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Author Topic: AD-2 Air Dryer  (Read 6891 times)
Ross
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« on: May 28, 2006, 01:50:34 PM »

OK...I just got done rebuilding the air dryer and purge valve.  No problems at all.  Done in less than an hour, but I have a question regarding it's operation.  When the bus is airing up, there are no leaks and the purge valve is tight.  When the bus hits 120, the purge valve pops, releases a large surge of air, then continues to "leak" a little until the bus air hits 95 or so, then the purge valve closes tight, bus airs to 120.  It cycles like this.  To me, this seems normal and is probably how the system regulates it's pressure, but every time I take the bus for inspection, they tell me I have to fix that "leak".  So far I've been able to explain that it is normal, but one of these days, I expect I'll run into someone who will just insist that it is a "leak".

Actually, everything I took out looked really good.  The dryer probably didn't need rebuilding, but now I know it's up to par, so worth doing I suppose. Smiley

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Dallas
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« Reply #1 on: May 28, 2006, 03:37:17 PM »

Ross,
you shouldn't have ANY leaks in your air system.
It sounds as if you have some grit or debris in the purge valve of your air dryer.

Dallas
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NCbob
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« Reply #2 on: May 28, 2006, 03:40:14 PM »

My understanding, and I might be wrong, is that every time the Compressor cycles it's suppose to 'pop' the valve.....at least that's what I'm hoping for mine when I get it installed.

No doubt ther are more learned minds out there with more experience, but I too, am hoping to get my bus through 'Inspection'
without too much anguish.

NCbob
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NCbob
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« Reply #3 on: May 28, 2006, 03:42:23 PM »

Dallas, please give me a call.  (828) 524-6812.  Need some advice.

NCbob
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Ross
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« Reply #4 on: May 28, 2006, 03:53:23 PM »

Ross,
you shouldn't have ANY leaks in your air system.
It sounds as if you have some grit or debris in the purge valve of your air dryer.

Dallas

If there were grit in the valve, it would leak all the time.  It doesn't.  It only leaks or bleeds air through the purge valve after it releases at 120PSI.  When it hits 95 PSi or so, it closes and is quiet and "leak" free until it pops again at 120PSI.  Is this bleeding of air a means of regulating system pressure?  It was doing it before I rebuilt the purge valve and it's still doing after rebuilding the valve.  Same thing, no better, no worse.

How does this system work?  How does the control line open and close the purge valve?  It sounds as if the purge valve is staying open just a whisker after it releases at 120PSI, but then is closes all the way at 95PSI.
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rv_safetyman
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« Reply #5 on: May 28, 2006, 05:17:15 PM »

Ross, I have the Bendix CD with all of the various manuals on it.  This includes the AD-2.  I can send it to you if you need it (PDF large in size - about 5meg).  In lieu of that, here is the purge text:

Purge Cycle; (Figure 4) When desired system pressure is
reached, the governor cuts out, pressurizing the unloader
cavity of the compressor which unloads the compressor
(non- compressing cycle). The line connecting the governor
unloader port to the end cover purge valve port (bottom of
the air dryer) is also pressurized, opening the exhaust of
the purge valve to atmosphere. With the exhaust of the
purge valve open, contaminants in the discharge line and
dryer sump are purged, or forced past the open exhaust out
to atmosphere.
The reverse air flows across the desiccant and starts the
removal process of moisture from the desiccant surface.
Dry air flowing from the purge volume through the purge
orifice and across the drying bed further dries the desiccant.
The combination of these reverse flows strips the water
vapor from the desiccant (drying bed). This normally takes
between 15-30 seconds.
The desiccant becomes activated from this cycle and is
now ready for another charge cycle, which occurs when the
compressor returns to the compressing cycle. It is for this
reason the air dryer must be purged for 30 seconds, after
receiving moisture saturated air for a maximum of 90
seconds from a 12 CFM compressor
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Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
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« Reply #6 on: May 28, 2006, 05:19:44 PM »

I have a dryer which does the same. Pruges at 120 and continues to bleed a small amount, but if I turn off the master switch for a second thn it stops bleeding air.
MCI 5C
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Ross
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« Reply #7 on: May 28, 2006, 05:31:02 PM »

