Air Dryer Question

Glennman · October 23, 2025, 10:54:41 PM

I have a series of green air lines fitted together below the driver's seat that were apparently at one point connected to an air dryer there. On the wall next to them where they are connected together, there is a label showing the direction of the valves for on/off. However, there are no valves, only fittings where it appears that someone took the dryer out and bypassed it by connecting the hoses together. What would that air dryer look like and is it the kind with a filter assembly? I would like to install another one and get it back to OEM. Any thoughts? (2002 MCI D4500, 12.7 Detroit)

luvrbus · October 24, 2025, 05:46:53 PM

It will be a AD-9 unless it has been changed over to a Haldex pictures are all over the internet, it will have a in line from the compressor ,a out line to the tanks and 1/4 inch control line from the compressor governor they are simple ,they are easy to rebuild unless the cover is rotten with rust none have valves ,you can buy a aftermarket dyer for 200 bucks they work good I have installed dozens with no problems, The Bendix will be around 600 bucks

Utahclaimjumper · October 25, 2025, 12:40:05 PM

Haldex is the only way to go and the rebuild kits are available and cheap..>>>Dan ( If you have a floor in that compartment,, you will have to create a hole for the dryer to purge)

Glennman · October 25, 2025, 05:40:54 PM

The "compartment" that the lines are in is under the driver's seat where there is an air tank and the windshield washer fluid. The problem is that since the one that (apparently) was there is gone, so I don't even know what it is supposed to look like. I wouldn't even know how to hook one up.

I'll do some more research on the matter. There is a tour bus outfit down the road that will probably let me take a look at one of theirs. They still have a few D4500s in their yard.

Glennman · December 28, 2025, 10:02:57 PM

Ok, so while under the front of the bus working on replacing the shocks and a bearing seal, I found the air dryer! I thought it was under the driver's seat where the washer fluid tank is because there is a 3-way ball valve there with a diagram showing which way to turn it the valve to bypass the air dryer. This valve has been reinstalled in a haphazard way such that it appeared that maybe the air dryer USED to be there, so I thought it had been removed or something.

At any rate, the 3-way valve bypasses the air dryer such that I suppose you can get by for a few miles if the dryer malfunctioned. All that being said, I removed the air dryer and the steel cover is badly rusted and it looks like it hasn't been serviced in a long time. I'm planning to get a rebuild kit for it, but they do not include the steel cover.

Today's question is this: Does the cavity between the desiccant filter and the steel cover work under pressure? If not, I think I can clean the rust off the cover and reuse it, as it does not appear to be rusted all the way through, and the inside is actually pretty clean. If it's under pressure, I will need to find another cover. Maybe that won't be too difficult, but I'm considering my options. Thanks everyone!

luvrbus · December 29, 2025, 04:45:34 AM

The cover is under pressure, a kit will not come with the cover.Buy you a replacement (after market ) they are good I been using them for several years without any problems, you can buy from Amazon or Fleetpride with or without mounting brackets or heaters for $150 bucks or less ,the Bendix or Haldex are made in China also and can run in to 4 or $500.00 range rebuilt.FWIW the location of the dryer on a MCI is not a good location for the dryers with spin on filters lol BTDT ,I would not worry with a dryer over 20 years old myself, you have it off that part is done

Glennman · December 29, 2025, 09:46:46 PM

I cleaned up the cover and it's not as bad as I thought. I took a hammer to it and there are no weak or thin spots, so I will paint it and it's good to go. It's about 1/8" thick steel. I have a rebuild kit on the way, but now that I've reread your earlier responses, I should have just ordered an aftermarket one as you suggested. I need to go to the local Fleet Pride tomorrow anyway, so I'll look into that. Also, I figured out the sequence of the 3-way bypass ball valve. It's actually a handy thing to have if there's ever a problem on the road. The air dryer is between the front wheels, so it's not something you're going to service without jacking up the bus and removing a wheel for access. I wish it was in a better place! Thanks again!

luvrbus · December 30, 2025, 02:55:14 AM

Quote from: Glennman on December 29, 2025, 09:46:46 PM
I cleaned up the cover and it's not as bad as I thought. I took a hammer to it and there are no weak or thin spots, so I will paint it and it's good to go. It's about 1/8" thick steel. I have a rebuild kit on the way, but now that I've reread your earlier responses, I should have just ordered an aftermarket one as you suggested. I need to go to the local Fleet Pride tomorrow anyway, so I'll look into that. Also, I figured out the sequence of the 3-way bypass ball valve. It's actually a handy thing to have if there's ever a problem on the road. The air dryer is between the front wheels, so it's not something you're going to service without jacking up the bus and removing a wheel for access. I wish it was in a better place! Thanks again!

