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Old 04-20-2019, 07:29 AM   #1
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Air brake air dryer was purging a lot then stopped.

I've been driving my 2003 Thomas back and forth to work sometimes to work on it a little at a time. Yesterday on the way home I kept hearing an air sound pshhh, every 20 seconds or so while driving. When I got home I realized it was coming from the air dryer purge valve. I googled and youtubed for a while and had a few things I would check when I got to work today. It did the same thing for about 5 minutes on the way to work then it stopped doing it and seemed to work normal. I guess I can eliminate a leaky air line or some other major damage to something. The pressure gauge never dropped. I don't know if I have a real problem or just forget about it?
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Old 04-20-2019, 10:24 AM   #2
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Sounds to me like you were driving during humid weather and it had a lot of moisture to purge. I'd consider getting a replacement cartridge for the air dryer if it hasn't been replaced recently.


Air dryers do fail and the main cause is neglecting to change the cartridge. The main symptom is the purge valves stick open and you won't build/maintain air pressure, and you get all the moisture in your air tanks.
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Old 04-21-2019, 08:49 AM   #3
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Your symptom can be caused by two things. One is the governor could have failed, the other is the check valve between the dryer and the wet tank.

Your air system works like this.
1. Compressor builds air psi up to around 125psi
2. The governor, which senses system air psi from the wet tank, signals to the compressor to stop pumping and signals the dryer to purge.
3. The dryer purge valve opens, backflushing the desicant in the dryer with dry air.
4. That backflush dries the desicant and pushes the moisture out of the discharge port, preparing the desicant for the next cycle.
5. Through normal use or leaks, the air psi in the wet tank falls below 100psi.
6. The governor tells the compressor to start pumping and the dryer purge valve to close.
7. You start back over at step 1

*The air for the purge comes internally from the dryer itself. After purge, the air psi in the dryer should be 0, while the check valve maintains wet tank psi at 125. If that check valve fails, air from the wet tank will leak backwards though the dryer until the governor kicks the compressor on, sealing off the purge valve, thereby stopping the leak.

You can diagnose this by inspecting the discharge port while the engine is running. If the check valve has failed, there will be airflow after the initial woosh from the purge. Normally, there should only be air flow for 1 or 2 seconds, basically the woosh and that's it.
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Old 04-21-2019, 08:51 AM   #4
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The frequency of the dryer purge isn't humidity dependent. The system has no way of sensing air humidity like your automatic drains present on a household compressor. It purges only when the compressor kicks off.
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Old 04-21-2019, 08:55 AM   #5
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Quote:
Originally Posted by Booyah45828 View Post
Your symptom can be caused by two things. One is the governor could have failed, the other is the check valve between the dryer and the wet tank.

Your air system works like this.
1. Compressor builds air psi up to around 125psi
2. The governor, which senses system air psi from the wet tank, signals to the compressor to stop pumping and signals the dryer to purge.
3. The dryer purge valve opens, backflushing the desicant in the dryer with dry air.
4. That backflush dries the desicant and pushes the moisture out of the discharge port, preparing the desicant for the next cycle.
5. Through normal use or leaks, the air psi in the wet tank falls below 100psi.
6. The governor tells the compressor to start pumping and the dryer purge valve to close.
7. You start back over at step 1

*The air for the purge comes internally from the dryer itself. After purge, the air psi in the dryer should be 0, while the check valve maintains wet tank psi at 125. If that check valve fails, air from the wet tank will leak backwards though the dryer until the governor kicks the compressor on, sealing off the purge valve, thereby stopping the leak.

You can diagnose this by inspecting the discharge port while the engine is running. If the check valve has failed, there will be airflow after the initial woosh from the purge. Normally, there should only be air flow for 1 or 2 seconds, basically the woosh and that's it.
This is a super informative post. Man, I really need to figure out a way to save posts like this for future reference!
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Old 04-21-2019, 09:54 AM   #6
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Quote:
Originally Posted by Booyah45828 View Post
The frequency of the dryer purge isn't humidity dependent. The system has no way of sensing air humidity like your automatic drains present on a household compressor. It purges only when the compressor kicks off.
That's what I was thinking from my understanding of how the system works. To have an intermittent problem like I had, my guess would be the check valve may have got hung up for a while?? Without knowing the last time the air dryer was serviced, like the first reply stated it probably would be good to service it Im guessing. Can the filter be replaced without taking the whole air dryer off?
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Old 04-21-2019, 12:30 PM   #7
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it can also be caused by the air dryer sense line being either leaky or in the wrong port of the governor..



I had that issue where the sense line that tells the dryer to go to purge cycle was leaking.. another symptom was even though my air gauges were high, the air alarm would occasionally beep for a moment during the purge cycle...



-Christopher
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Old 04-21-2019, 01:44 PM   #8
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Quote:
Originally Posted by aridgedell View Post
That's what I was thinking from my understanding of how the system works. To have an intermittent problem like I had, my guess would be the check valve may have got hung up for a while?? Without knowing the last time the air dryer was serviced, like the first reply stated it probably would be good to service it Im guessing. Can the filter be replaced without taking the whole air dryer off?
Good thread. I have the same intermittent problem. I don't suspect any leaks downstream from the air drier as my system will almost hold full pressure sitting overnight. It may be a check valve or a leak between the compressor-wet tank-air drier. Let us know what you find out. If I figure mine out I'll let you know.

Ted
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