Author Topic: Persistent Air Release  (Read 12312 times)

JCMillerSr

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Persistent Air Release
« on: September 02, 2015, 02:20:06 PM »
During our last outing we noticed a persistent air release while driving.  When driving at highway speeds (55 to 65 mph) we hear the sound of an air release about every 15 seconds.  If sitting still while idling the engine the same noise occurs about every minute or so.  It sounds as though there is some overpressure valve being released and only last a second or two.  No problems with using the air leveling except that there seems to be a slow leak on the right side which requires a redo about every 24 hours or so.  Pressure gauges on the dash are all in the normal range.  Any suggestions where to look?  Coach is currently in the shop for some routine maintenance and I hate to pay an RV Tech spend hours looking for the source without some idea of what/where to look.

Dave Atherton

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Re: Persistent Air Release
« Reply #1 on: September 02, 2015, 03:16:01 PM »
J C Miller, the noise you hear sound like the air dryer is popping off, the reason you hear the
noise because air Dryer filter needs to be replaced. With your motorhome it one of the old style
type and my suggestion would put on the new style air dryer instead ( cost wise )
Dave Atherton Retired Cat Mechanic

JCMillerSr

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Re: Persistent Air Release
« Reply #2 on: September 02, 2015, 03:34:05 PM »
J C Miller, the noise you hear sound like the air dryer is popping off, the reason you hear the
noise because air Dryer filter needs to be replaced. With your motorhome it one of the old style
type and my suggestion would put on the new style air dryer instead ( cost wise )
Dave Atherton Retired Cat Mechanic

Thanks Dave.  I will pass this information on to the service tech and see what he comes up with as well. So far the only thing they have confirmed is that the noise is coming from the chassis air tank pop off valve but did not have an explanation as to why it is occurring.

Dave Atherton

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Re: Persistent Air Release
« Reply #3 on: September 02, 2015, 04:10:08 PM »
 J C, if confirmed air release from pop off valve on the air tank, than problem can be two things.
First your Air Compressor is putting out too much air and the pop off valve is a safety valve releasing
the excess air pressure. Your air pressure gauge on dash may have faulty reading ( should read 120
PSI ) or the pop off valve is faulty. And is a easy repair and does not cost much. If air pressure too
high than your air governor is stuck and that is reason pop off valve is opening. Cost for Air Governor
$ 60.00. Hope this helps. Dave Atherton Retired Cat Mechanic

Edward Buker

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Re: Persistent Air Release
« Reply #4 on: September 02, 2015, 07:36:07 PM »
Dave,

The air popping off and cycling is a normal event and does not indicate the need for a dryer cartridge change, it has to do that to discharge moisture and go through a regeneration cycle. As far as how often it should occur varies with driving conditions. That being said, if you are just idling, not using brakes, not moving then it should not be repeatedly popping off given there should be little air used by the chassis. The point Dave made about the overpressure valve could be defective, opening too soon, or the air governor needing replacement or being set too high is excellent advice from our favorite Cat mechanic :-)

Later Ed

Dave Atherton

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Re: Persistent Air Release
« Reply #5 on: September 02, 2015, 09:15:17 PM »
Hi Ed back again, sitting here with the good book right in front of me ( Air Brake System Troubleshooting )
Air dryer purges at intervals 25 seconds or less,  air dryer filter must be changed, or every three years.

Air Govenor cut off should be at correct pressure 125 to 135 psi if not reset air governor to meet pressure.

Buildup time, pressure should build from 85-110  within 40 seconds

Air Compressor buildup time greater than 40 seconds, examine inlet filter or carbon buildup in discharge
line just off compressor.

Air Governor cannot adjust air pressure at pressures 120- 135 psi replace air governor

Air Governor cutin: reduce the service air pressure to governor cut-in. the difference between cut-in and
cut-out pressure must not exceed 30 psi.
Air Pressure drop ( A 4 psi pressure drop within 2 minutes is allowable and can remain in service )
Dave Atherton Retired Cat Mechanic

Dave Atherton

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Re: Persistent Air Release
« Reply #6 on: September 02, 2015, 09:48:08 PM »
Ed, forget one last thing and have made several service calls, Motorhome stops for lunch along road
and cannot build air pressure and release brakes. Root cause of problem air dryer popping off at fast
cycles ( less than 25 seconds ) shut engine off,  now air dryer the purge valve on air dryer will not close
so air system will not build air. The repair untill owner can get air dryer filter,  we by pass the air dryer
with a piece of 5/8 plastic air line. there is a by-pass kit that can be bought doing the same thing I have
done. some thing that will come in helpful if problem happens along road.  Dave

Keith Moffett

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Re: Persistent Air Release
« Reply #7 on: September 02, 2015, 09:53:25 PM »
Hey Dave, this info might work for me too on the other thread.  Thanks
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Joel Ashley

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Re: Persistent Air Release
« Reply #8 on: September 02, 2015, 11:43:48 PM »
Ed, also note Dave mentioned in his first reply that Jack may have an older style dryer, older perhaps than what we're used to.  Though the drier may purge and regenerate regularly on the road, I sure as heck don't hear mine every 10-15 seconds at the normal highway speeds Jack mentioned;  even his '92 coach I wouldn't expect to hear cycling that often either.

