Skip to main content ↑ ↓
Topic: Air System Problem (Read 483 times) previous topic - next topic

Air System Problem

I am having a problem with my air system.  I suspect there is a check valve somewhere that is sticking periodically but studying the air schematic hasn't helped me figure it out.

The symptoms:  The air system pressures up normally upon engine start up and seems to operate normally for a while.  Then with a slight application of brakes or with the use of the retarder or sometimes for no discernible reason at all the pressure in the aft air tank will ease its way down to about 60psi.  It doesn't happen with every application of brakes, only occasionally.  Of course the dinger alarm sounds off because of the low pressure.  The protection valve seems to be doing its job as it doesn't allow the pressure to go lower.  I can continue driving, sometimes for miles, with the dinger blasting and then for no discernible reason the pressure in the aft tank will return to the normal range.  After a while the sequence will repeat. 

Does this situation point to a specific valve to look at or some other problem? 

As an aside, various items on my air schematic are identified by numbers 1 through about 61 and the letters A through U.  I have not found a key to these.  Some are also labeled, some are easy to figure out but most are not. 

Richard

Re: Air System Problem

Reply #1 –
When the rear tank goes to 60, what is it saying the front tank has?

If you pay attention to the pressures, when compressor kicks in the pressure in both tanks should go somewhere around 125 psi. Then when one of the tank's pressure gets to around 105 then the compressor should kick in again and bring them both up to 125 or so.  If you watch and give us the additional information about the front we might be able to nail down what is going wrong.

I know on some years there was/is a problem with the pressure sending unit.

Re: Air System Problem

Reply #2 –
Might be a false reading on pressure.

Re: Air System Problem

Reply #3 –
The front tank varies between about 100psi and 125psi as it should.  During the times that it is acting normally, the rear tank varies right along with the front tank.  It is just periodically that the rear tank pressure drops and holds around 60psi and the warning ding chimes in. 

I have the mechanical gauges with air lines connected directly to the back of the gauges so I doubt that I am getting false readings. 

Thanks for your responses. 

Richard

Re: Air System Problem

Reply #4 –
When parked but running - any air leak sound near the retarder accumulator valve?  I believe it gets its air from the rear tank and it's been a reported leak point in the past with symptoms similar to yours.  Here's one of several prior incidents:

Air pressure leak

and this one

Big Air Leak Hydraulic Accumulator

Air leak

Mike's got a good write-up on disassembly and cleaning Allison retarder air valve

Dick S's topic on replacing it Replacing transmission retarder valve (split from What did you do to your coach)

Re: Air System Problem

Reply #5 –
Quote from: Jan & Richard –
I have the mechanical gauges with air lines connected directly to the back of the gauges so I doubt that I am getting false readings. 

Never say never.  You might swap the gauges and verify the problem is not the gauge.

Need to watch your gauges.
1.When everything is working perfect aprox. how long does it take to draw down from 125 before pressure starts rising again?
2.When rear tank is at 60, how long does it take for the front to draw down from 125 before pressure starts rising again?
Same time or when problem shows front tank spends way more time around 125?

Based on pressure staying at 60 psi on the rear tank. The leak would have to be a big leak to draw that tank down that bad and it would be on the output side of the protective valve.

In looking at schematic for a 2009 coach coming off the rear tank protective valve is retard, air hose connection, & air supply to the rear height adjusters.

So if problem is not the gauge it could be
1.retard  (See Michelle''s post above) (you also said when you hit the brakes. The retard if turned on will work every time you press down on the brakes)  (Try turning the retard off and see if problem disappears)
2.Air hose connection (very low probability)
3.one of the rear height controls (I had to replace both of them on my 1997 U295 at different times. This was over 6 -12 years ago. )

Re: Air System Problem

Reply #6 –
If the alarm goes off I would think that the gage is working now it may be off a pound or 2. On that age coach the gage and pressure switch are manual reading not like the later ones that are electronic.

Mike

Re: Air System Problem

Reply #7 –
Try changing your air governor. They can stick and cause unusual problems.

Re: Air System Problem

Reply #8 –
Something about this "problem" does not make sense.  Air pressure flows from the air compressor to the dryer, then to the wet tank, and then to the front and rear brake tanks.  The engine driven air compressor cuts in and out based on signals from the D2 governor.  The D2 governor gets its pressure signal from the wet tank.  If the rear brake tank is actually dropping down to 60 psi that would mean the wet tank is simultaneously also at 60 psi.  But the wet tank should never get to 60 psi because the air compressor would cut in at around 100-105 psi in that tank.  I would say the problem is with the mechanical pressure gauge, except for the fact that the low air pressure alarm also goes off when the gauge drops to 60 psi.  This would seem to indicate that the pressure in the rear brake tank IS actually dropping to 60 psi.  Like I said, doesn't make sense.

I would like to know exactly where in the air system the low air pressure sensors tie into the air lines.  Are they tied into the same air hoses that feed pressure to the front and rear brake tank pressure gauges?

Re: Air System Problem

Reply #9 –
Quote from: Chuck & Jeannie –
Something about this "problem" does not make sense.  Air pressure flows from the air compressor to the dryer, then to the wet tank, and then to the front and rear brake tanks.  The engine driven air compressor cuts in and out based on signals from the D2 governor.  The D2 governor gets its pressure signal from the wet tank.  If the rear brake tank is actually dropping down to 60 psi that would mean the wet tank is simultaneously also at 60 psi.  But the wet tank should never get to 60 psi because the air compressor would cut in at around 100-105 psi in that tank.  I would say the problem is with the mechanical pressure gauge, except for the fact that the low air pressure alarm also goes off when the gauge drops to 60 psi.  This would seem to indicate that the pressure in the rear brake tank IS actually dropping to 60 psi.  Like I said, doesn't make sense.

Odds of buzzer pressure switch and gauge both being bad is slim.  If the leak is big enough it could overpower the compressor and keep the wet and rear tank at 60 psi. The problem comes to play is the front tank would not be getting air since the wet tank is at 60 psi. Unless the check valve going into the rear tank has a restriction and leak is greater then the restriction at the check valve but not so big that it is letting the wet tank and front tank fill and compressor shut down. This is the only way the wet tank could get the front tank back up to 125 psi.  If he tests that I have requested could help solve this mystery.


Re: Air System Problem

Reply #10 –
Richard,

Has this ever happened when you have been setting still and were able to get out and listen for air leaks?  Or have you herd any air leak after you stop at a campsite after killing the engine? 

Mike
 

Re: Air System Problem

Reply #11 –
The heavy rear end of the coach is always moving up and down and side to side for lots of good reasons, changing each of the ride height valves.  Every movement up dumps airbag air, and every movement down causes the front air tanks to refill an airbag, causing air tank losses even without touching the brakes. So that could be the cause of your "no discernible reason" comment.

Cheap to replace the air governor, (good to carry a spare) and maybe a ride height valve is leaking or out of adjustment as they are known to do. You could disable both rear ride height valves to diagnose by disconnecting the rods and setting the valve to neutral so it will not let air in or out from both rear airbags.