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Discussion Starter #1
Hello,

Started up the engine today after many months. One air came up quickly, but the other remained on zero and after about 3 minutes at idle I kept thinking something was leaking or stuffed so I turned engine off and on again numerous times checking things.

Then eventually I let the engine run for more than 5 minutes, including revving up and holding at 3kRPM, and suddenly the other came up, Both on about 16 bar.

Is that normal? should I have to wait that long before the second one comes up? I'm wondering if something wrong with the compressor?

Also

After about 1/2 hr, they have both dropped to 3 bar. I'm wondering if I have a leak (I'd expect to hear it) or if thats normal?
 

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You have a leak somewhere - happy hunting.
Cannot speak for the ADF 1700, but on mine, when starting from zero, the primary circuit charges up first, then the second kicks in at about the half-way point. I beleive, it is by design.
 

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On our U1300L, the primary circuit airs up to ~7 bar before dropping back to 5 bar at the same time the secondary circuit starts to gain pressure. This is the same behavior that I have observed on the past 3 Mogs I have owned (435, 424s). Hope that helps give you some idea. Your ADF 1700 may be different, maybe not.
 

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You have a leak somewhere - happy hunting.
Cannot speak for the ADF 1700, but on mine, when starting from zero, the primary circuit charges up first, then the second kicks in at about the half-way point. I beleive, it is by design.
some of the ADF ones did do this, some didn't mine does at the start but gives up when it gets to about 7 bar then the second one kicks in
Have fun finding the leak, I'm in the process of replacing the instrument cluster gauge as mine is well had it after 35 years, a very slow leak that's been getting worst since I got the truck last year.
In the Army out Field I've had leaks before in the air lines running to the rear of the truck I found that someone had put black tape on it to fix it ! so your leak could be anywhere, sorry.
 

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The second circle is for the trailer operation and is filled only when there is enough pressure for Mog's brake.
After turning off the pressure should last several hours, better for days.

for finding leaks a hand sprayer for flowers with plenty of dishwashing detergent in the water is helpful,
it will be formed at leaking bubbles.
There is also special leak detection spray at the technical trade.

trans by engine! Sorry!
 

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Discussion Starter #6
Ran engine again today after fitting intercooler. Primary tank never got above 5.5 bar and the other stayed on zero. No obvious large leaks.
Some times after I start it, it comes up as it should, the next time it might not come up. Nothings changed since I last ran it a few weeks ago (and vehicle is still up on jacks). Tried pumping the brakes, but no change to air pressures. Checked the air tanks, same as on the gauges.
 

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Ran engine again today after fitting intercooler. Primary tank never got above 5.5 bar and the other stayed on zero. No obvious large leaks.
Some times after I start it, it comes up as it should, the next time it might not come up. Nothings changed since I last ran it a few weeks ago (and vehicle is still up on jacks). Tried pumping the brakes, but no change to air pressures. Checked the air tanks, same as on the gauges.
If truly no obvious large leaks, then the compressor must be on it's last legs, unable to deliver pressurized air. Are you dialed in to the sound the unloader valve makes when it reaches full pressure, and dumps the compressor output? That is definitely a "large leak", but it is relatively quiet, at least on my truck (not counting the "whoosh" when it opens at max pressure). Possible the unloader is always unloading, maybe not fully closing.

Before panicking too greatly, I'd get an independent air supply/ compressor and round up a fitting to put air into the truck system downstream of the unloader valve. There should be an auxiliary port on the unloader, probably with a fitting for the tire fill hose; I use it on my truck. There have been numerous threads on this subject, and many ways to skin cats. If the check valve function on the unloader is good, you can charge up the tanks, and do some listening/ soap spray/ gauge watching on the air distribution system, without benefit of engine operational noise. A compressor with even 9 bar output should let you cycle the brakes, and verify no big leaks in use. That will at least get you one thing known, and if all checks out "acceptable", then it is the compressor, unloader, or the air line between. If you can get 5 bar out of the truck compressor, a soap spray will find a leak in the line or fitting. The reality is that a maximum output that low, without a clearly audible leak, says "compressor" to me; valves sticking, most likely. At that point, I'd be thinking of disconnecting the air output line, and cobbling a connection directly into a remote tank with a good gauge and good safety relief valve on it. If you are an 18 bar system, that is extremely high relative to typical air compressors, as used in construction, for instance. High pressures are not to be trifled with.
 