It's likely Dave's on target again ;) .  Now if only I remember his bypass advice when mine starts doing that some day.  My wife learned long ago how to bypass when I start popping off :) .

-Joel
« Last Edit: September 02, 2015, 11:48:51 PM by Joel Ashley »
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Edward Buker

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Re: Persistent Air Release
« Reply #9 on: September 03, 2015, 03:20:42 AM »
Dave,

I guess the struggle I am having is the purge valve vs the coalescent cartridge as to what is needing replacing. They serve two functions and as long as there is no moisture in the system the coalescent cartridge is doing its job. They talk about 100 to 300K mile life to those filters. The purge valve is the component that may not cycle as it should and could blow off at the wrong pressure or not cut off as it should but that has little to do with changing the filter regarding a cure.

Later Ed   

Dave Atherton

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Re: Persistent Air Release
« Reply #10 on: September 03, 2015, 04:05:30 AM »
Ed, that is correct, in most cases when the purge valve needs kit is because of the cycling so often,
oil/water will not seat purge valve. this is where motorhome cannot build air pressure after stopping
for lunch, takes about 5 minutes to put o-ring on purge valve assembly. Location different parts of country where it is very humid the filter will not last as long in area like AZ. etc.  Note: cycling of Purge valve
on air dryer. When the air pressure becomes greater than that of the preset cut-out, the governor
controls the unloader valve mechanism of the compressor, stops the compressor from building air, and
causes the air dryer to purge. As the service reservior air pressure drops to the cut-in setting of the
governor, the governor returns the compressor back to building air and cycles the air dryer back to
air drying mode. As the atmospheric air is compressed, all water vapor originally in air is carried along
into the air system ( as well as small amount of the compressor lubricating oil ) as vapor.
Dave

David T. Richelderfer

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Re: Persistent Air Release
« Reply #11 on: September 03, 2015, 04:38:49 AM »
I think Ed meant desiccant, not coalescent.  His spell checker may have found the wrong word?  That would be my excuse.
« Last Edit: September 03, 2015, 04:44:08 AM by David T. Richelderfer »
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Edward Buker

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Re: Persistent Air Release
« Reply #12 on: September 03, 2015, 05:01:08 AM »
David,

Thanks, you are right, the coalescent filter removes oil and some of the newer Meritor desiccant filters are dual purpose and include a coalescent filter and therefor remove both oil and water. I should have used desiccant filter in this case.

For the other David, our resident Cat mechanic, is there an interval for changing the compressor governor, how long are they expected to last? I have never seen that mentioned as a maintenance item.

Later Ed

Lee Welbanks

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Re: Persistent Air Release
« Reply #13 on: September 03, 2015, 05:23:44 AM »
Ed,  In all my trucking days I think I have replaced one governor, the one on the PT stuck once and had to cycle the jack screw to get it to start acting right. Has been fine ever since. If you air system stays clean they will last a long time.
I would suggest that for this air dryer problem get under the coach find out what dryer is on it and download the manual to be able to troubleshoot what is going on. We are all shooting in the dark as we don't know what is actually on the coach.
As for this bypass kit, on my PT you had better have some big wrenches, the compressor discharge line is a 3/4 Jic fitting which takes a 1 1/4" wrench and a cheater pipe on it to break the nut loose. Just info.

Dave Atherton

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Re: Persistent Air Release
« Reply #14 on: September 03, 2015, 06:12:23 AM »
Joel, by-pass air dryer, what you need look at your air dryer and should be enough slack in
hose to take both 5/8 feed and discharge line and remove them from fitting on air dryer. take
and remove both fitting one end will have a male jic end and other end will be pipe male thread.
there are 2 fittings. buy a pipe nipple and connect both fitting together and connect both 5/8
air lines to each end of fitting connected together. if there is no slack in air line than do the same
thing but add a piece of 5/8 air line and connect both ends of present air line.  any questions
give me a call. Dave Atherton Retired Cat Mechanic