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Eng air over builds pressure, air dryer constantly releasing
#1
Symptoms;
Engine compressor causing Air dryer to constantly blow off after it hits 120psi. pst,pst,pst,pst. very rapidly.
it builds pressure fine and holds pressure, but once to 120 (+/-) it continually releases until the engine is shut down.

How to diagnose.
On mine, the D2 governor is located behind the air dryer, inboard of the left rear tag wheel. it has a 2 lines 1/4" lines attached.  One line comes from the primary tank, the goes back to the compressor. If you remove the 1/4" line at the compressor at start up, and the chassis has no air built up yet, the line will not have air coming out of it. As the system build pressure and the governor recognizes 120psi, it will start to release air, back into the compressor into the unloading valve. This valve uses air to open the compressors valves so that it, in a sense, free spins or does not build pressure until a pressure drop to 90 ( or what ever your governor is set at) is recognized at the governor. 

Repair
The Unloading valve is on top rear center of my Bendix compressor with 2 small Torx bolts holding it down the cover. Its is qn upper plunger inside of a larger plunger, all with about 4-5 O rings
My unloader valve had a bad O ring and leaking internally therefore it was not moving correctly, therefore, allowing the compressor to run all the time and making (forcing) the air dryer to open its relief/pop off valve to control the pressure.

I went into CAT with my Bendix air compressor part numbers, prepared to get my exact part. I was told, I was better off giving them my engine serial number. For that reason, I do not offer you my part number since it correlates directly to your engine.  About $120 and 1/2 hour and I was back to running normal.

I also watch several good YouTube Video's using the search "unloading valve", how to install, ect.

I'm not an expert, but this is my best understanding. please feel free to add or correct this post.
Hope this helps somebody.
Dave, Karen and buddy. 
06 Newell #784 towing a 05' Featherlight enclosed  trailer for toys and tools
A bunch of stuff on wheels, including 59' Nash metropolitan conv., KTM 250 dual sport. 

Carpe Diem. 
Tomorrow is not guaranteed.

#2
Nice write up Dave.
Richard and Rhonda Entrekin
99 Newell, 512
Subaru Outback Toad
Inverness, FL (when we're home Cool )
#3
Thx Dave!
1993 Newell 45' 8V92,towing an Imperial open trailer. FMCA#232958 '67 Airstream Overlander 27' '67GTO,'76TransAm,'52Chevy panel, 2000 Corvette "Lingenfelter"modified, '13 Grand Cherokee.
#4
That is really good to know thanks
Joe Galowitch
1998 Newell
Coach 459 45 foot 1 slide
#5
I have this continuous pop-pop-pop under load (I think when turbo is spun up) now, too. Newell over the phone thinks it is the popoff valve acting up and needing replacement.

It sounds like there are a lot of reasons it could be happening.

We're going to Miami in a few weeks. I'll try to learn what you have figured out by then, so I can "help" diagnose.
2008 Newell #1234
Boulder, CO

#6
Wink 
Nebster,

My problem was occurring when the engine air pressure reached 120 or higher like 140 on the dash gauge. This would happen at idle while stopped and no load or driving. I could walk around the left rear tag wheel and hear it pop,pop,pop at the air dryer filter. Then the top rear of the air compressor would psssssss air over the, what I now know to be, the overload valve. Once the engine was off, this overload valve would PSSS for a few more seconds until pressure lowered to ???120psi I think.

I have a Cat C15, and I've not seen a pop off valve for the progressive turbo's. Pop off valves on Turbo's blow off excessive pressure after maximum boost has been reached, or just before an upshift, but never under Jake braking.

My 2 cents Big Grin
Dave, Karen and buddy. 
06 Newell #784 towing a 05' Featherlight enclosed  trailer for toys and tools
A bunch of stuff on wheels, including 59' Nash metropolitan conv., KTM 250 dual sport. 

Carpe Diem. 
Tomorrow is not guaranteed.

#7
Yeah my situation is a bit of a mystery. I definitely hear it more when accelerating or under load.

But I also hear it sometimes when I pull into a campground or rest area and the engine is coming back to idle. Just not all the time.

And then, when I shut down now, I hear a pssssssssh for 30 to 90 seconds afterwards, but my sound is coming from the engine bay, not underneath by one of the axles.

Maybe I have more than one thing going on.
2008 Newell #1234
Boulder, CO

#8
What do you notice looking at the Air Supply gauge when this is happening?
Richard and Rhonda Entrekin
99 Newell, 512
Subaru Outback Toad
Inverness, FL (when we're home Cool )


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