Ross, I have the Bendix CD with all of the various manuals on it.  This includes the AD-2.  I can send it to you if you need it (PDF large in size - about 5meg).  In lieu of that, here is the purge text:




I have the manual, but it doesn't really answer the question.  Should the purge valve bleed a small amount of air while the compressor is "off". When the governor turns the compressor back on, the purge valve stops hissing.   Near as I can figure, the governor cuts the compressor off and pressurizes the dryer control port.  How long should this line stay pressurized?  Is this controlled by the compressor governor?  Seems that if the governor is somehow not fully releasing the pressure on the control port, that could cause it to stay open a little.  Maybe I have a bad governor?Huh   

The manual states that the purge cycle should last 15-30 seconds, which leads me to believe that it should bleed a little air after the initial "pop" as part of the normal purge cycle.

Glenn....I don't have a master, just the key switch, which if turned off shuts off the motor.  That said, the purge valve does stop bleeding when I shut the bus off.
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pvcces
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« Reply #8 on: May 28, 2006, 06:36:25 PM »

We had one of these AD-2 dryers, and it gave us exactly the same symptoms. One result is that our compressor was running a lot of the time.

We had the purge valve overhauled and it made no difference whatever.

On asking around, we were told at two different shops that the AD-2 wasn't worth the trouble to overhaul. When we were offered an upgrade to AD-4, we took it. It has worked perfectly ever since.

For what it's worth.

Tom Caffrey
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Ketchikan, Alaska
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Tom Caffrey PD4106-2576
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« Reply #9 on: May 28, 2006, 10:02:42 PM »

What we were told in class was the after the burst of air, there should be some small amount of flow (the 15-30 second flow) which finishes the "drying" of the collection material.  This air volume comes from a drying air chamber in the air dryer and should not come from the tanks.

As a side note, the"collection" material does not actually absorb the water, it just provides a surface for the water to collect according to Bendix. 

In any case, there is no way that the pressure should drop more than a couple of PSI and as note above, it should not drop at all.

What bothers me is that the gauges on the dash are for the front and rear (secondary and primary) brake systems.  These tanks have check valves and you should not see any drop in pressure on the gauges even if there was a drop in the wet tank. 

This would not be a governor issue.  Sounds like the dryer check valve is staying open.  However, you need to make sure the brake tank check valves are doing their job.  If these tanks are dropping, you have more than one problem.
« Last Edit: May 28, 2006, 10:07:06 PM by rv_safetyman » Logged

Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
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Ross
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« Reply #10 on: May 29, 2006, 10:48:13 AM »


This would not be a governor issue.  Sounds like the dryer check valve is staying open.  However, you need to make sure the brake tank check valves are doing their job.  If these tanks are dropping, you have more than one problem.

I pulled the dryer check valve and it is not leaking, so I know that the air bleeding through the purge valve is not backfeeding through that valve.  So now I'm kind of at a loss.  Keeping in mind that it only bleeds air like this (1) while the bus is running and (2) while the compressor is unloaded, the only thing I can think of is that the governor is keeping the purge valve slightly open when it should be completely closed.  This raises another question....

Should the unloader port/control port be pressurized when the compressor is unloaded or should it pressurize for 15-30 seconds, open the purge valve then depressurize closing the purge valve?  If the unloader port stays pressurized all of the time that the compressor is unloaded, what closes the purge valve when the purge cycle is done?  I mean, if the control port on the dryer stays pressurized during the entire unloaded cycle, it makes sense that the purge valve would stay open, right?

When you refer the the "brake tank", which tank is that.  Up front I have a wet tank, a parking tank and an accessory/air ride tank.  Out back I have a dry tank.  Which one of those is the brake tank and where is it's check valve located?   

Thanks
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Len Silva
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« Reply #11 on: May 29, 2006, 11:25:06 AM »

Near as I can tell from my 4104 book (lots of detail there), the governor applies pressure to the unloader valve on the compressor all the time that it not calling for air.  When the tank pressure drops, the governor release the air from the unloader valve.  So, yes, the unloader port should be pressurized all the time that the compressor is unloaded.

