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Old 10-14-2022, 01:29 PM   #1
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How does an air dryer work??

Iím pretty sure my air dryer works. I drove 4K miles this summer and the air tanks built up pressure and purged as they should, I think.

Hereís what happened. I was under my bus today and after bumping things around a bit I noticed a hanging wire.
Itís one of the two wires to the air dryer. I believe itís an A9.

I couldnít immediately find the other end of the wire to hook it back up. I found that the other wire is bolted to the frame. The hanging wire has a butt connector on it but or doesnít look like its been connected in a long time. In fact it hadnít been crimped.

So. Can an air dryer work(somewhat) without the wires connected. Can it purge on its own without the aid of electricity?

I was planning on servicing the dryer this winter anyway so I guess it just jumped to the top of the list.

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Old 10-14-2022, 02:06 PM   #2
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Dryers are all mechanical. The only electric in it would be a heater in the base.

That wire isn't necessary, as I've found the heater isn't really needed unless it's near or below 0 out. A lot of the new ones we find the fuse missing for them.
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Old 10-14-2022, 02:26 PM   #3
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Dryers are just a cartridge in-line from the compressor to the tanks. The cartridges capture moisture to keep the tanks "dry" (although its still a good idea to drain any moisture from your tanks once a day). We replace our cartridges once a year right before winter. Once the cartridges get "used up" in freezing temps moisture can get trapped in the lines and freeze the air lines solid so the tanks won't build air.

The wires you're seeing might be for a sensor.

Heads up though if you ever get a frozen air line, stay clear of the line if you open it up when you're trying to thaw and clear it out. An ice bullet coming out of there at 120psi can do some serious damage!!
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Old 10-14-2022, 04:41 PM   #4
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the heater on my red bus air dryer quit working.. I was good on the road but once the bus sat all night and then I started it next day, it was hard to get pressure till the warmer somewhat compressed air made its way.. the purge valve had stuck partially open from ice.. temperatures were low teens.



the heater on the dryer in my DEV bus does work and I was in minus 25f weather this past new years and never had an issue.. I really didnt plan on being in minus 25 weather but it happened so im glad the heater is connected and I had checked to make sure it worked in the fall
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Old 10-14-2022, 08:35 PM   #5
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how a drier works

Have you ever played a trumpet or french horn... trombone? your breath has water vapor in it and as the breath goes around the tubes is cools off and you get water condensate inside the instrument.... My trumpet and bass both had "spit valve" where you could dump out the water that condensed inside the tubing.

The drier on the bus works on a similar principle. I dont know this, I am part guessing... the outlet of the compressor dumps into a tank and that tanks has spit valves on the bottom so you can dump water once a day. The drier has an inlet line up high and out let line up high... the drier is like a pot with an automatic dump valve at the bottom... you know water collects at the bottom.
so every time the pressure hits 120 psi dump valve opens and spits out a little mist of water and some air... compressor goes and then the pressure gets to 120 psi and dumps some air with a little water vapor... and on and on and on.


I hope that if I have this wrong Cadillac will correct me... but this is the basic version of how it works.

you should open valves on the air tanks at the end of the day of driving... keep the water out and also will allow to inspect for oil - oil will indicate a problem with the air compressor. Gives you time to plan for repairs, before you HAVE to replace it. A lot of water -- more than usual -- may indicate problems with a coolant leak inside the air compressor. Again this gives you warning to problems with the compressor.

Did I get this mostly correct?

If you are asking this question, it is time you spend three or four hours on youtube university learning about air brake systems and the parts in the system and how to take care of air brake systems.

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Old 10-15-2022, 07:45 AM   #6
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Thanks yíall. This southern boy wonít be anywhere near cold weather but Iíll fix it anyway. Ha. I kept the snow chains. Ya never know!

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Old 10-15-2022, 10:17 AM   #7
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Thanks y’all. This southern boy won’t be anywhere near cold weather but I’ll fix it anyway. Ha. I kept the snow chains. Ya never know!

[emoji3522]Dave
IIRC your bus should have a Bendix AD-9 Drier. If you remove the whole dryer and remove the cover you will find a cartridge screwed down into the base. It's recommended air driers are serviced annually. The AD-9 especially it's recommended to replace the purge valve (which is held in by 3 tiny bolts on the bottom) at the same time. The purge valve is the part that has the heater in it as well.

