the coach is at idle, and you can not hear while driving. It also serves as a relief valve when the coach suspension is compressed for some reason, like speed bumps or rail road tracks or what have you, as opposed to popping the air bags or air lines.
It has an internal spring-loaded diaphram that opens at 120 psi + -, releases or spits out a burst of air, and then springs shut again.
This regulates the top end of the air system, while a pressure switch at the engine compressor regulates the lower end (engages the compresor clutch at 95 psi + -) So the proper pressure range is 95 (turned on by engine compressure clutch) to 120 (regulated by the unloader unloading) You can watch this happen by sitting with the enige running in park and pumping the foot brake. You will drop about 5 psi with each brake application. Once you get down to around 90 psi on the gauge, stop breaking (the compressor is now engaged) and watch the needle run up to 120 (until the loader pops) Although this may not help impress the girls, it is fun to do for a few hours.
Here is my guess on your specific issue:
Based primarily on location and description of noise, I think the spring loaded diaphragm inside the unloader is stuck partially open and "moaning" out air. For the age of coach it is very normal and common for the spring / diaphragm to get fouled with fine dirt or similar debris, causing it to not seat fully closed after opening.
Most often when this happens it fails in the fully open state, causing the coach to rapidly loose air and you are stranded. I think in your case it is only cracked open a bit and creating a "kazoo" noise. Or perhaps there is not enough pressure form the convertor's aux air compressor alone to pop it fully open (?) Either way, I think your unloader is not seating.
The good news: You do not have to replace the entire device. There is a kit for the spring and diaphrm alone (about the size of a tuna can) that will screw directly into the bottom of the unloader and that should do it. I mentioned the unloader should be a Bendix brand, if not it will be a Haldex brand, since those are the only 2 Prevost has ever used.
Any decent truck shop can do this and it should be familiar to them, as it is straight DOT pneumatic stuff. Do NOT let them sell you the whole device, just try the kit first.
Hope that helps and hope that's it!
Pat Sprenger
Coach Pro LLC
800-918-7172
Pat your reply sounds like it may be the answer. I notice the noise when my Aux compressor has reached at least 100psi. I will have to say that the noise is not as loud as the noises when the engine compressor pops off.
Thanks I will check this out when I get home.