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Old 07-26-2018, 12:34 AM   #1
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Loosing air pressure only when engine is running

I took my coach out today to fuel up and noticed my air pressure drops constantly when the engine is running. When I shut down it holds solid. Any idea what takes or requires so much air only when running ??
Or what could be leaking, it is going to be hard to listen for a air leak under the coach when running the engine.
Thanks for any input or ideas

TC
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Old 07-26-2018, 12:59 AM   #2
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Parking brake off, or on?
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Old 07-26-2018, 01:11 AM   #3
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Hmmm may be a combination of several things..
The check valve at the wet tank (this is the tank that the air from the compressor goes to)(rare thing to happen normally see a failed check valve due to desiccant contamination caused by the air dryer not being serviced in a timely manner) Causes the tank air to back flow out of tank and back to the dryer. (under $20 to replace)

D-2 air governor bleeding back (under $50)

Air dryer needs service)
cracked fitting or line from the air dryer to wet tank
cracked or split line or fitting from the air compressor to air dryer.
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Old 07-26-2018, 06:35 AM   #4
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Sounds like the inversion valve. With the coach parked build air pressure up chock wheels realese the park brake and listen for a leak by left back dual if leaking there its probaly the inversion valve.
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Old 07-26-2018, 12:00 PM   #5
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This is really strange.

I assume, from your description, your gauges only drop to zero when engine is at an idle, and you can build up normal air pressure with engine running at a higher rpm.

With the engine running (compressor pumping) the air supply must be able to overcome any small to mid-size leaks otherwise you could not build any pressure. And if the leak is large enough that your gauges are dropping while the compressor is pumping (at idle speed) you must have a pretty large leak. So I would think you could build up air pressure, then shut off the engine, and get to back of coach fast enough to hear the leak (it must be a pretty big/loud leak).

The only difference between air system operation with engine off and the engine running is the "supply circuit" of the air system. Past the wet tank (supply reservoir) the air system would function the same whether engine is running or not.

So the problem must be in the Compressor, Air Dryer, Governor, or connecting hoses.

I would bet you will find the leak in the discharge hose from compressor to dryer - and you have a leaking check valve at the Dryer.

EDIT - thinking about this more, if your dash gauges are dropping then you're losing air out of the "primary" and "secondary" air tanks too. Those have check valves to prevent air lose out the direction of the supply circuit. Sop either those are leaking too, or there is some other information missing from your description.
Are you releasing the parking brake or stepping on brake valve or doing anything else while the air is dropping?
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Old 07-26-2018, 12:21 PM   #6
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Quote:
Originally Posted by 96 Wideglide View Post
Parking brake off, or on?

Wideglide Is on to something here, Please answer the question?

After reading the post again it sounds like you DO have a brake can diaphragm leaking when the brakes are released.
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Old 07-26-2018, 01:00 PM   #7
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If he is seeing the pressure drop, he is either releasing the parking brake and has a leak in that system or has some air powered accessories, that are actavated with the key. Air powered step, door seal or anything like that.

There is a turbo control air valve modulator that could leak, while the engine is running, but its for ISX and ISM engines.Click image for larger version

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Old 07-26-2018, 01:33 PM   #8
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Thinking this over...The air leak has to be "down river" from the front or rear air tank, otherwise the air pressure gauges would not register the air loss...due to the check valves at each tank.......
There is not a gauge for the wet tank, just the front and rear tanks..
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Old 07-26-2018, 03:28 PM   #9
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Parking brake is off, this happens while driving.
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Old 07-26-2018, 03:47 PM   #10
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Hi guys thanks for all the replies, here is more info. My coach is powered with the Cummins 400 ISL, the compressor pumps fine, while driving down the road no brakes being pressed I see my dash gauge ( both needles) slowly drop to under 90-100 psi, compressor kicks as required, pumps up to 130 and I hear the drier “blow off”, compressor shuts off. Gauge slowly starts dropping ant the process repeats its self. All within 2-3 min. In the refrigeration trade we call this “short cycling” not a normal function. I reciently acquired this coach and performed all th maintenance my self, I did replace the drier cartridge and do not remember this happening even after replacing the cartridge. Seems like it just started.
I will check the dump solenoids and the previously mentioned inversion valve (just have to find it )
I do not have air powered step cover or doors, I do have air powered exhaust brake, PAC style.
Thanks again for all the replies and input ������
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Old 07-26-2018, 03:59 PM   #11
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One quick test. With air pressure at full, shut off engine. You said air pressure does not drop.

Now release the parking brake - (air pressure will drop some but should remain steady). Does air continue to drop?
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Old 07-26-2018, 04:14 PM   #12
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The driving down the road comment makes me think he may have a leak in one of his air bag ride height valves. Does the air leak down with the coach running and stationary or only when going down the road. Find a nice level spot, and release the parking brake and see if it still runs down without moving or pressing on the brakes.

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Old 07-26-2018, 04:19 PM   #13
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Quote:
Originally Posted by tc914-8 View Post
Hi guys thanks for all the replies, here is more info. My coach is powered with the Cummins 400 ISL, the compressor pumps fine, while driving down the road no brakes being pressed I see my dash gauge ( both needles) slowly drop to under 90-100 psi, compressor kicks as required, pumps up to 130 and I hear the drier “blow off”, compressor shuts off. Gauge slowly starts dropping ant the process repeats its self. All within 2-3 min. In the refrigeration trade we call this “short cycling” not a normal function. I reciently acquired this coach and performed all th maintenance my self, I did replace the drier cartridge and do not remember this happening even after replacing the cartridge. Seems like it just started.
I will check the dump solenoids and the previously mentioned inversion valve (just have to find it )
I do not have air powered step cover or doors, I do have air powered exhaust brake, PAC style.
Thanks again for all the replies and input ������
Did you do the check i sugested above if you did you should be able to here the leak.
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Old 07-26-2018, 07:04 PM   #14
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Hi Walt, I just finished work, I’ll go check the options now, where do I find the inversion valve ??
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