FWIW

Len
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« Reply #12 on: May 29, 2006, 11:48:11 AM »

Ross, Tomorrow, (Tuesday) you can call the Bendix Tech Help line and speak to a Bendix guy about your dryer. The toll free # is on the website, as well as repair info on all Bemdix products.
WWW.Bendix.com
The techs are very nice. I just go ahead and tell them I don't know 'nuthin (big suprise there!) and they take it from the basic standpoint to troubleshoot or provide info.
Hope this helps-

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« Reply #13 on: May 30, 2006, 07:58:59 AM »

Ross, Tomorrow, (Tuesday) you can call the Bendix Tech Help line and speak to a Bendix guy about your dryer. The toll free # is on the website, as well as repair info on all Bemdix products.
WWW.Bendix.com
The techs are very nice. I just go ahead and tell them I don't know 'nuthin (big suprise there!) and they take it from the basic standpoint to troubleshoot or provide info.
Hope this helps-



The tech guy knew exactly what it was, and the verdict is....Bad compressor unloader valve.  The governor is sending pressure to the dryer purge valve and the compressor unloader, but the unloader valves are not seating, which means the compressor is still compressing.  Since the purge valve is essentially in purge cycle, that air coming from the compressor is just bypassing the purge valve, hense my leak....

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Dallas
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« Reply #14 on: May 30, 2006, 08:04:35 AM »

Ross,
Thanks for posting the verdict!
Now I think we've all learned something.
I for one was looking in the wrong place, but am always willing to learn.

Thanks Again1

Dallas
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rv_safetyman
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« Reply #15 on: May 30, 2006, 08:52:53 AM »

Ross, I would hope that you have the answer, but I kind of doubt that this is the problem.  In your first post, you said that the air dryer continued to vent until the pressure dropped to 95 PSI.  I can't see how the unloader valve on the compressor would cause that to happen.

I take it from your post that you have not actually done the valve repair work yet.  If you have, and it solved the problem, I will be in terrible shape trying to figure out why that would cause your problems (can't tear out my hair, as I don't have any) Wink

Upon further thought, I guess that, if the unloader valve is bad and gets into some sort of leak mode the problem you described could happen.  I know that my compressor had some sort of leak in it (could hear it when everything was off) and I just decided to replace it rather than rebuild it myself - thus I don't know for sure what was leaking.  As I recall, it would leak down to some pressure and then the leak noise would stop. 

Please keep this thread up to date so that we can all learn.
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Jim Shepherd
Evergreen, CO
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« Reply #16 on: May 30, 2006, 09:22:33 AM »

Ross, can you tell that this thread is driving me crazy(ier?)? Smiley

I went to my Advanced troubleshooting Guide for Air Brake Compressors (BW1971) to get some more information.  You can download this document from bendix.com (http://bendix.com/downloads/BW1971.pdf).

They have a test that you should be able to perform fairly easily:

Test 6: Compressor Unloader Leakage
Bendix® Compressors: Park vehicle, chock
wheels, and follow all standard safety procedures.
Remove the governor and install a fitting to the
unloader port. Add a section of air hose (min 1ft
long for a 1/2" diameter line) and a gauge to the
fitting followed by a shut-off valve and an air
source (shop air or small air tank). Open the shut
 off and charge the unloader port by allowing air
pressure to enter the hose and unload the
compressor. Shut off the air supply and observe
the gauge. A steady reading indicates no leakage
at the unloader port, but a falling reading shows
that the unloader mechanism is leaking and needs
to be serviced.


If your compressor is like my original 6V92 application, it is a devil to get to and replace.  Mine was a bit easier than some, because I had cut out the floor area (after lots of reinforcement) in anticipation of someday installing a Series 60 (came sooner than later).  This test might be worth doing rather than having to pull the air compressor.
« Last Edit: May 30, 2006, 09:25:14 AM by rv_safetyman » Logged

Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
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gumpy
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« Reply #17 on: May 30, 2006, 10:22:16 AM »

I take it from your post that you have not actually done the valve repair work yet.  If you have, and it solved the problem, I will be in terrible shape trying to figure out why that would cause your problems (can't tear out my hair, as I don't have any) Wink

Upon further thought, I guess that, if the unloader valve is bad and gets into some sort of leak mode the problem you described could happen.  I know that my compressor had some sort of leak in it (could hear it when everything was off) and I just decided to replace it rather than rebuild it myself - thus I don't know for sure what was leaking.  As I recall, it would leak down to some pressure and then the leak noise would stop. 