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Discussion Starter #8
If truly no obvious large leaks, then the compressor must be on it's last legs, unable to deliver pressurized air. Are you dialed in to the sound the unloader valve makes when it reaches full pressure, and dumps the compressor output? That is definitely a "large leak", but it is relatively quiet, at least on my truck (not counting the "whoosh" when it opens at max pressure). Possible the unloader is always unloading, maybe not fully closing.

Before panicking too greatly, I'd get an independent air supply/ compressor and round up a fitting to put air into the truck system downstream of the unloader valve. There should be an auxiliary port on the unloader, probably with a fitting for the tire fill hose; I use it on my truck. There have been numerous threads on this subject, and many ways to skin cats. If the check valve function on the unloader is good, you can charge up the tanks, and do some listening/ soap spray/ gauge watching on the air distribution system, without benefit of engine operational noise. A compressor with even 9 bar output should let you cycle the brakes, and verify no big leaks in use. That will at least get you one thing known, and if all checks out "acceptable", then it is the compressor, unloader, or the air line between. If you can get 5 bar out of the truck compressor, a soap spray will find a leak in the line or fitting. The reality is that a maximum output that low, without a clearly audible leak, says "compressor" to me; valves sticking, most likely. At that point, I'd be thinking of disconnecting the air output line, and cobbling a connection directly into a remote tank with a good gauge and good safety relief valve on it. If you are an 18 bar system, that is extremely high relative to typical air compressors, as used in construction, for instance. High pressures are not to be trifled with.
I don't believe that it has an unloader (unless its built inside somehow)? My home compressors do have them externally, but nothing obvious on this compressor to do that job. It just runs whenever the engine is running. Its a relatively new compressor, only about 2 years old. Sounds good and I can hear it pumping every cycle ok. I'm thinking something to do with the 4 way valve or regulators?
 

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Okay, I’ll try and compress (compress….ha-ha) the tech weenie lecture to the essentials.

Mog airflow:

1) Air filter to compressor intake port
2) Compressor output port into long, fairly large dia. metal tube entering
3) Pressure regulator/ compressor unloader valve/ return check valve feeding
[3A. Air dryer/ water separator, if so equipped, which feeds: ]
4) a. Smaller pipe going forward through check valve to glad hand on front bumper (can be used to fill truck air tanks)
b. 4 circuit protection valve, which uses dual feed lines to the two main reservoirs; other outputs to various locations.
5) Main air tanks provide feedback loop to the press reg (3) to open unloader when max fill (18 bar) is reached, dumping compressor output to atmosphere, and close unloader when either tank falls below 10 bar, sending compressor air output into the truck system (3A-4)
6) From here on, things get messy, complicated and likely rather variable, depending on the truck and equipment.

All constant-run compressors MUST have an unloader. If one contained the output, there is a real limit to ultimate pressure that can be achieved by any compressor, based on physical characteristics, like compression ratio (surprise) and valving. Most piston-type compressors use reed valves, in which down stroke sucks the intake open, and pulls the exhaust valve closed, and vice versa. The air in front of the piston cannot achieve a pressure higher than is created by the compression ratio, because if there were to be higher pressure downstream, the piston could not force the exhaust valve open to push more air downstream. In practical terms, every compressor has a “stall point”, in which only damaging heat can be created, but no additional air storage. One can disconnect power before that point is reached, or one can dump the output; with no back pressure, the compressor free-wheels with minimal heat generation; the output air needs very little pressure to push open the exhaust valve, and there is no restraint in the line, so there can be almost no pressure build-up. The Mog dumps the output to atmospheric, through the pressure regulator/ unloader valve (and the aux port that I use for inflation has no air pressure unless the unloader is closed for the 10-18 bar fill range).

My assertion stands: if you cannot go above 5.5 bar, either the compressor is failed, or there is a way that large amounts of air are being bled off unintentionally. My reasoning for checking the unloader first is that if it is indeed partly open, pressure will not likely get high enough to generate a really load hiss. I would do the remote fill of the twin tanks as the easiest first “section” (4, 5, 6) of the system to evaluate. This should also quickly tell you if the check valve function of the pressure reg is holding. You should be looking for the first device on the feed line from the compressor. It may look like just a pressure regulator, but it is an active device, and it opens, as an unloader, to limit max press. I amp quite sure the first check valve in the air delivery is also part of this device.

