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Topic: Air Dryer Pop off valve (Read 1331 times) previous topic - next topic

Air Dryer Pop off valve

Having a discussion about a friends coach air pressure goes from 128-115 almost instant after the compressor shuts down.  Both tanks are showing the lower air pressure, pressure stays around the 115 mark for a while and slowly goes down to the cut in pressure around 107. My coach when it cuts out at around 128 will slowly go down from there until compressor cuts in.

We are trying to figure out what is causing this which brings up my question as to how the pop off valve on the air dryer works.  Does the pop off stay open until the compressor starts back up or does it open on compressor shutdown and then closes back up?
2014 ih45  (4th Foretravel owned)
 1997 36' U295 Sold in 2020, owned for 19 years
  U240 36' Sold to insurance company after melting in garage fire
    33' Foretravel on Dodge Chassis  Sold very long time ago

Re: Air Dryer Pop off valve

Reply #1 –
Sounds like he may have two issues:

Check valve(s) not working. Irrespective of a malfunction of the dryer/purge valve, the check valve(s) should keep air from going from tanks back to dryer.

Purge valve sticking open-- it should only open momentarily when cut-out pressure is reached, then close (remember that "sneeze" from the dryer-- it is close to instantaneous.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Air Dryer Pop off valve

Reply #2 –
Quote from: wolfe10 –
Sounds like he may have two issues:

Check valve(s) not working. Irrespective of a malfunction of the dryer/purge valve, the check valve(s) should keep air from going from tanks back to dryer.

Purge valve sticking open-- it should only open momentarily when cut-out pressure is reached, then close (remember that "sneeze" from the dryer-- it is close to instantaneous.

That is what I was thinking but I was not sure exactly how long the purge valve stayed open.
2014 ih45  (4th Foretravel owned)
 1997 36' U295 Sold in 2020, owned for 19 years
  U240 36' Sold to insurance company after melting in garage fire
    33' Foretravel on Dodge Chassis  Sold very long time ago

Re: Air Dryer Pop off valve

Reply #3 –
Quote from: turbojack –
That is what I was thinking but I was not sure exactly how long the purge valve stayed open.

It is a "sneeze"-- no longer.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Air Dryer Pop off valve

Reply #4 –
Learning a lot on this project.  Believe I have most of the external leaks under control--lot of bubble bath.  Now looking at internal check valve or PPV valve leaks--harder to identify.  No check valve between wet tank and air dryer--only one is on the Cummins air line.  Believe front DOT tank check valve is leaking back, rear tank holds at 52 PSI.  As mentioned, Cummins compressor cut off at about 125lbs but DOTs quickly settle to about 115lbs.  Suspect air dryer is OEM--thinking about just replacing the whole assembly.  Ref attached, any additional thoughts would be appreciated.
 
Old Scout
2015 IH45
New Braunfels Tx
 

Re: Air Dryer Pop off valve

Reply #5 –
Quote from: OldScout –
No check valve between wet tank and air dryer...
Strange there is no check valve at the wet tank inlet port.  I always assumed that was standard air line plumbing practice.  There is an internal check valve on most (if not all) air dryers.  It is part of the dryer delivery port assembly and should be replaceable.  See "typical" AD-9 dryer schematics below.  I realize you probably have a different model dryer on your 2015 model coach.

Bendix® AD-9™ Air Dryer

PS:  I took the liberty of adding your air schematic to our Forum library.  THANX!

https://www.foreforums.com/index.php?action=media;sa=item;in=5020#viewitem
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"