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SBU - Air dryer

47K views 117 replies 26 participants last post by  Mog8575 
#1 ·
I don't have an air dryer on my U1250 - do I need one?
 
#4 · (Edited)
I think it is a good thing for a mog; especially SBU/MBU...even more expensive air valves and controls than earlier models. This increases your chance of succeeding and lowers your chance of failure. The only fluid I like to put in air system is isopropyl alcohol. Do a search on this forum: air dryer
 
#6 ·
It's a really good idea to have a heated air dryer with the big heavy filter/water absorber. Bill Caid told me horror stories of older Mogs without them that have to do expensive replacements of hard to find and hard to diagnose air valves, and have air tanks full of oily/water/filth emulsion. The oil comes from the compressor, and is filtered by the big Wabco filter item. the water is from condensation and is heated and removed by the system.

Charlie
 
#8 · (Edited)
Products & systems - WABCO
Check out all their air products, but...
Looks expensive. You could try a junkyard specializing in heavy trucks. Sometimes there are retail shops specializing in heavy truck components that include air system parts, basically a heavy truck auto parts store.
You could also ask one of the German Mog part dealers such as Unimog, 4x4, Spezialfahrzeuge,Geländefahrzeuge, militärfahrzeuge und v.m.-MEREX Autovertrieb GmbH if this part was ever a factory option on you model.
Maybe other readers who actually own older SBUs could suggest where to look so that laying down $$$ on an air protection system won't be "shutting the barn door after the horse left the barn" (I'm sure there's a similar Australian expression).

Charlie
 
#9 ·
Hey guys, I'm in the process of buttoning up some odds and ends on my partial restoration job on the 1550. I am now at the point of having a new air line made between the compressor and the air dump valve thing-a-ma-jiggy, and am looking into whether or not I should install an air dryer. I had no idea they were so complicated to set-up! After looking around the web, and here, it looks like the bendix AD-9 is the simplest setup and reasonably priced. I am a bit confused as to why the unit couldn't be plumbed between the compressor and the air dump valve. I understand that the purge cycle is powered by the exhaust from the dump valve , and so how would one make a connection to this port as it is smooth and was not intended to have a connection made to it. Would a new unit be required, or is the ticket one of the wabco units that has the dump valve built in and replaces the mog dump valve. The thing that scares me about the wabco is the whole notion of having an other tank for "re-gen". Whats makes that system worth the hassle?
My biggest quandary is why the dryer can't go between the compressor and the dump valve. Is a dryer worth the hassle when I live in the arid S.W.?

Thanks in advance!
 
#10 · (Edited)
Oh I forgot, here is the diagram for the Bendix unit. As you can see all of these units are designed around air brake systems that are layed out a bit different from the mog. On mine, 437, dump valve is before the reservoir, and obviously the unloaded air goes to atmosphere, not back to the compressor. This dumped air would need to be routed back to the con port of the bendix unit, and then it would dump the air.
 

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#13 · (Edited)
There should be a schematic around for an air dryer in your 437. Does your system run at 18 bar or is it the lower pressure? 11 bar? or something like that. That is important because the dryer needs to be matched. There are very few air dryers that are rated for 18 bar. I studied different versions before I put one in my 1700 and got a lot of help here on the forum. I tried to plumb it the way they intended it and I used the MB spec'd part. We get temps down to -20 C here and I have never had a problem and this dryer does not have the heater installed. I do however have a alcohol injector plumbed in after the dryer.
I need to re-plumb the dryer on my 437 2400 however; I think it was done wrong (alcohol injector BEFORE the air dryer...that does not make sense to me) and it is pissing air. It looks a little more complicated than the earlier versions; the exhausts are all remoted to little mufflers/ filters? so it is a bit more of a bugger to trace the lines. If I recall (and I am doing this from memory) You need to deal with 4 ports on the factory style dryer. I think the port number designations are in previous posts.

Coming from the compressor you have the line IN to the dryer
Coming out from the AIR DRYER you have the line OUT to the regulator and rest of system.
There is also one line to the 'Regeneration Tank' (if you do not have the tank, it can be ordered or contrived.)
The other OUT is the exhaust out the bottom of the AIR DRYER (on my newer model plumbed to a remote 'muffler')

Maybe your schematic would plumb it differently and I have forgotten something. Someone correct me if I have erred.

HTH rostige
 
#14 ·
One key part you need to remember is on the newer systems the air compressor actuall stops pumping air, so the PSST of air is actually the dumping of the line + signal that we have enough air and stop pumping so the systems with the line returning to the pump create this. FYI unimogs.. atleast the older ones say 1990 and older allways pump air it just where does the air go in the tank or to the atmosphere...

The debate about dryer or no dryer is going to be a $$$ Q... I would love a dryer but its not worth the extra $$ so I'll just use the alcohol injection for protection.

Unless you have failing parts or antisipate being in really cold areas and don't have an alsohol injection I would just keep what you have, its worked for how long now?
 
#23 ·
There's one benefit of not having a drier that I haven't seen mentioned. That is, being able to flush out components with a light lubricant occasionally. I don't know how many times I've cleared 406 mog air system problems by injecting a little lube oil (like Marvel Mystery Oil) ahead of a device, even the compressor. Of course MB recommends flushing the diff locks occasionally, but the same technique will keep crud out of valves and regulators, too.

