[DK on the road]

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.

[Richard]

Nice write up Dave.

[HoosierDaddy]

Thx Dave!

[Joe Galowitch]

That is really good to know thanks

[Nebster]

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.

[DK on the road]

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 

[Nebster]

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.

[Richard]

What do you notice looking at the Air Supply gauge when this is happening?

[Nebster]

I finally made it to Newell and got some time to look into this.

Here are the things I observed over the summer:

1)  Stuck on side of road, one of the air lines coming out of the distribution manifold (what Newell calls the "air dryer", although I think really it is more like a governor + distribution + air dryer assembly) blows out at its fitting.  Replaced that line and drove on.

2)  Stuck on side of road again three days later, female coupler at shop air inlet ruined (gasket/spring mechanism mangled inside the fitting) and air blowing straight out of it at full bore.  Replaced with new fitting and drove on.

3)  Intermittent issues when pulling into a campground and getting ready to level:  seal deflation ran out of air many times, no supply air being produced.  Went back to manifold and safety pop-off valve was stuck open.  Pushed valve back in and pressure builds again.  I think that safety valve is designed to stay open when it pops.

4)  Intermittent issue with air hissing under front when underway, usually five minutes into a drive at a stop light or something.  This is definitely the height control trying to release air, but for some reason it "releases" air for five or seven minutes before stopping, instead of 5 or 7 seconds.  The coach never got out of proper travel height.

5)  When we drive, we hear constant pop-pop-popping from the rear.

6)  When we come to a stop and shut down the engine, there is a blow off of air in the engine area that lasts 15-60 seconds every time.

When we got to Newell this week, immediately they noticed that the 1/4-inch air line between the governor and the air compressor was blown off the compressor fitting and just hanging around loose in the engine compartment.

They checked a few things and noticed that the compressor was running constantly rather than just until the tanks were full.  (That's #5 above.)

Newell decided to swap the entire dryer/manifold/governor assembly with a new one ($600).  This new piece did nothing to change the behavior, so they backed it out and put my old one back in.

Newell called CAT and someone there told them to replace the whole air compressor ($1600).  I actually went to Joplin to pick up this part, and I talked to one of the techs there.  He said that these compressors rarely fail, and that the problem is probably elsewhere.

The first thing he suggested was to put a fresh 1/4" air line into the control port on the compressor and then pressurize it with a standalone tank or compressor, to test to see if the airflow into that line shuts the compressor off.  We did that, and indeed the compressor just kept pumping.

The next thing we did is to replace the unloader valve.  This is the exact same thing that Dave did above.  The unloader valve kit is what you need, about $100.  If you have a CAT engine, it is the same kit for all the engines and air compressors that they ever made or installed, so it's pretty easy to source.

The new unloader valve took about 10 minutes to install, and all the problems went away.

Except the height control related issue (#4 above), which we haven't had an opportunity to test yet.  I'm not quite sure how the problem in the rear could induce this problem, but I guess my flimsy theory is that the overpressure in the lines was somehow preventing the raise solenoid from fully closing.  Because this problem requires driving and is intermittent, it'll take a while to know whether it was also a symptom of the failed unloader valve.

Easy peasy!  Except for the driving back and forth to Joplin, intermediating the conversation with CAT, and keeping Newell from throwing $1500 parts at the problem.  

[Richard]

I think you are on the right track in how to look at this, by asking the question “what could have been damaged or compromised by constant overpressure.

Clearly any safety relief valves.
Possibly the height control valve itself
Any of the fittings in the air system
Valving in the air systems

It’s good that the seals, potties, and doors are all protected from this situation by the design of the system and feeding those systems through a standalone regulator.