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)
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
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)
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.
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!
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
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!
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
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?
There is a small rubber disc at the tapered end of the fitting. It can break off. Are you having air loss issues?
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
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.
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!
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!
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
Quote from: luvrbus on 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
What I did was follow the manual's test procedures. I put the device in the freezer and tested the ohms, then I heated it to 100 degrees and tested it again. There was no difference. According to the manual, it's supposed to go from around 6.5 to over 1000 ohms. Mine had no difference at all, as it stayed at 6.5 (the warm amount) and didn't change at all when I froze it.
Ok, so I retested the heater with a real test! I pulled the heater element and snap disc out of the purge valve housing. It's amazing that they got all that in there! I checked the snap disc by heating it up and then cooling it down. You can actually hear it engage. No continuity when warm, then after it snaps, it has continuity (after pulling it from the freezer). Then I connected 24 volts to the little element. It has a ceramic covering about the size of a pencil, is about 2 inches long, and gets hot when applying 24 volts.
I already ordered a rebuild kit from NFI. If it includes everything I need, I'll go with that. However, if it doesn't include a new check valve (I viewed a check valve at Fleet Pride today in it has a spring loaded device inside it, unlike mine that has nothing in it), then I'll pick up Fleet Pride's 12v AD-9, install my heater in it, install 1/2" to 3/4" port adapters and be done with it.
Just to close this out for now in case anyone cares, I finally got the air dryer put back together. I ordered a new check valve besides the rest of the plunger kit. The plunger kit came with the heater element and the plunger already assembled and installed in its housing. That part was pretty cut and dried. It came with a filter too.
My main question regarding the check valve was answered. I ordered a new one, and lo and behold, it had an actual check valve inside it (if you recall, the old one was only a brass fitting with nothing in it. It must have come out during a service at some point). So, now it's all assembled, but I have yet to check it for leaks. Every time I climb under the bus I find something new. Climbing out yesterday after installing the dryer, I noticed some cracks in the brake linings that I didn't notice earlier. Now the bus will be on blocks for another 2 weeks while I install new shoes (relined) and brand-new drums.
You should check the MCI air schematics for your serial number not all applications used the check valve on the discharge port with the AD-9, some check valves were installed at the tank,Van's H-40 Prevost AD-9 just had a fitting on the AD-9 discharge port.I don't know what difference it would make but I used the fitting and left the check valve on the tank
Every time I climb under the bus I find something new.
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[/size]Isn't that so true! These old buses are never 100% ship shape. There is always something that is old and needs maintenance.
Quote from: luvrbus on January 20, 2026, 05:46:03 AM
You should check the MCI air schematics for your serial number not all applications used the check valve on the discharge port with the AD-9, some check valves were installed at the tank,Van's H-40 Prevost AD-9 just had a fitting on the AD-9 discharge port.I don't know what difference it would make but I used the fitting and left the check valve on the tank
Thanks luvrbus, I'll check for that next week when I get back under the bus, since I still have it on blocks anyway. The local tour bus outfit ran a copy of a manual off on the AD-9, but it was from a '98 DL3. It included a breakdown of the AD-9 components with an actual check valve in it. I'll check the tank for any kind of a check valve on it. I'm assuming you are speaking of the (2) tanks that are near the dryer (?).
Clifford sent you a PM for the wife.
Dave
Quote from: Dave5Cs on January 22, 2026, 08:01:55 PM
Clifford sent you a PM for the wife.
Dave
I don't see a pending PM anywhere (?)
Dave sent the PM to me
I finished intalling the air dryer, as stated in a previous post. However, based on luvrbus's comment on the possibility that the check valve on the bottom of the air dryer may not have been installed (or functional. The fitting was there but it was hollow allowing air to pass both ways) due to a check valve at the tank instead. I did more research and also finally got back under the bus today, and indeed, there is an inline style check valve between the two tanks. My research tells me that one of the two tanks adjacent to the air dryer between the front wheels is supposed to blow off with the air dryer. This certainly does sound right. When the air dryer pops, there is quite a bit of air that comes out, more than what the dryer holds, so it appears to be designed to blow off the "wet tank" (one of the two) with its purge.
All that being said, I'm going to put the old hollow fitting back in, since it appears that it was designed to work that way. Nothing like creating a problem where there wasn't one to start with. It appeared to function just fine before I decided to take it all apart. At least now I have a new filter and purge valve. I feel pretty good about that, and I did receive an education working on it all! Thanks everyone for all your help. If I have any problems when I fire it up, I'll let everyone know. That won't be 'till probably next weekend, as I'm in the process of installing new brake drums and shoes in the meantime.
Quote from: Glennman on January 26, 2026, 12:36:07 PM
I finished intalling the air dryer, as stated in a previous post. However, based on luvrbus's comment on the possibility that the check valve on the bottom of the air dryer may not have been installed (or functional. The fitting was there but it was hollow allowing air to pass both ways) due to a check valve at the tank instead. I did more research and also finally got back under the bus today, and indeed, there is an inline style check valve between the two tanks. My research tells me that one of the two tanks adjacent to the air dryer between the front wheels is supposed to blow off with the air dryer. This certainly does sound right. When the air dryer pops, there is quite a bit of air that comes out, more than what the dryer holds, so it appears to be designed to blow off the "wet tank" (one of the two) with its purge.
All that being said, I'm going to put the old hollow fitting back in, since it appears that it was designed to work that way. Nothing like creating a problem where there wasn't one to start with. It appeared to function just fine before I decided to take it all apart. At least now I have a new filter and purge valve. I feel pretty good about that, and I did receive an education working on it all! Thanks everyone for all your help. If I have any problems when I fire it up, I'll let everyone know. That won't be 'till probably next weekend, as I'm in the process of installing new brake drums and shoes in the meantime.
I kind of figured that was the reason because on the later buses the ping(wet) tank was moved closer to the air dryer with a check valve, Bendix lists a parts number for the pass through valve
Exactly luvrbus! Thanks to you I looked deeper, located the pass through check valve and now I've removed the new check valve and installed the old fitting back where it belongs. Thank you!