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Old 03-20-2018, 02:02 PM   #1
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Air Purge Every 30 Seconds

I'm hearing a burst of air from my coach about every 30 seconds when the coach is idling, it sounds like it's coming from the rear of the coach. The air pressure in both front and rear tanks stays constant at 120 PSI so it's not leaking down causing the air compressor to cycle. The air pressure has bled off the front and rear tanks in 2-3 days since I've owned the coach, but the constant cycling is a new development. I rebuilt the air dryer last fall, but I only used the DQ6026 General Service Kit which didn't include the purge valve, but did include the relief valve. What should I check first?
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Old 03-20-2018, 02:07 PM   #2
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I was going to suggest the air dryer when I began reading your post, but I see you rebuilt it. Still, if it wasn't a complete rebuild, that may be the issue. I know that my MH had the same issue before I bought it. The PO spent the bucks to replace the air dryer ($$$) and all is well. Can't say for sure that's your problem, but it's worth a check, IMO.
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Old 03-20-2018, 02:16 PM   #3
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Could be the air governor is bad.
Try disconnecting the "Unloader Line" at the Air Dryer or Governor end, whichever is easier, and see if there is a continuous air pressure (leaking) from the governor. If yes, then replace it.
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Old 03-20-2018, 02:23 PM   #4
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There is a third tank that does not have a gauge. Thats the tank that is loosing pressure and cycling your compressor.

It may be a section of one of your tanks. Usually the longer tank is two tanks in one. Look for a weld seam around it.

First check for a drain valve on it. It could be open and dropping your pressure.

If this started after your working on it, check for a bad check valve, either near the air dryer or on the tank, that the dryer line connects to. A bad check valve will let the air reverse out of the tank and blow out the air dryer port.

Next place to check is the small 1/4 " air line that comes from that first tank, called the wet tank, and goes to the governor.

The governor is mounted on or near the compressor. If mounted on the compressor there will only be two lines. From the governor, another 1/4 " line runs to the air dryer purge valve.

If either of those lines are leaking or your purge valve is passing air, the air tank will drain, causing the compressor to cycle.

If the governor is mounted remotely, there will be another 1/4 " line from it to the compressor head. Again, if leaking, it will cause the cycling.
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Old 03-20-2018, 08:04 PM   #5
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Roger,

I have a vague idea of how the air system works. Half my front tank is wet and the other half is dry. The air compressor supplies air to the wet tank and the air dryer dries the air for the front and rear dry tank. The air for the brakes and suspension comes from those two tanks. The governor monitors the wet tank to control the compressor?

The dryer should only purge the system one time when the motor home is operating properly and there is no demand from the suspension or air brakes?

From where you've told me to look I think I understand the system a little better so I'll take a look at it later this week. I ordered a lower service kit for the air dryer--I probably should have done that when I serviced the top side.
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Old 03-20-2018, 08:05 PM   #6
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Quote:
Originally Posted by CountryB View Post
Could be the air governor is bad.
Try disconnecting the "Unloader Line" at the Air Dryer or Governor end, whichever is easier, and see if there is a continuous air pressure (leaking) from the governor. If yes, then replace it.
Thanks Mike, that's a very useful picture.
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Old 03-23-2018, 05:26 AM   #7
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Denis check your inversion valve for the leak , it should be on the driver rear past the rear duals leave coach in N with engine running and chock wheels.
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Old 03-23-2018, 12:08 PM   #8
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Quote:
Originally Posted by walt2137 View Post
Denis check your inversion valve for the leak , it should be on the driver rear past the rear duals leave coach in N with engine running and chock wheels.
Thanks Walt, I didn't find an issue there.
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Old 03-23-2018, 12:11 PM   #9
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Okay, I finished the other projects and had time to look at the air problem today. The wet tank doesn't have any leaks from the air drain valve. I found that the air dryer purge valve stays open a long time after the initial purge. It stays open at least 15-20 seconds, then closes. I'm thinking its the purge valve, but could something be causing it to stay open that long or is just likely bad?
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Old 03-23-2018, 12:46 PM   #10
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The purge valve opens from a signal from the governor. That same signal stops the compressor from pumping.

Both will remain open until the wet tank drops to 90 psi.

Is air coming out of the purge valve, the whole time ?

If it is, you need to follow the air dryer large discharge line to a check valve. It may be right on the air dryer or right on the tank.

That check valve is probably bad and letting the air back thru it. They can often be opened and cleaned.
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Old 03-23-2018, 12:49 PM   #11
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Look for it in either spot.
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Old 03-23-2018, 12:50 PM   #12
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Quote:
Originally Posted by twinboat View Post
The purge valve opens from a signal from the governor. That same signal stops the compressor from pumping.

Both will remain open until the wet tank drops to 90 psi.

Is air coming out of the purge valve, the whole time ?

If it is, you need to follow the air dryer large discharge line to a check valve. It may be right on the air dryer or right on the tank.

That check valve is probably bad and letting the air back thru it. They can often be opened and cleaned.
It comes out for about 20 seconds. Starts out blowing hard and fast and tapers off until the valve closes.
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Old 03-23-2018, 01:36 PM   #13
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That long is enough to drain the wet tank down to 90 psi. That in turn starts the compressor pumping again.

Once the dryer purges for 3 seconds, it should have no pressure in it. The purge valve remains open to let the moisture drip out.

Find the check valve and open it up. You may find a small price of rubber holding it open.
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Old 03-23-2018, 01:37 PM   #14
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Picture ofcheck valve, looks like a long fitting.
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