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Old 10-14-2022, 02:29 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, 03:06 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, 03:26 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, 05:41 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, 09:35 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, 08:45 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, 11:17 AM   #7
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Originally Posted by Meathead View Post
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!

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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