At a MCI training class the instructor told the dryer was installed there so the air would be cooler and the dryer would not need servicing as often,LOL he was right because no one services the dryer on MCI,I remove the driver's side wheel to get to those dryers and air tank in that location that always rots out

Glennman · December 30, 2025, 09:43:07 AM

Quote from: luvrbus on December 30, 2025, 02:55:14 AM

At a MCI training class the instructor told the dryer was installed there so the air would be cooler and the dryer would not need servicing as often,LOL he was right because no one services the dryer on MCI,I remove the driver's side wheel to get to those dryers and air tank in that location that always rots out

That's actually funny luvrbus!

I have another question for you, if you will. The so-called "check valve" in the AD-9 is a cone shaped fitting with about a 3/8 inch hole in the top. The main air line attaches directly to it. When I pulled the valve out, there are no moving parts or anything that would indicate that it is a check valve at all. How in the world does it function as a check valve?

azdieselman · December 30, 2025, 11:33:55 AM

There is a small rubber disc at the tapered end of the fitting. It can break off. Are you having air loss issues?

Iceni John · December 30, 2025, 02:52:44 PM

I replaced my AD-9 a year ago because the old one wasn't purging correctly. I bought a genuine Bendix made in Mexico for less than $200 because its box was missing (BFD!), but then realized it had no internal check valve, so I installed a 3/4" check valve made in Italy that works well. It was cheaper to do this than to buy a Bendix dessicant kit and a purge valve assembly to fix my old AD-9. I now have a dryer that purges with a PSSSTT instead of with a long drawn-out PPSSSSSSSSSSSSSSSSTT. Yeah! I also had my TuFlo 700 compressor rebuilt while my engine was out - it was passing a lot of oil.

John

Glennman · December 30, 2025, 04:30:43 PM

Quote from: azdieselman on December 30, 2025, 11:33:55 AM
There is a small rubber disc at the tapered end of the fitting. It can break off. Are you having air loss issues?

I'm not having any air loss issues. I'm only working on it because it looks like it hasn't been serviced in quite a few years. Now that I have it off, I'm wanting to do a complete maintenance on it. The manual shows something that looks like a spring and some kind of a rubber cap or something, like what you describe. It could be that someone removed it in the past.

azdieselman · December 30, 2025, 07:15:33 PM

Most likely it "rotted" away.
A few years ago, I purchased some new Bendix AD 9's from a local heavy truck dealer for less than I could buy the components to rebuild them.
It might be worth a quick search, 2 bolts and a band clamp is the fastest repair!

Glennman · December 30, 2025, 09:00:23 PM

Quote from: azdieselman on December 30, 2025, 07:15:33 PM
Most likely it "rotted" away.
A few years ago, I purchased some new Bendix AD 9's from a local heavy truck dealer for less than I could buy the components to rebuild them.
It might be worth a quick search, 2 bolts and a band clamp is the fastest repair!

After looking at dozens of AD-9's, not all are equal. I have yet to find one with 3/4" ports AND a 24 volt heater. NFI has what appears to be one for $1300. In looking at the internal air passageways, I'm thinking I can pick up one with 1/2" ports and install 1/2" to 3/4" reducers without hindering anything. Also, I performed all the tests on the heater element and found that it is not working. I can pick up an AD-9 at Fleet Pride for around $200, take out the 12V heater and install a 24V one for $50, then install the 1/2" to 3/4" adapters, and good to go!

If the check valve is only a rubber or plastic ball or whatever, I can see how it would disintegrate after a while. I've watched a bunch of You Tube videos on the subject, but not one covers how the check valve is supposed to function. All the videos are on the purge valve function. Thanks!

luvrbus · December 31, 2025, 03:31:53 AM

Those heaters don't heat till the temperature is below 38 degrees then it is automatic makes one hard to test, some people use the 12 v tap on the MCI and new wire and don't change the heaters, I change the heaters, I bought 3 off Find it Truck Parts a few years ago for less than 50 bucks