No air Pressure going to Front and Rear air tank

[Larry Dedrick]

I have a 2008 Beaver Marquis with a Cat C15. Today, started engine and my air pressure in both tanks was around 90 psi and would not build above that. Pumped the brakes to lower air pressure. Pressure dropped to 65 PSI but will not build back up. Tried to source some information, such as a related fuse or relay, but no luck except the relay in the rear run box labeled "AIR". replaced relay but no affect. So, I'm seeking some ideas, anyone?    Also, I don't believe the AUX Compressor is operating either with a good fuse.  Possibly a faulty low pressure switch problem?

 
         Thanks in advance,  Larry D.

[Frank Bergamo]

Larry,
assuming no obvious air leaks in system, there is a Bendix air governor      http://www.newtruckspring.com/wholesale/replace-bendix-d-2-air-compressor-governor/                                                       that may need replacing. On my 2007 it is mounted in the engine compartment on the cross rail. These do fail occasionally and need replaced. Also, there is an unloader valve on the engine compressor that get gummed up and sticks. There are rebuild kits available,         https://parts.cat.com/en/catcorp/335-6328  or just by taking them apart and cleaning them up you can get them to work again. Hope this helps.

[Larry Dedrick]

Frank

         Thanks for the information. I traced the air line from compressor to the regulator? to ck for anything obvious, found nothing unusual. After that I had DW start engine so I could monitor the compressor and make sure there was no popping of air from the top of compressor (I don't remember what that device is called) but it was not releasing any air so all was good there. DW came out of coach to report air pressure was building, so apparently all is good, At this point. Hope there is no more problem.

  Thanks again.

[Keith Moffett]

Larry, any chance this could be a sticky pressure relief in the air dryer.  I believe the engine air compressor is mechanical so if a sticky valve seems like one might hear that at the air dryer. 

[Larry Dedrick]

Keith
          There is an unloader poppet valve on top of compressor that I was checking, but it was not abnormal I believe. Located the regulator but didn't do much except physically checking an electrical connector goin to it. After that Laura cranked up the engine and found air pressure building up.

       Thanks Keith

[Frank Bergamo]

Larry,

it’s been my experience the unloader valve sticking is intermittent, and more than likely will happen again. As I stated above the valve can be disassembled and cleaned, then reassembled with satisfactorily results. The air governor would be the first thing I would replace, $20-$40. If the problem persists after governor replacement, then the unloader valve, $100, would be next on my list. Both tasks are fairly simple with limited mechanical abilities. Hope this helps.

[Dave Atherton]

Larry, with many possible area to cause the low air build up. Over time the steel braided hose from
the air compressor to inlet on wet air tank break down over time and the inner hose will deliver less
volume of air such as in your case. We have run into this problem several times in the past years and
Is very hard to pin point. My suggestion, take shop air and charge your air system to system pressure,
than listen for air leaking around length of steel braided hose from air compressor last thought on that.
Take and wiggle hose around the hold down clamps because that is most likely area where air leak is
located. Note: with shop air into motorhome air system will not build pressure beyond 90 psi there is
a check valve at the discharge fitting between the wet air tank which is first air tank to receive air from
air compressor that connects to second air tank known as the dry tank and in between the connection there is a 90 psi one way check valve ( possible stuck ) and will not allow air to build up within air system over 90 psi. Some times there is only one air tank that is called a split wet and dry air tank,
If that is the case the check will be out side on the air tank going into fitting on each tank.
Dave Atherton Retired Cat Mechanic