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Topic: Frequent air dryer purge (Read 656 times) previous topic - next topic

Frequent air dryer purge

I had an issue with my air dryer purging way too often, especially under load.  Too often, as in, every 4 seconds or so.  The air dryer / dessicant was serviced this fall and the symptoms showed up about a month later.

Took it to my local Cummins certified shop, they had fits figuring it out.  Replaced governor and purge valve and that fixed "about 70%" of the problem, but it was still purging too often.  Disassembled, cleaned, and serviced the air dryer, and still not completely fixed.  They had several calls with Haldex and didn't get a good suggestion for a fix.

They ran a line to the front so they could monitor the valve from the front of the coach while watching pressures and strangely, the problem went away.  So... the "fix" was to run a line about 3 ft off the air dryer and hang the valve off of there.  No one has a good explanation for why this works, but I wanted to pass it on - both for discussion and in case it helps someone in the future.

Attached pics show (3) red hose on bottom of air dryer (driver side),  (2) hose running behind engine, (1) purge valve tied up to curb side.
Steve and Cassi
2007 Foretravel Nimbus 42'

Re: Frequent air dryer purge

Reply #1 –
Well, if it works...then that's a win.  :thumbsup:

That's a new one on me.  What model air dryer?
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Frequent air dryer purge

Reply #2 –
I guess it's a win, but I don't know if I trust it.

Haldex DA34100 - had to bust out the borescope to read it!
Steve and Cassi
2007 Foretravel Nimbus 42'

Re: Frequent air dryer purge

Reply #3 –
I can't see the bottom of the dryer in your photos, but I don't think that is the purge valve that is remote mounted on the red tubing.  That looks more like a pressure relief valve to me.  The purge valve is the black thingy with two electric wires (that power a heating element).  If I'm right, then the "solution" to your rapid purging problem is even more curious.  The safety valve doesn't really have anything to do with the purge function.  It's just there in case the dryer sees excessive pressure for some reason (like a frozen purge valve).

https://www.haldex.com/en/na/air-dryer/air-dryers/haldex-newremanufactured/purest-air-dryer/da34100-403aa8cd/

1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Frequent air dryer purge

Reply #4 –
Good eye, Chuck.  I never looked at the plumbing and just assumed one purge was sufficient.  Am I right in assuming that the standard function is for the purge to activate at a lower pressure than the safety valve?  If so, the safety should pretty much never activate unless there's a problem with the purge valve...?

Here's pics showing the safety with red hose and the purge on the other side.
Steve and Cassi
2007 Foretravel Nimbus 42'

Re: Frequent air dryer purge

Reply #5 –
Your air dryer should purge when the engine air compressor cuts out - typically at about 120-130 psi.  The safety relief valve on the air dryer should open at about 175 psi.  You have another safety relief valve on your wet tank that should open at about 150 psi, so it is unlikely that you would ever see the air dryer safety valve open.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Frequent air dryer purge

Reply #6 –
There's a thread from about 6 years ago that brings up a similar sounding problem that seems to be related to the wrong safety valve being installed - 175 psi vs 200 psi.

Haldex calls out 175, but for some reason, FT put on 200 psi safety valves.  Looking at my invoice, they put on a 175 psi valve (KN31527).  I'll put a call into FT tomorrow if I can't find anything in my manuals. 

Still seems to be some smoke and mirrors has to why my fix seems to work, hopefully the real fix is this simple. 
Steve and Cassi
2007 Foretravel Nimbus 42'

Re: Frequent air dryer purge

Reply #7 –
Last year my purge valve started releasing every few seconds, I had rebuilt it 2 years prior and really didn't think it would last too long. this time replaced with OE rebuilt, looked new to me and has solved the problem. I don't think new are available. Jim.
Jim and Dona
2002 U295 36'
Lily, blue healer. Our fury person
Manx mid travel Ecotec turbo
2022 Bronco badlands Sasquatch
Trail 90 and 110cc

Re: Frequent air dryer purge

Reply #8 –
The purge chamber is directly connected to the governor  line. This line should not have any pressure in it until the govt piston slides over and tips the unloader valve. At the same time the piston uncovers the purge line port and pressurized the purge chamber in the dryer . The purge chamber is ported to the top of the purge valve and pushes the valve open .
For the dryer to purge before the compressor runs loader valve opens , there must be a leak between the dryer air chamber and the purge chamber. This leak is most likely to be one of the seals on the purge piston  .
To help find the issue , disconnect the purge line and verify that it's only under pressure when the governor unloads. If it has pressure before that , the governor is faulty .
If the dryer purges without the hose connection , the dryer is faulty .