I know little about air systems, so just trying to learn something new, but I was under the impression that when the compressor is unloaded by the governor, the purge valve is kept open, so is continuously open while the compressor is unloaded.

Then, the governor will not kick back in until the pressure falls to the cut in point (isn't that supposed to be around 85 lbs? or is it 95?). At that point, it will kick depressurize the unloaded line, which will also allow the purge valve to close.

So, if there is a leaking unloader valve and the compressor is essentially still compressing a small amount of air (which I don't quite understand yet), then it seems to make sense that any air coming out of the compressor and through the dryer would continuously escape through the open purge valve, and not enter the air system past the dryer. Then, as the system air falls off to the cut in pressure, the governor would re-load the compressor, and the valve closes, and air builds as normal, and the whole cycle starts over.

Sure, clear as mud!!

Is any of this even close? 

So, how does the compressor unloader valve work? How does sealing a valve cause the compressor to not compress? This part I don't understand yet.

craig
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Craig Shepard
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Ross
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« Reply #18 on: May 30, 2006, 10:40:16 AM »



I know little about air systems, so just trying to learn something new, but I was under the impression that when the compressor is unloaded by the governor, the purge valve is kept open, so is continuously open while the compressor is unloaded.

Then, the governor will not kick back in until the pressure falls to the cut in point (isn't that supposed to be around 85 lbs? or is it 95?). At that point, it will kick depressurize the unloaded line, which will also allow the purge valve to close.

So, if there is a leaking unloader valve and the compressor is essentially still compressing a small amount of air (which I don't quite understand yet), then it seems to make sense that any air coming out of the compressor and through the dryer would continuously escape through the open purge valve, and not enter the air system past the dryer. Then, as the system air falls off to the cut in pressure, the governor would re-load the compressor, and the valve closes, and air builds as normal, and the whole cycle starts over.

Sure, clear as mud!!

Is any of this even close? 

So, how does the compressor unloader valve work? How does sealing a valve cause the compressor to not compress? This part I don't understand yet.

craig


That's it in a nutshell...according to the Bendix tech guy.  It made sense after he explained it.  Based on the fact that the amount of air bypassing the purge valve changes with engine RPM, there was no doubt that the problem was the unloader valve.  The air would have to be coming from the compressor.  With these symptoms there is no other option.
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rv_safetyman
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« Reply #19 on: May 30, 2006, 02:31:07 PM »

Ross, I don't want to beat a dead horse here, but so far nothing explains why air would flow through the air dryer exhaust and drop the pressure in the system to 95 PSI.  That is not right and sure does not sound like an unloader problem to me.

Keep us posted.
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Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
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« Reply #20 on: May 30, 2006, 03:05:41 PM »

Ross, I don't want to beat a dead horse here, but so far nothing explains why air would flow through the air dryer exhaust and drop the pressure in the system to 95 PSI.  That is not right and sure does not sound like an unloader problem to me.

Keep us posted.

Pressure drops to 95 when you apply brakes or anything else air driven, at which point the governor kicks off the compressor amd pressurizes the unloader.  It will drop by itself, but it takes 30 minutes or so, which points to a very minor leak somewhere, but not worth chasing down at this point.  I don't think the slight leak and the air bypassing the purge valve have anything to do with eachother.  The system is basically functioning normally other than the air bypassing the purge valve while the compressor is unloaded.

If it were not compressor related, it would leak all the time and not just when the engine is running and compressor unloaded.  Luke is sending me a new unloader valve.  He says that it can be changed easily without removing the compressor.  It's only $16 so it's worth a try.  My compressor is just inside the curb side rear door, so it looks like an easy job.
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NCbob
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« Reply #21 on: May 30, 2006, 07:28:38 PM »

There is a lesson for us all here...carry a spare unloader valve.  I shall from now on.  Chalk it up to a learning experience.

And we all know...when you carry a spare...you carry a spare for life...you never need it.  Grin

NCbob
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Ross
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« Reply #22 on: June 01, 2006, 04:26:52 PM »

Just an update.  The unloader valves were the problem.  They were both siezed in the closed position.  I had to soak them in PB Blaster and make a special tool to drive them out from the governor port.  New valves now installed and no more air bypassing the dryer purge valve.  The compressor was basically running full time and never unloading.