Shop around for the cartridge, the prices can vary wildly and some have a core charge. There is a large spring inside the cartridge, don't try and open it.

You should have a vice and a large strap wrench ready, the cartridge can be a fight to get off if someone didn't lube the o-rings. And they apparently didn't lube the o-rings on my last bus.
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Old 04-13-2023, 04:41 PM   #8
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Ok. Iím back. I changed my ad9 cartridge (Napa) and everything went smoothly. I drove about 40 miles. When I got back I left the bus running to check transmission fluid.

Then I heard the air dryer. It went PSSSSSsssssssssss. Different from the usual PSSST. Oh Crap!

Then I hit the brakes and every time I did I could hear PSST coming from the general direction of the air dryer.

So I go to the skoolie.net search engine and found my own thread from a few months ago. [emoji846]. I didnít plan on asking about this but since Iím hereÖ.

Iím thinking purge valve. I didnít check it when I had it on the bench as I was directed to do. It wouldnít be the first time something else broke when I fixed something. [emoji1743]

My pressure builds up nicely to 120psi.

[emoji3522]Dave.
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Old 04-13-2023, 05:02 PM   #9
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A faulty purge valve wouldn't cause an extended drain like you're hearing. Your problem likely lies in the check valve on the outlet of the drier. If you want to diagnose it, you can remove one of the plugs in the wet tank and watch how the pressure acts during purge. The check valve should prevent air from back flowing into the dryer, maintaining pressure in the tank.

Or you could have an air leak somewhere that's causing frequent compressor cycling's/dryer purges.

Don't get me wrong, you should have rebuilt the purge valve. But unless you have a constant leak from it, it's likely not your issue.
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Old 04-13-2023, 05:06 PM   #10
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Oh, btw, the dryer/compressor operation should be largely unaffected by brake actuation, aside from maintaining system pressure.

Be sure that you're hearing air from the purge valve and not from a relay valve, treadle valve, double check or somewhere else.

Might help to have a friend work the pedal while you crawl underneath listening around. And as always, be sure to chock the wheels.
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Old 04-13-2023, 05:51 PM   #11
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When my purge valve went, I couldn't even get up to 50 PSI. Just heard the PSSSSSSSSS from underneath forever.
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Old 04-13-2023, 08:15 PM   #12
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Quote:
Originally Posted by Booyah45828 View Post
A faulty purge valve wouldn't cause an extended drain like you're hearing. Your problem likely lies in the check valve on the outlet of the drier. If you want to diagnose it, you can remove one of the plugs in the wet tank and watch how the pressure acts during purge. The check valve should prevent air from back flowing into the dryer, maintaining pressure in the tank.

Or you could have an air leak somewhere that's causing frequent compressor cycling's/dryer purges.

Don't get me wrong, you should have rebuilt the purge valve. But unless you have a constant leak from it, it's likely not your issue.

Okay. Lots to do.

Test check valve

New or rebuild purge valve.

Pump brakes and listen.

The whole dryer assembly was on the bench for 10 days while Napa shipped the cartridge. I shoulda maybe plugged up those hanging hoses. [emoji848]

I also found this:
Click image for larger version

Name:	IMG_2371.jpg
Views:	10
Size:	110.2 KB
ID:	71482
Worth a look.

Thanks yíall

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Old 04-13-2023, 09:53 PM   #13
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Definitely need to crawl back under there and check for the source of the noise before changing parts. Should be reasonably easy to track down the source since it's audible walking around the bus.

