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Thread: FLU air compressor, what actuates compressor? (and other air system questions)

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    Default FLU air compressor, what actuates compressor? (and other air system questions)

    I know on a normal truck with air brakes the governor cuts the compressor 'on' and 'off', but how does the air system work on a FLU? It seems the compressor will just pump all the time, so where does the excess pressure go? I understand there is a unloader valve on the passenger side framerail that I guess acts as a sort of an air drier and spits out excess moisture, but does it constantly bleed off pressure? I have a FLU that was not building pressure so I have been going through it and am really debating removing much of the stuff that isn't needed such as front/rear gladhands and all associated hoses (such as the manual trailer hand valve, and the one in between seats). I say it is unneeded because why in the world would I attempt to pull a trailer with this thing...

    I am also thinking about making the 4wd a manual lever, and making the diff locks just a ball valve. It is ridiculous how over engineered these things are, cool yes, but way too complicated.

    Thank you for any assistance.

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    I'm no expert on the air system (either), but yes, the puny compressor runs all the time. Once the max pressure is reached, that unloader valve spits out the excess.
    As you've noticed, there are many, many places for air leaks to exist, and it doesn't take much of a leak (or a few very minor ones) for the compressor not to keep up.
    The drain valves on the tanks, for example, are a notorious leak source.

    Before you remove what is most likely innocent parts of the system, use the front glad hand to pressurize the system making it easier to listen for leaks. And you never know when you may want to tow a trailer in the future, so why not leave that part of the system intact?
    Why attempt to pull a trailer with a SEE? One reason would be that they're rated to pull 17,000 lbs., if I remember correctly. Also, there's nothing wrong with having the weight of the SEE as a safety net, compared to a 3/4-ton pickup pulling 17,000 lbs. For going down the highway it would be a different story, but that's not where a SEE belongs anyway.

    While I haven't used a SEE as a yard goat yet, that's only because of a lack of time. With half the turning radius of my tractor, the SEE and a con gear will be my preferred method of moving 5th wheel trailers - until I need to back up.

    Making a lever for 4WD and a dash mounted locker valve has been on my list for a while. I will also put a valve in the line that pressurizes the axles when engaging 4WD so that I can choose when they're pressurized or not. At least in my case, engaging 4WD doesn't equal fording, and fording doesn't equal using 4WD.

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    I also don't understand why that if the 'unloader' or 'regulator' on the passenger frame rail 'regulates' the pressure, why is there (what appears to be) regulators on each tank on the drivers side rail? I have looked through the manuals and can't really figure out what any of this stuff does. I have already replumbed (and will eventually replace) the regulator on the passenger side. I used flexible air brake line that I had on hand, I got rid of the 'evaporator' as they call it that sucks in the alcohol to prevent freezing. I would rather just pull a line off a quick-connect and add alcohol myself as needed (rarely). I removed the rearmost tank as it was badly dented and will replumb everything off the single large tank and hopefully simplify things in the process. Since I will never use this thing to tow a trailer (as in ANY trailer) I have no need for the extra air reserve that may be needed for trailer brakes. I may leave the 4wd switch if I can get it functioning, it won't rotate at all currently (does it require air pressure to even be able to turn?)

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    Quote Originally Posted by NewEra View Post
    I also don't understand why that if the 'unloader' or 'regulator' on the passenger frame rail 'regulates' the pressure, why is there (what appears to be) regulators on each tank on the drivers side rail? I have looked through the manuals and can't really figure out what any of this stuff does. I have already replumbed (and will eventually replace) the regulator on the passenger side. I used flexible air brake line that I had on hand, I got rid of the 'evaporator' as they call it that sucks in the alcohol to prevent freezing. I would rather just pull a line off a quick-connect and add alcohol myself as needed (rarely). I removed the rearmost tank as it was badly dented and will replumb everything off the single large tank and hopefully simplify things in the process. Since I will never use this thing to tow a trailer (as in ANY trailer) I have no need for the extra air reserve that may be needed for trailer brakes. I may leave the 4wd switch if I can get it functioning, it won't rotate at all currently (does it require air pressure to even be able to turn?)
    I would guess that the other regulators in the system are there to provide the desired maximum pressure for the specific part of the system they feed. For example, I would not want 80-90 psi going to the axles.

    While removing a damaged air vessel makes perfect sense, having less tank capacity with a low output compressor does not. To me.