I'm also one step closer to finding all of the minute leaks that you can't really detect.  Although the unloader valves were siezed in, the rubber o-rings were gone, which was allowing some air to bypass the unloader valves during the "un-loaded" cycle.
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« Reply #23 on: June 01, 2006, 06:29:48 PM »

Ross, thanks for letting us know the result. 

I am still amazed.  I would think that you would hear the overpressure valve letting go or see the overpressure in the gauges.

Those Bendix folks are great folks and you sure got a good answer.

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Jim Shepherd
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’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
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« Reply #24 on: June 01, 2006, 07:37:21 PM »

Jim,

As I understand it, you wouldn't see overpressure because when the governor says stop compressing, it also opens the purge valve and keeps it open, so any air that the compressor keeps putting out will just exit the dryer through the open purge valve.
That's why he kept hearing air coming out of the purge valve continuously.

craig
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Craig Shepard
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« Reply #25 on: June 02, 2006, 05:26:32 AM »

Jim,

As I understand it, you wouldn't see overpressure because when the governor says stop compressing, it also opens the purge valve and keeps it open, so any air that the compressor keeps putting out will just exit the dryer through the open purge valve.
That's why he kept hearing air coming out of the purge valve continuously.

craig


Exactly...If the governor had failed, I would have seen an overpressure on the guages and the overpressure valve would have popped. 
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« Reply #26 on: June 02, 2006, 05:27:08 AM »

Though I sort of understand what happened here, I still don't understand how the unloader valves work. I'm going to have to take some time to read up on this. I was under the impression that when the governor says "enough", the valves open and vent the compressor to atmosphere, so it can't compress, but your description seems to be 180* off that.

So I still have some learning to do. This is an area I've not had to do much to on my bus. Had a stuck governor once, and couldn't build pressure. It would build to about 65 psi, blow off the dryer purge valve, and vent the air supply till it had fallen to about 30 and start all over. I got it to work by tapping on the governor with a screwdriver handle. When I got home, I took the governor apart, cleaned it, replaced the o-rings, greased the moving parts, put it all back together, and haven't had any more problems in about 3 years.  I carry a spare governor now, though, so don't expect to ever have another problem with the governor.

« Last Edit: June 02, 2006, 05:35:52 AM by gumpy » Logged

Craig Shepard
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« Reply #27 on: June 02, 2006, 06:08:27 AM »

Though I sort of understand what happened here, I still don't understand how the unloader valves work. I'm going to have to take some time to read up on this. I was under the impression that when the governor says "enough", the valves open and vent the compressor to atmosphere, so it can't compress, but your description seems to be 180* off that.

So I still have some learning to do. This is an area I've not had to do much to on my bus. Had a stuck governor once, and couldn't build pressure. It would build to about 65 psi, blow off the dryer purge valve, and vent the air supply till it had fallen to about 30 and start all over. I got it to work by tapping on the governor with a screwdriver handle. When I got home, I took the governor apart, cleaned it, replaced the o-rings, greased the moving parts, put it all back together, and haven't had any more problems in about 3 years.  I carry a spare governor now, though, so don't expect to ever have another problem with the governor.



The unloader valves are connected to pushrods that go up into the head.  When the governor pushes the unloaders those pushrods open the inlet valves.  With the inlet valves held open the compressor can not compress.  Would be about the equivalent to holding the intake valves on an engine open.  That's what I was able to ascertain from the manual anyway.
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« Reply #28 on: June 02, 2006, 06:36:55 AM »

Ok, that makes sense.

Does anyone know offhand what the compressor and governor numbers are on an MC9? I was looking at the service data sheets on the Bendix site and found the DD-3 and AD-2 information, which I've uploaded to my web site so I'll have it (see other thread). I'd like to find the governor and compressor information and put that there, also, so I have it in one place should I ever need to refer to it.

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Craig Shepard
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« Reply #29 on: June 02, 2006, 12:03:00 PM »

Craig, I can't speak for MCI, but both the 6V92 (I think) and Series 60 (for sure) in my Eagle had Tu-Flow 550 compressors.

I continue to struggle with this thread.  We talked about a similar situation in class, and I am sure we concluded that the system would go into an over-pressure condition. 

