Humming noise near front axle

[Jim Blackwood]

So I got my 110v compressor installed, it takes awhile to pump up but switches off at 130 and the automatic drain blew at the exact same time. So that's all good. But while it was pumping up I got some funny noises from up front.

First, at around 80psi there was a fairly loud buzzing noise, which went away when I switched off 28v power. Then at about 120psi there was a humming noise regardless of power and when I touched a stethoscope to the curb side air tank behind the axle it sounded a lot like an electric motor running. Then when the compressor shut down and the drain purged all went quiet. Main power on, main disconnect on, no noise at all that I could hear.

For those of you who have lived with the DL3, does this sound normal? Anyone know what was making the noises?

I'll get a photo of the compressor install and post it.

Jim

[chessie4905]

Ive heard that sound when pumping up the coach from external shop air. Mine comes from the protection valve under driver in compartment. Goes away once it opens to allow air to air suspension and accessories.

[hogi6123]

My bus has several 1-way air valves, which hum and buzz and make other noises while the air is flowing through the system (while airing up).  Yours probably does too.

A loud buzzing noise that is on with the power is likely a solenoid?  I'm not sure what it would be for though.  Could also be a low air alarm, but I would guess you know what the low air alarm sounds like on your bus.

[richard5933]

Could you be hearing some of the sound of the compressor itself telegraphing though the bus? Especially if you used hard piping to connect?

Otherwise, my bus makes similar noised up front as the engine airs things up. Quite noticeable just before the auxiliary tank opens and just after it opens.

[luvrbus]

That is normal for the DL the air dryer is in the front by the axle the low air buzzer is in the panel on the drivers side and are adjustable for different sounds when the pressure was built up the buzzer went off is why there was no difference with power off or on 

[Jim Blackwood]

OK thanks. Pretty much normal then I guess. The compressor cycles a lot sooner than will be acceptable for camping so I must have leaks somewhere. Here's a shot of the compressor and one of the bus up on stilts. I'm going to let it sit like that for a day or two before getting underneath. The stands are sitting on 18" diameter 1/4" plate just to spread the load a bit. The rear has two 6x6 blocks up under the frame on each side to lift it up. I used the "rear raise" switch to lift it and then block it. Been sitting like that for a couple weeks now.

Are there some places these are more prone to leaks than others? Right now it takes about 1/2 hr to pump up and probably less than 10 minutes to cycle back on again.

Jim

[richard5933]

Where to look?

I started at one end and tried to follow every line I could, even in the tunnel and behind panels if they were removable. Surprising how a 1/4 turn on a fitting can make such a big difference.

Wiper motors and especially control valves (if you have air wipers) are notorious. Air bags, height control valves, etc, etc.

Remember that it's possible for height control valves to leak internally if the check valve is failing - it's difficult to find the leak since it sends the air back to a tank and not externally leaking.

Does your bus have air controls/lock for the door?

[ol713]

   HI;
      I also have an MCI (7).  When using shop air to air up the system,  I also hear it "humming".
      I just call it '' the bus is singing". Stops after the air bags are up.
                                       

[hogi6123]

Anything could be leaking.  The previous owners would have fixed the "most common" leaks already, so I think you just have to listen for escaping air, I would expect it to be pretty obvious at that leak rate.  From my experience, valves with internal seals are prime suspects, but any tank could have a hole rusted in it, or any line could have a hole poked in it.

General locations to listen:
- Front axle
- Spare tire compartment
- Under the dash
- Rear axle
- Engine compartment

[luvrbus]

The little compressor take longer to air up the suspension,the engine compressor is at least 12cfm your portable is probably4,5 or 6 cfm

[Jim Blackwood]

Yeah, maybe even 3. I'd have to look. At least it's made from good old cast iron so it should live awhile.

Messing with the compressor, jack and jack stands has made my back a little sore so I'm going to give it a rest for a day or two, maybe play with a CAD program a little on those tank drawings. It'd be nice if I can give them something better than sketches to work with. I'm thinking 3/8" plastic might be the way to go on those.

So if I'm interpreting the suggestions right, maybe my first move is to go over all the lines and snug them up at the ends while listening and feeling for air. I have this neat gadget that's used to check for water leaks in buried lines... It's kinda like an amplified stethoscope. Maybe it can help. If there's a valve leaking I hope I might be able to hear it with that. The air bags all look to be in pretty good condition. I already gave the tire well a once over. Well, I should know more in a couple days.

Jim

[hogi6123]

My leaks in my 1981 were:
- rusted through parking brake tank
- inversion valve internal seal
- an air bag
- push/pull valve interval diaphragm
- rear suspension leveling check or relief valves
- wiper controls when the air pressure is around 70-90 psi

They all hissed quite noticeably to someone standing next to the bus, except maybe the push /pull valve.  However it was frequently a lengthy search to pinpoint the actual leak.

I did not find any leaks at fittings.  But others have.

I would recommend starting by listening for escaping air.

If you become serious about every leak:
Others have talked of soaking all their lines with soapy water.  This will help for slow quiet leaks but sounds very tedious.

If you can identify how the subsections connect, you can isolate them with caps and plugs and a pressure gauge and hopefully reduce the amount of lines that need to be checked.

[buswarrior]

In the self inflicted wound dept...

"Tightening" fittings for no apparent reason is way down my list of remedial actions.

Snap one and you've got unnecessary project distraction.

Go find the noisy ones first, and see where you're at before making work?

Happy coaching!
Buswarrior

[chessie4905]

I went through the brake system on mine last year. Found several leaks. 3 were lp3's. Two were an incredible bitch to replace. Up behind last step to main floor. No longer accessible  from above, had to get to them from below. Another leak was a main inline check valve. Used Harbor freight stethoscope without the amplifier attached along with leak detector liquid that ac supply stores carry. It works better than soapy water. Has glycerin in it, so it stays in place longer. Bendix recommends every air system component to be rebuilt every several thousand miles. When was the last time yours were done or any other coach here? I rebuilt or replaced the majority of mine. The worst was the module at rear axle. Several different components there. Or we can wait till many of the components are finally unobtanium. I ran into this with the inversion valves. The one type is no longer available, even from Luke. I ended up buying a new one and swapped spring and shims from old one. Even so, it took three months to get the new one. They had to make more.

[Busted Knuckle]

BTW your air bags and such don't start filling until the brake system has about 80-90 psi so that could be why some of the noise starts them as the "accessory" system starts filling.
  BK