One thing I would especially check is the brake chambers because it sounds like your noise gets worse when you step on the brakes. Because your drier is most likely mounted near the rear axle you may actually be hearing a leak from the service side of a brake chamber in addition to a leak from the drier.
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Old 04-14-2023, 09:54 AM   #14
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do not forget the governor {bendix d-2} controls the purge valve but i dont think its your issue as when the purge valve opens it dumps all the air in your drier so it can expel the moisture that collect on the cooling fins inside. purge valves stay open until the governor releases the pressure to your unloader valve and the pump cycle starts again. after the volume in your air drier is expelled it gets quiet which is misinterpreted as closing. it dont close til the compressor starts pumping again. so a extended discharge cycle has to get extra air from somewhere so as booya said replace that check valve and quit wasting time
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Old 04-14-2023, 10:44 AM   #15
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A faulty purge valve wouldn't cause an extended drain like you're hearing. Your problem likely lies in the check valve on the outlet of the drier. If you want to diagnose it, you can remove one of the plugs in the wet tank and watch how the pressure acts during purge. The check valve should prevent air from back flowing into the dryer, maintaining pressure in the tank.
Is it possible to not have a wet tank? I have traced to copper line back toward my engine. It turns into braided line and goes to the top of the air compressor.
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Old 04-14-2023, 10:49 AM   #16
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Definitely need to crawl back under there and check for the source of the noise before changing parts. Should be reasonably easy to track down the source since it's audible walking around the bus.
Hey Truth. We have the same bus from same VA county. What does your wet tank look like. Itís between the dryer and the compressor. Right?

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Old 04-14-2023, 11:07 AM   #17
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wet tank is after the drier and is requires by the dot in case the drier fails. its possible that it was removed (illegally) because it rusted out and not replaced. its also a smaller one however on all late model trucks it was in the end of the primary (one tank with a divider wall inside) and could be recognized as a large line came out of the front and went into the back of the same tank. this will have (required by law
) another check valve (sometimes built into a elbow) to prevent backflow. so you might not see a separate tank however if you get a good picture of your tank i can identify which one is your wet tank. one last clue it will be the tank with a drain valve on each end
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Old 04-14-2023, 11:35 AM   #18
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Ok gotcha. I mis understood/misread what booya said. I know where my tanks are with valves on them. I have two. I had just let air out of those valves right before I started hearing the weird purge sound. Maybe that proves the point about the check valve. Btw I never get any water when I turn the valves on the tanks.

Iím not understanding how releasing pressure beyond the dryer tests the check valve. A check valve prevents back flow correct?

Iím just try to understand. Iíll watch one of the Bendix videos. Maybe save us all some time. Thanks for the help!

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Old 04-14-2023, 01:14 PM   #19
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Compressor-Dryer-Wet tank-primary/secondary.

Air flows in that order. There are check valves keeping air backflowing from wet tank into dryer, and primary tank into wet, and secondary into wet. Your "tanks" might just be one tank with individual compartments, that's pretty common to find on buses actually.

Governor controls purge and compressor on/off. Governor reads pressure from wet tank.

Here is the scenario.
Governor sees pressure at wet tank hit 120(thereabouts). Governor valve opens, flowing air/pressure to purge valve and compressor unloader valve. Compressor stops pumping air, and purge valve opens, discharging air in dryer cartridge backwards out the valve. That should be a small volume of air, giving you a quick pssst during the purge cycle.

If you have a long psssssstt, that shows the check valve in the dryer isn't working, and is allowing air from the wet tank back through the cartridge and out the purge valve, until wet tank pressure drops to 90(thereabouts), where the governor valve closes, removing air/pressure from the compressor unloader and the purge valve. Which starts the compressor pumping again and the purge valve to close and stop exhausting.

If you install a pressure gauge on the wet tank, you can watch this decrease/increase cycle and see that it coincides with the purge cycle. Once system is up to pressure for the governor to open, that pressure should maintain, unless it's used or leaked by the system somewhere.
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Old 04-14-2023, 04:35 PM   #20
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Ok guys, I guess this is my week to learn about air brakes. Does everything break at once or do you just notice em all at once?? I feel fortunate to have found all of this bf my trips this summer.

Ok. So I ordered a check valve and Iím going to rebuild the purge valve.

Checking the brake chambers, I found at least 2( one front. one rear) hissing and making the REEONK! sound. I read that they a may be good for 10OK miles and Iím at 195K so itís prolly time.

I will be replacing all 4 of them. Any tips on purchasing and installation.
Do they just bolt on? One brand better than another?

Thanks for all the help. I remember back in the day, working on Vdubs, all I had was a friend, friends uncle, and a JC Whitney catalog! [emoji849][emoji20][emoji23][emoji85].

Oh, and a half a$$ set of tools!

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