Hey Dave, where do you buy that alshole oil?
LOL

Bob



...chop...
Unless you have failing parts or antisipate being in really cold areas and don't have an alsohol injection I would just keep what you have, its worked for how long now?
 
#15 ·
My 94 2400 looks like it has no control ie. always pumping and unloading through regulator at max pressure. I never had luck rebuilding air parts (at least in this corrosive climate) since I had a 406 right into various SBU. I have had 100 percent success with replacing defective modules/ valves. For approx. $500.00-$1000.00 that gets the dryer and then add on plumbing if your mog does not have it already. Each major air component on the SBU brake lists at around $400.00 (uncanny that)...give or take a few hun. I guess it depends on how long you plan to keep your mog and personal preference. It is a pay them now or pay them later deal...I think of it as preventive maintenance and a good investment.

rostige
 
#16 · (Edited)
Hey guys, sorry for the delay, I've been a bit busy this week.

My truck is an 18 bar system, and the compressor is always on.

Rostige, I've been seeing re manufactured units on ebay in the $150 range. There is a Bendix AD9 unit that, supposedly, doesn't require an additional tank. Unfortunatley, it's range ends at 150psi. Do you know what units are capable of handling 18bar? I bet those get up in price huh.

After examining my regulator a little closer this weekend, it appears that the discharge isnt completely smooth after all. It has two ribs that suggest is could except a hose with some type of clamp and then route it to the dryer's re-gen port.

Back to one of my original questions, can the dryer go between the compressor and the regulator?

I think I will proceed to have the air line built with a union at a point where I think a dryer unit could mount, then I can simply insert what ever unit I find down the road. I do need to know if anyone feels it shouldn't be plumbed between the compressor and the regulator.

Thanks guys -James
 
#17 ·
the only issue you may have with plumbing in a air dryer is that you may never get a setup to actuall purdge its self. the all in one devices, or the ones with extra lines get input from these extra lines telling it to purdge, or in the case of the all in one it purdges its self by simply unloading the compressor. you may find one that will purdge when the PSI drops all of the sudden when your current unloader valve unloads now but I dobt it. + you will also dry all the air while the engine is running seeing as its allways moving air. minor issue but still going to happen.

One idea you may look into ... a little expensive but they are cool is an all in one device. Unloader, dryer, 4 way protection valve all in one device. This would really simplify a few things and may help to save space...

Products & systems - WABCO
 
#20 ·
OK, I see an issue with mounting a unit like the Bendix AP9 between the compressor and the regulator.
Basically the AP9 harnesses the air normally dumped to atmosphere on the UNIMOG as it's purge pressure. If the AP9 is plumbed before the regulator, it will always have pressure flowing through it, not allowing the purge valve to open. I imagine the result would be a blow out as the UNIMOG regulator would force it's dumped air into a valve that is stuck shut w/i the dryer.

So, that answers one major question. It's not to say is wouldn't work well after the regulator though. Except it isn't rated to 18bar.

So... I can proceed with my air line with out interference, and I'll work something between the regulator and the tank later.
 
#21 ·
Do not use an AD-9

Been there, done that. Do not use the Bendix AD-9 as the pressure in Unimogs will destroy the relief valve. I had it changed twice and still had problems all the time until I installed a WABCO unit. Use a Wabco unit with the valve properly adjusted.

Saludos
 
#24 ·
With regard to the above post:
The lack of an air dryer/filter is precisely why crud builds up in earlier Unimog air systems to begin with, The "crud" is a mixture of oil that blows by the compressor's rings that oxidizes in the presence of high air pressure (= high O2 concentration), and condensation.
A proper air treatment system will prevent this.
One doesn't hear about modern trucks with many many engine hours having air system problems, but it's a common complaint of owners of earlier Unimogs.
Injecting Marvel Mystery Oil into the compressor seems like a way to measure MMO's cetane number, at the risk of a big explosion.

Charlie
 
#25 ·
All good points Charlie. However, if a Diesel engine could be made to compression ignite at 150 psi then there'd be no need for such high compression in a Diesel engine?? Anyway, no big bangs yet.

I totally agree that if a truck had a dryer from when it was new, that'd be the best. I'm suggesting solutions for when that's not the case.

My experience has been with old mogs with 30 years with no dryer. The crud I find inside air lines is mostly rust from condensation. That's what trashes o-rings and keeps valves from closing. A little oil in there will carry the particles to a low point and trap them there.

Bob
 
#26 · (Edited)
Good counter points. Although the air storage tank is ~10 bar, what is the compression ratio of the compressor? Remember that a piston is adiabatic compression, pressure may go up X 10 (probably more, that's why there's a relief valve), but temp also goes up:
T1/T0 = (comp ratio)**(gamma-1.0), gamma = adiabatic constant of diatomic gas = 1.4, so ratio of temps (in absolute) = comp ratio**0.4. If comp ratio is 20:1, then T1/T0 ~ 3.1 = 600 C. enough for self ignition.
At comp ratio of 10:1 (I think it's higher), T1/T0 = 2.5 ~ 480 C.


Charlie
 
#29 ·
Compression ignition (diesel engines) results from the temperature increase that happens when compressing air but the compresion ratio has to be high enough so the temprature goes to or above the ignition temperature of diesel, that is exactly why a diesel engine will not fire in very cold weather. The elements of fire are: combustible material, oxydizer (oxygen) and a heat source (hot compressed air).

Saludos
 
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