    There is a diagram of the air system out there, somewhere. I've seen it. You may want to find it.

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    Before you go ripping and tearing, the air system does run a few things like

    Brake Assist
    Remote Throttle control switch air cylinder
    Air shifter
    4WD
    DIff Locks
    Diff pressurization
    Trailer Brakes

    I believe you are correct one air tank is for the vehicle supply and the other is backup for the trailer brakes so you may be able to strip one off. I agree on the alcohol injector get rid of it. There are far simple replacements and the alcohol just gums up the works downstream. I have the parts to switch to air dryer but my 1300 project has waylaid the installation.

    Yes the 4wd switch should be able to rotate without air.
    Unimog SEE, Unimog 1300L Ambulance

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    I messed with it a little this morning and it appears to be filling the secondary system (I need to get some fittings to connect the primary lines to the tank). It got to 60psi and then the valve that sends air to the high-idle cylinder started blowing off air. The 'regulators' on the tanks had 5.2br on them, 5.2 bar is 75 PSI, but I am not sure if they were actually set at 5.2. I honestly don't care if I don't even have the high-idle, but I will try to make it work. I think I will likely leave the 4wd shift cylinder IF it works without leaking too bad.

    I may put a ball valve on the aux stuff so it doesn't have any air pressure unless I cut it on, that way if driving down the road something springs a leak it won't be robbing air from the brake system.

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    Quote Originally Posted by NewEra View Post
    It is ridiculous how over engineered these things are, cool yes, but way too complicated.
    The problem is that Mercedes Benz has always striven for a higher level of operator comfort. Today, this can be achieved relatively easily and inexpensively with micro-controllers. However, when the 416 was designed micro-controllers were not an option and trying to achieve the desired functionality with electro-pneumatic/hydraulic hardware lead to these convoluted systems we are fighting with today.
    1990 FLU419 SEE
    1986 M1009, 6.2L engine, 700R4/NP241, 4.10:1 14 bolt 8 lug rear, 10 bolt 8 lug front, 4" lift, 34.5" tires
    1993 M998 2-man troop carrier - SOLD
    MEP002A generator
    Gichner Expandable ISO shelter = my machine shop

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    Quote Originally Posted by alpine44 View Post
    The problem is that Mercedes Benz has always striven for a higher level of operator comfort. Today, this can be achieved relatively easily and inexpensively with micro-controllers. However, when the 416 was designed micro-controllers were not an option and trying to achieve the desired functionality with electro-pneumatic/hydraulic hardware lead to these convoluted systems we are fighting with today.
    To me, that higher level of comfort would be a lever, or a simple valve. The more sophisticated a system is, the less comfortable I am with it.
    With a lever I can feel if everything seems normal, and a ball valve is simplicity itself.
    Not that everything on a FLU can be easily changed over to something simple (the high idle, for example), but many functions could. Hopefully I'll be able to change several of the systems, one by one, to something more basic as time permits. For now, if it still works it'll remain as is.

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    Quote Originally Posted by The FLU farm View Post
    To me, that higher level of comfort would be a lever, or a simple valve. The more sophisticated a system is, the less comfortable I am with it.
    With a lever I can feel if everything seems normal, and a ball valve is simplicity itself.
    Not that everything on a FLU can be easily changed over to something simple (the high idle, for example), but many functions could. Hopefully I'll be able to change several of the systems, one by one, to something more basic as time permits. For now, if it still works it'll remain as is.
    I agree 100% and would love if MB had left all the unnecessary cr@p out of the military version of an agricultural implement. At least they refrained from adding a shutoff solenoid, electric warm-up idle/advance, and other fluff that would reduce the reliability of a mechanically injected diesel engine.
    1990 FLU419 SEE
    1986 M1009, 6.2L engine, 700R4/NP241, 4.10:1 14 bolt 8 lug rear, 10 bolt 8 lug front, 4" lift, 34.5" tires
    1993 M998 2-man troop carrier - SOLD
    MEP002A generator
    Gichner Expandable ISO shelter = my machine shop

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    Quote Originally Posted by alpine44 View Post
    I agree 100% and would love if MB had left all the unnecessary cr@p out of the military version of an agricultural implement. At least they refrained from adding a shutoff solenoid, electric warm-up idle/advance, and other fluff that would reduce the reliability of a mechanically injected diesel engine.
    Speaking of, did you find a SEE to your liking yet? Looks like GP's supply is drying up.

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