Re: Frequent air dryer purge

Reply #9 –
Good info, Mike.  I'm not sure it applies to the problem I had though.  This was the safety valve popping off, not the purge - I learned about the presence of more than one source of the air pop off sound during the course of this thread.

Specifically, my governor was replaced, new safety valve, and air dryer was disassembled, cleaned, and a new desiccant cartridge installed.
Steve and Cassi
2007 Foretravel Nimbus 42'

Re: Frequent air dryer purge

Reply #10 –
SOLUTION - I heard back from Foretravel.  Per James, the safety valve should be 200 psi.  This was the solution to my exact issue of the frequent opening of the safety valve.  One work around they had found was to put a length of hose between the dryer and the 175 psi valve - just like my shop found by accident!  They were surprised that 3 ft did the job - when they installed a couple of these, they coiled it (so who knows if that was 6 ft or 26 ft).

No one seems to have a good understanding of why this works, just that it does.  Seems to me that the hose would shrink and contract with air pressure changes that helps soften the peaks and valleys of the pressure changes (ie, less like a saw tooth profile and more like a sine wave). 

Not that any of that matters, just happy to know I should be in good shape. Hopefully this helps someone else down the road!
Steve and Cassi
2007 Foretravel Nimbus 42'

Re: Frequent air dryer purge

Reply #11 –
Makes absolutely no sense to me.

Your air compressor cuts out at 120-130 PSI (perhaps 140 if D2 is adjusted a little high).  There is a safety valve on the wet tank that should open at 150 psi.  So somebody explain to me how the air dryer safety valve, rated to open at 175 psi, can ever see enough pressure to open.

The "fix" is installing a 200 psi safety valve, because although the pressure in the dryer CAN get above 175 psi, seems it's not likely it will reach 200 psi.  Why not?

Somehow, inside the air dryer, air pressure is being magically "amplified" from 130 psi to 175+ psi.  AMAZING!  The scientists at DARPA should be notified!

1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Frequent air dryer purge

Reply #12 –
My safety valve opens around 140
Peter and Frieda Morin
1999 36ft. U320 Foretravel
Build # 5436
1998 Suzuki Sidekick Sport

Re: Frequent air dryer purge

Reply #13 –
I went back and read the old thread referenced in Reply #6.  I participated in that discussion 6 years ago.  I didn't like the way that thread ended because we never found a logical answer to the air dryer safety valve question.

Pure Air Plus Air Dryer

The best answer anybody on the Forum could come up with was a scenario where some kind of weird compound blockage in the air dryer would allow the pressure to rise inside the dryer without a commensurate rise in wet tank pressure.  Since the wet tank pressure never reaches cut-out pressure, the air compressor would just keep on pumping until the 175 psi safety valve in the dryer finally opened.

Problems with this scenario.  If a runaway air compressor could pressurize the air dryer to 175 psi, why would installing a 200 psi safety valve solve the problem?  Why wouldn't the runaway compressor just keep on pumping up the dryer until it got to 200 psi and opened that safety valve?  Also, if the air compressor on a coach was regularly pressurizing the air dryer to 175+ psi don't you think we would notice other problems in either the compressor or in the dryer.  Changing the safety valve on the dryer from 175 psi to 200 psi would not "cure" the blockage inside the dryer.

Besides, in this case Steve's dryer is a freshly rebuilt unit, so a massive internal blockage is a pretty remote possibility.

Nope - sorry - I didn't buy the explanation last time, and I don't buy it this time.

1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Frequent air dryer purge

Reply #14 –
Could be that the compressor pumps with a piston and the air pressure bumps up enough to pop the safety