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Low Air Pressure?


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Pulled into new spot for about another week. Spent two days with wife in emergency (all ok). Today decided to set up. Fired up engine to build up pressure to extend slides. Pressure was at 93 lbs and built up to 123 before slowly dropping to 111 lbs. Holds steady there but will not raise and no popping of at dryer. Have 40 miles up into mountains to my home base. Where should I be looking for this issue? Would it be safe to finish to destination in a week if issue is not solved?

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Just a couple of thoughts.

Compressor usually has a cut in pressure. 111 may be too high for the compressor to engage. Try hitting your brakes ancouple of times. Mine cuts in around 75-80 and cuts out around 115. At what pressure do you normally hear the air dryer chuff? Mine will get to about 120 rear and 115 front then chuff and drop to 115/110 as the air dryer is regenerating. Other than not hearing the chuff, your account could be normal operation. There is no air control board, it is all controlled by air pressure, springs and diaphragms (on my rig at least). I would pump the brakes to lower the air pressure to see if it will charge up above 111.

If you should be at higher pressure, my money is on the compressor governor (sets the purge/chuff pressure)or the air dryer preforms the purge.

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1 minute ago, KevinH said:

Last time I had a similar problem it was the air governor

I’ll check to see where it’s at on my 360 Cummins. Maybe give it a thump?

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One other thought, ony rig I need 65 psi to work the brakes (make them release) as I recall. If you can maintain pressure above the amount needed to use your brakes and the compressor will pump air into the system, then you should probably be ok to proceed, but I would get it looked at asap. If your pressure drops too low, your brakes will engage (and without the air pressure to release them)and you will need to be towed.

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2 minutes ago, JohnC3 said:

Just a couple of thoughts.

Compressor usually has a cut in pressure. 111 may be too high for the compressor to engage. Try hitting your brakes ancouple of times. Mine cuts in around 75-80 and cuts out around 115. At what pressure do you normally hear the air dryer chuff? Mine will get to about 120 rear and 115 front then chuff and drop to 115/110 as the air dryer is regenerating. Other than not hearing the chuff, your account could be normal operation. There is no air control board, it is all controlled by air pressure, springs and diaphragms (on my rig at least). I would pump the brakes to lower the air pressure to see if it will charge up above 111.

If you should be at higher pressure, my money is on the compressor governor (sets the purge/chuff pressure)or the air dryer preforms the purge.

Mine purge’s around 122-123 and settles down to 117-119 while driving. Now that I’m supported by jacks I’ll drop the pressure below 65 at which low alarm activates and see if she will rise to the occasion. Hmmmm!!

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2 hours ago, Tim-AZ said:

When was the last time you had the air dryer serviced?

Tim

9 mo ago

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Gary, attached is a good pre-trip check list for air brakes.  See how your system performs especially under step 3.  This should help you decide if the coach is safe to drive.

 

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If your system normally unloads at 123psi, I wouldn't expect to see it build pressure again till it drops to about 100psi.

You will usually lose about 10psi right after unloading (regen cycle), then you will usually have another 15psi for braking / suspension etc, before the governor will close your compressor valves.

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Posted (edited)
6 minutes ago, 96 EVO said:

If your system normally unloads at 123psi, I wouldn't expect to see it build pressure again till it drops to about 100psi.

You will usually lose about 10psi right after unloading (regen cycle), then you will usually have another 15psi for braking / suspension etc, before the governor will close your compressor valves.

Normally after I start engine and no matter  what psi is at, it unloads at  122-123psi and then drop to 112-113 then build up to 118-119. I brought rpm’s up but it would not raise pressure at all.

Edited by Gary M
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5 minutes ago, Gary M said:

Normally after I start engine and no matter  what psi is at, it unloads at  122-123psi and while rim doing checks it will unload several times more by the time I get in to drive. I assume that’s normal!!!

I'm guessing your using air filling your air bags during this time?

Those of us with air leveling are a little different. We start our engine and the suspension stays where it is till we press 'travel mode', so, if I start my engine and my air pressure is above 100psi, it doesn't build and purge.

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46 minutes ago, Frank McElroy said:

Gary, attached is a good pre-trip check list for air brakes.  See how your system performs especially under step 3.  This should help you decide if the coach is safe to drive.

 

Thanks Frank👍

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Based on what you described, it sounds like your system is functioning normally.  Running through a brake pre-trip checklist will let you know for sure that the system is operating the way it should.

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38 minutes ago, Frank McElroy said:

Based on what you described, it sounds like your system is functioning normally.  Running through a brake pre-trip checklist will let you know for sure that the system is operating the way it should.

Yes but why won’t it build pressure from 111 with rpm’s up just sitting. Unless my memory is off it should build up.

1 hour ago, 96 EVO said:

I'm guessing your using air filling your air bags during this time?

Those of us with air leveling are a little different. We start our engine and the suspension stays where it is till we press 'travel mode', so, if I start my engine and my air pressure is above 100psi, it doesn't build and purge.

Well unless my memory has failed, when ever I raised rpm my pressure would build to max and then purge. 

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52 minutes ago, Gary M said:

Yes but why won’t it build pressure from 111 with rpm’s up just sitting. Unless my memory is off it should build up.

Well unless my memory has failed, when ever I raised rpm my pressure would build to max and then purge. 

The likely reason why the compressor didn't cut in at 111 is that you are still a few psi above the cut in pressure.  Try pumping the brakes once or twice and I think you will hear the compressor start and build pressure.

Engine RPM does not control the compressor cut in and cut out pressures.  The governor does.  Higher engine RPM will cause the compressor to build pressure faster.  On my coach the cut in pressure is 108 and the cut out pressures is 134.

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On 6/30/2022 at 9:30 PM, Frank McElroy said:

The likely reason why the compressor didn't cut in at 111 is that you are still a few psi above the cut in pressure.  Try pumping the brakes once or twice and I think you will hear the compressor start and build pressure.

Engine RPM does not control the compressor cut in and cut out pressures.  The governor does.  Higher engine RPM will cause the compressor to build pressure faster.  On my coach the cut in pressure is 108 and the cut out pressures is 134.

Low Air Pressure? 09 Knight 38PKQ.

Update: Dropped pressure down to 60 lbs and then raised rpm. Pressure built up to 122 and dryer purged. Let engine idle and pressure slowly dropped to 111 again and held there. My next test will be the air brake pre tip test.

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On 7/1/2022 at 9:59 AM, 96 EVO said:

Yep, I'm with the others!

You're not going to know if you have a problem till you get your tank pressure below about 100psi.

Dropped pressure down to 60 lbs and then raised rpm. Pressure built up to 122 and dryer purged. Let engine idle and pressure slowly dropped to 111 again and held there. My next test will be the air brake pre tip test.

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