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Dryer purging repeatedly
Hi guys. I recently noticed that my air dryer purges every couple of minutes after building pressure without using any brakes. That is, pressure remains at high cut-out but the air dryer will purge a little every two minutes or so. Pressure is a bit high as well (around 130 psi).
Is this a governor or air dryer issue? I was planning on replacing the governor but thought I’d ask.
Gerald
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23 Replies
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It’s normal. No need to worry.
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I don’t think a purge every few minutes is normal. The purge valve assembly on the dryer may need a rebuild.
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you may also have excessive air leaks .
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Guys, Listen to your coaches. Every coach I had did what Gerald described.
I have a brand new dryer on mine and it purges periodically just like all the others that preceded it.
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x2 what Tom said.
But, I once had a pretty big leak in the aux system, then the aux pressure would drop to 90 psi in a minute or two, the engine driven compressor would rebuild the pressure and the dryer would purge.I fixed the leak.
Does your bus have an aux pressure gauge? Mine didn’t so I installed one.
I’ve never measured the time between purges with the bus just sitting there, but I will soon.
1999 XLV X-6872, liberty #440 -
Approximately same year coach. At 245K during pre purchase the system was purging quite often. The easiest fix was to replace govenor …very accessible. Turned out to be not so lucky and not the problem. We had to put a new unloader valve repair kit in. Their are two versions of this kit. One just the top end..and one for complete system. More complicated to do this repair as they had to removed manifold. Its a half days worth of work for an expert mechanic.
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3X what Truk said
AL
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Jon,
Obviously if it purges and the aux gauge is cycling it’s a leak, if it’s pissing air every few minutes (that’s what I understood) there’s something wrong with the dryer. I had to rebuilt one of mine due to the purge valve not sealing. Not saying that is the problem, but sounds logical.
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I listen to my coach. I have listened to every coach. I know my coach and watch my gauges. I cannot remember my first coach, but I remember the last two and after the dryer sneezes when it reaches the pressure set point it will sneeze periodically following that.
My aux gauge and brake gauges are at 120 PSI when that happens.
My 97 had the Wabco dryer and the governor on the compressor. My current coach has the Haldex dryer with an internal pressure governor.
I replaced the Haldex dryer last summer and it purges in the same manner as the original that was replaced.
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Just another something to check…Soon after purchase of our coach, I had the same symptom on our maiden voyage and after soaping the Haldex dryer itself, I found 6 small allen head bolts on the underside of the dryer were not tight. Those 6 small bolts secured a part of the dryer to itself and had a seam. I tightened them and has not leaked since, nor does it cycle repeatedly as you describe.
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Here is Bendix troubleshooting guide:
https://www.bendix.com/media/documents/products_1/compressorsgovernors_1/troubleshootingguide.pdf
2025 Marathon H3 / 4th Prevost
21 Ram 1500 EcoDiesel 4x4 -
Thanks for all the feedback. I really like Jon’s explanation but I don’t remember the bus doing this in the past. I’m at high idle, stationary with parking brakes on and the bus will purge every two-three minutes.
I’m NOT losing air pressure between the purges. At least, both brake reservoir gauges stay at high cut-out pressure.
Could I have a leak in the wet tank or dryer that I cannot see on the brake gauges? I understand there is a check valve in the dryer that prevents wet tank air from feeding back into the dryer after purge. That could be the culprit.
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My wife listens to our coach too. Every other day she says she “hears something” that I can’t hear. Then she wants me to call Jeff at ETLC. I stopped doing that a few years ago.
2000 Featherlite H3-45
2024 Jeep Grand Cherokee
Leesburg, FL -
Hi! It sounds like you have a leak in the signal line going from the compressor to the dryer. Hope this helps. Peter in Maine.
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While I have been reading some very valid comments and reasons regarding this purging, perhaps someone can help me understand what I see as an enigma.
It is suggested regular purging is an indication of a leak somewhere in the pneumatic system. Potentially the signal line because that would explain why the gauges do not show a pressure drop.
But what has me confused is the dryer purges every time the pressure drops and is restored. Once the pressure reaches the cut out point there is a purge. Makes sense. But here is where I cannot get my brain to understand all the causes proposed. My coach air gauges, brake and accessory do not cycle often which indicates I have no gross leaks while going down the road. But they do cycle. The cycling is mostly limited to the accessory gauge and over a significant period of time, far greater than the occasional purge upon start up cycle so if the purge is an indication the pressure has satisfied the governor it would suggest I would never note the cycling of my compressor when driving.
But it does cycle because over time I see the accessory pressure gradually drop and then it is restored to 120. I do not have a leak free accessory system, but I do suspect most of the pressure loss while driving is the suspension working, and exhausting and adding air.
So if the purging when idling after start up is due to a malfunction or leak, and the purging is initiated at the end of a compressor cycle then why do I see normal air pressure cycling on my gauges? If purging every few minutes is restoring system pressure I should never see cycling on my gauges.
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