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Learning about the engine air compressor
#1

Gurus,

I was minding my own business when a fellow guru commented to me his concern regarding the length of time it took my coach to build air.  My coach has always taken several minutes to build up to 120 psi from a low air pressure situation.  That simple conversation caused me to learn about my air system and to make some needed repairs.  Take what I am going to tell you with a grain of salt.  I really don't know what I am talking about and am putting this out there to cause you to think about your own coach and how its air system works.

Before I go into the details of what I found and what I did, I would like to jump to the bottom line things that I learned....
  • The governor is critical to the overall health of your air system.  They are cheap and easy to replace and in my opinion are often overlooked.
  • If you are not building air quickly, a simple and small leak can stop you on the side of the road leading to an expensive tow or worse.
  • Everyone knows that you should change the cartridge in your dryer regularly, but it is important to test the dryer to see if it is leaking air.
  • Finally, because of the conversation with the fellow Guru, I now know much more about my air system and with the same Guru's help, I now build air quickly and my engine air system works as designed.

When I first looked at my engine service brake air system on my coach, it would take 5-6 minutes at high idle to go from 0 psi on the engine gauge to 120 psi.  At that point the system would "pop-off" and tell me that I was full of air.  I thought this was normal.  Well it *might* be normal if your air bags are fully deflated.  In my case with the bags at travel height and 70-80 psi in the system it would still take a while to reach 120 psi.  Something was definitely amiss.

I had another weird symptom as well.  Instead of a nice PSFT! when the dryer popped off, I would sometimes get a psst-psft-psft-psft-psft-psft sound.  In other words it had a series of small pop-offs instead a single purge.  Hmmmm.

Time to do some learning!   

The main components of the engine air system are the Compressor, the Governor and the Air Dryer.  Of course there are other parts, but these are the stars of the show.   

The compressor makes air, the air goes thru the dryer and onto the service tank where it is used to operate the brakes.
   

The compressor is bolted to the engine and is turning whenever the engine is running.   It is cooled with engine coolant.  It obviously produces compressed air.   The dryer obviously dries the compressed air.  But the smarts of the whole system is the crazy little device called a governor.

   

The governor is the widget that tells the compressor whether to be making air or to be idling.   The compressor has an unloader circuit that allows it to turn easily without the energy required to make air.   This  unloader circuit is controlled by an airline from the governor.   The way that the governor determines when to unload the compressor is by using an airline from the service tank to measure pressure in the system.  The governor uses pressure from the service tank along with its cutoff pressure adjustment to determine when to send air to the unloader on the compressor -- essentially turning off the compressor.    At the same time it sends an air signal to the air dryer to tell the dryer to purge any liquid.   This is what is happening when you hear the "pop-off".

   

Consultation with the other Guru, led me down a path of testing and repair in the order of cheapest / easiest  to harder and more expensive.    In order of lowest expense and complexity are the governor, the dryer and the compressor.

The first thing I did was to replace the governor. 
.pdf d-2_governor_maintenance.pdf Size: 137.92 KB  Downloads: 28

This drastically improved the amount of time it took to fill the service tank...but the pop-off was still not working correctly.

The next thing was to order a new air dryer  We raised coach and blocked the coach frame as well as chocking the wheels..  The dryer is bolted to the wall just behind the tag axle on the drivers side.   Access the bolts in the cable reel bay.  I used an air wrench.    It took maybe 30 minutes to change the dryer.  (The new dryer came with a new desiccant cartridge).  

Here is old dryer.
   

At this point, the system was going from 0 psi to 120 psi in about 1.5 minutes.   (bags already full).  Success!  We found that the purge valve on the old dryer was rattling and had worn to the place where pieces were loose within the valve assembly.  

Since my coach has 250,000 miles and 16 years of use, I would say that I got my money's worth from the governor AND the dryer.   Smile

If we had found that we were not making enough air, we would have looked at the compressor next....but luck shined on me and my compressor was fine.

I said it above, and I will repeat here.  Your coach may be different and I may or may not be right about the way that I think about the air system.  What is absolutely true is that the air system is critical to the operation of the coach and that it would be a good idea to do a once over your system so you understand how it works.   Do yourself a favor and find your governor and buy a spare.  They are fairly inexpensive and it may be the one thing that gets you going again when you run out of air.

Thanks again to the unnamed guru who assisted on this project.   It really does take a village.

Cheers,
bill

Bill Johnson
Birmingham, Alabama
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#2

Bill: I'd say you got it right, good on ya. It seems this felt beyond you, but you proved yourself wrong - in a good way. You were open to learning, and to applying it. I love hearing this kind of story.

From a villager

Jon Kabbe
1993 coach 337 with Civic towed
Reply
#3

Jon,

Thanks for the comment! You're exactly right. It felt beyond me...but at the end of the day...It's not rocket science. I am now gonna be a better driver / owner because of this experience.

bill

Bill Johnson
Birmingham, Alabama
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