The 550 compressor produces 13.2 CFM at 1250 RPM and I don't think there is any way that the air dryer can dump that much air if the compressor is compressing full time.  Perhaps the unloader was stuck so that it somehow did not let the compressor produce full CFM, but that also doesn't make any sense. 

The thread is so long now and I don't have time to go back, but I thought Ross said that the problem came about after he rebuilt the AD-2.  IF that is the case, it really makes me scratch my old bald head.  Maybe it is a coincidence, but that would be weird.

If the fix worked, obviously I am out in left field - but I would sure like to find a way to make my thought process jibe with the solution.
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Jim Shepherd
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’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
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Ross
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« Reply #30 on: June 02, 2006, 12:50:11 PM »

Craig, I can't speak for MCI, but both the 6V92 (I think) and Series 60 (for sure) in my Eagle had Tu-Flow 550 compressors.

I continue to struggle with this thread.  We talked about a similar situation in class, and I am sure we concluded that the system would go into an over-pressure condition. 

The 550 compressor produces 13.2 CFM at 1250 RPM and I don't think there is any way that the air dryer can dump that much air if the compressor is compressing full time.  Perhaps the unloader was stuck so that it somehow did not let the compressor produce full CFM, but that also doesn't make any sense. 

The thread is so long now and I don't have time to go back, but I thought Ross said that the problem came about after he rebuilt the AD-2.  IF that is the case, it really makes me scratch my old bald head.  Maybe it is a coincidence, but that would be weird.

If the fix worked, obviously I am out in left field - but I would sure like to find a way to make my thought process jibe with the solution.


The problem was there before the AD-2 rebuild.  I was hoping that the rebuild would solve it, which it didn't.  It was dumping quite a bit of air through the dryer purge.  You would only have an overpressure situation if the compressor was compressing AND the purge valve was closed, but since the governor was working and the purge valve was opening, the compressor air was just blowing past the purge valve.
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« Reply #31 on: June 02, 2006, 08:50:39 PM »

After looking at the service sheets on the AD-2 dryer and D-2 governor, and with Ross's description of how the unloader valves work, I think I now understand what happened (maybe 90%), and it's as Ross explained.

The air pressure hit 120 psi within the air system. This caused the governor piston to move up and open to the unloader / purge port. Pressurizing that line opens the purge valve in the AD-2 dryer. This effectively opens the dryer chamber to atmosphere and anything coming into the supply side of the dryer will be vented out the purge valve. There's a check valve in the dryer outlet to keep air that's already in the down stream air system tanks from coming back out the dryer purge valve. In fact, the purge valve stays open the entire time the governor is in the cut-out stage.

The governor unloader port was also trying to unload the compressor, but because the unloader valves were stuck, they could not raise the intake valves and stop (fully) the compressor from continuing to compress air into the system, which goes directly to the dryer, which is now vented to atmosphere. So all the air coming out of the compressor which has stuck unloader valves is going directly out the purge valve on the dryer.

The check valve in the top of the dryer keeps the system pressure up, and since that's what feeds the governor, the governor thinks everything is hunky-dorey. After awhile, the pressure in the air system drops due to normal usage and general leaking, and eventually reaches the governor cut-in pressure which moves the piston back down and releases pressure in the unloader/purge line, which allows the purge valve to close, and takes the pressure (peer pressure ??) off the stuck (or sticky) unloader valves. 

Of course now, the air being compressed is passing through the dryer and into the air system, so the air exiting the purge valve stops (which is what Ross's original description said was happening). Once the system pressure reaches 120 psi, the whole vicious cycle begins anew.

I can't comment on the CFM output of the compressor vs. the volume passage ability of the dryer, but the scenario makes sense to me now.

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Craig Shepard
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« Reply #32 on: June 02, 2006, 10:12:18 PM »

Craig, thanks for the description. You have just made me realize that we have a bad check valve downstream of our air dryer.

When we shut off our engine, our air pressure gauge would show a pretty good leak by dropping down to cut-in pressure in about 15 minutes. During that time, I could hear air coming from the dryer purge valve.

That couldn't have happened if the check valve wasn't admitting air back into the dryer from the system. We had other problems, but those had already been fixed.