With the engine running, you should not hear any venting to atmospheric at the press reg/ unloader (3), assuming you start at less than 10 bar and haven’t risen to 18 bar max. There might be a very brief “dump” at start, but it can’t sustain that dump, unless there is a failed part. There is also likely to be an audible sound of air flowing within the piping.

If you are able to pump up to the 5.5 bar, that should give enough pressure for the soap test on the air line and fittings from the compressor (2)

At that point, you are left with the compressor itself (1). Of course, you can do the tests in reverse order, but I advocate a rigorous methodology, regardless of starting point. One thought: have you ever had the truck running with the compressor air intake hose NOT coming from after the air filter? Meaning, could you have admitted unfiltered air, and gotten a bit of grit inside, messing with a reed valve?

FWIW, I have scanned and attached some pages from my (OM 366A) workshop manual. Might help with diagnostics.
 

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Discussion Starter #10 (Edited)
Thanks Photobldr. I will follow through with testing. Yes I have run it for short while without filter, but only while vehicle sitting on concrete apron and no dust around. Hopefully should not be any grit.
I thought the device at each tank was a regulator. Now know that they are unloaders.
 

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The pressure regulator/ unloader is the FIRST, large device that the airflow hits, at the end of that larger pipe from the compressor. It is a single device. At each tank there are pressure relief (safety) and the feedback loops for the unloader. It is complicated, I had to re-read the manual, and crawl under my truck to make sure I was describing correctly. I would probably just positive ID the unloader, and just listen as the truck starts and tries to build pressure. If it is dumping when it should be closed, it should sound like a gentle whoosh, because there is not much pressure behind it, and you should definitely be able to feel the air being expelled. It should be coming from a serrated black plastic cap, on the bottom side of the reg/ unloader, and it should be oily from the lube oil in the air stream. Should be obvious enough not to need the stethoscope, but that could come in to play later. I'm done for tonite, but if you are still having ID issues, post or PM, and I will photograph and mark up.

Also, what about belt tension/ old or glazed belt? I put on new segmented belts (more grip, less heat) and tensioned and re-tensioned them all. The manual says 5-10mm deflection (unspecified force: how strong is YOUR finger?) for the compressor belt, which is pretty tight. A long shot, but you need to run through the easy stuff too.
 

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Discussion Starter #12
Ok. First device is unloader.
I have fitted the automatic water removal kit (has a tank) between the air compressor and the unloader. I suppose this means it has a lot of extra unloading to do now? My home compessors make a loud whoosh when they unload, but I don't recall ever hearing the truck unloader make a noise.
 

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The truth will out!! Did you fit the auto water remover yourself, and did that coincide with the time that the compressor started it's failure to come up to pressure antics? An essential part of a water separator is the ability to exhaust the water back out to the atmosphere, along with a bit of O2. I have not delved into the specific operational method of the M-B separator, so I must be careful about pontificating here. What I am pretty sure of though is, is that the water separator must be downstream of the pressure reg/ unloader/ check valve unit (that first device), so that it is operating on a compressed air stream. Most static separators use a high-speed swirl action in the airflow to fling the water out of the air (actually, it is drawing the air out of the water particles, centripetal force). In an automatic unit, there is then a draining action of the accumulated water. If I am understanding your description correctly, the very first thing I would experiment with is removing the water separator, re-establishing the original air connection/ supply, and run the truck. I have to speculate that the auto-drain function of the separator is bleeding off air, but that is just speculation at this time.

If you cannot come up to 18 bar pressure, and the press reg/unloader is functional, you will never hear the whoosh, because it does not unload until it has hit the high pressure limit. Any big leak at or prior to the unloader is operating in very low pressure air; the air stream is being pushed out of the compressor into a relatively unconfined space. Meaning, it does not dump 18 bar air into the pipe, but rather pushes air into the pipe up until it reaches its working limit of 18 bar in the pipe (and the tanks, etc). And the Mog compressor is optimized for high max pressure/ low volume; this is not going to be like listening for your leaf blower (high volume, low pressure). The whoosh on unloading is due to the stored energy in the air mass between compressor and unloader; at 18 bar, that is considerable, even though the volume/ mass is small.
 
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