Thanks a lot!

Tom Caffrey
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Ketchikan, Alaska
« Last Edit: June 02, 2006, 10:17:37 PM by pvcces » Logged

Tom Caffrey PD4106-2576
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Ketchikan, Alaska
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« Reply #33 on: June 04, 2006, 08:04:14 AM »

Tom, you may have other problems.  Each brake tank (primary and secondary) also have check valves to keep the air from flowing back to the wet tank and then, if that check valve is bad, to the air dryer.

The typical gauges on the dash are for the primary and secondary supply tanks.  They do not monitor the wet tank pressure. 

So, if your gauges are dropping you most likely have a check valve problem in one or both brake tanks and/or leaks in the brake part of your system. 

If it takes 15 minutes and both gauges go down, that is kind of serious.  The fifteen minute period suggest a fairly slow leak that might be a bit harder to trace.  If not the check valves, it could be the relay valves (you would hear it in the exhaust of the valve) or even in the diaphragms.

Bendix has a great video on air system leakage.  I am trying to get permission to put it on my safety video. 
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Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
Blog:  http://rvsafetyman.blogspot.com/
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« Reply #34 on: June 04, 2006, 08:14:28 AM »

Jim, which tanks are you refering to when you talk about the brake tanks?  I have 2 tanks behind the front axle.  A wet tank and a parking tank.  There is one under the driver, which is the air ride/accessory tank and there is one at the rear which is the dry tank.  Are the check valves built into these tanks or are they external, in-line type check valves.  It's hard to decipher from my scanned air schematic.
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« Reply #35 on: June 04, 2006, 01:10:43 PM »

Ross, it sounds like you have four tanks and that is typical for most buses.  There should be two brake tanks, one wet tank (usually in the rear close to the air compressor/dryer) and one or more accessory tanks.  I think the tank you are calling a parking tank is one of the brake tanks.

Each brake tank is usually close to the axles they serve (shorter lines to the brakes).  The front brake tank will generally not have a relay valve, but will have a quick release valve to dump the air when you let off the brake pedal

The rear axle/bogey tank is always (AFAIK) close to the axle.  It will be attached to a relay valve.  The relay valve takes a signal from the brake pedal and opens the valve from the tank to the brakes (again, the objective is a a short run from the tank to the brake chamber from  the air supply).

The best way to tell which tank is which, is to trace the air lines from the brake chamber through the valve(s) to the tank for each axle.

Every one way valve that I have seen is a screw in valve to the tank.  They are usually about 1.5 inches in diameter and maybe 1.25 inches long.  They are made to be replaceable.  They can be tested by airing up the system, then drain the wet tank and take the air line from wet tank to the brake tank off (can remove from either end) and no air should escape.  The other way to test it (easier) is to air up the system and then drain the wet tank using the water drain valve.  The gauges for both brake tanks should remain unchanged for several minutes.  As noted earlier, most buses have two air gauges that are labelled front and rear and are connected to the BRAKE tanks.
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Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
Blog:  http://rvsafetyman.blogspot.com/
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« Reply #36 on: June 04, 2006, 01:41:57 PM »

Thanks Jim.  I think you're right.  I think what the manual calls the parking tank is the front brake tank and rear tank (dry tank in the manual) goes to the rear brakes.  If it ever stops raining I'll get back under there and trace everything back to the tanks and check all the valves. 
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« Reply #37 on: June 04, 2006, 06:09:39 PM »

Ross, I have lost track of what all I have said in this lengthy thread.  If I have not said it already, be sure to air up the system before you crawl under the bus (you are blocking it, right?) and open the drain on EVERY tank in the system for a few seconds.  When you do, look for unusual amounts of oil (say three or four ounces).  You need to drain these tanks anyway to make sure that oil and water are not accumulating and reducing the volume of the tanks.  You will always have some oil, since the design of the compressor is such that it needs to pass a bit of oil. 

If you find very much oil in any of the tanks, it is not necessarily a death pronunciation for the compressor.  What it means is that you should check the wet tank (and maybe all of the tanks) every month or so of use and see what builds up.  If you see an ounce or two of oil after a month of use (at least a couple of thousand miles), I would begin to worry. 

Bendix has a special method for checking for excessive oil.  I think it is on the mybendix.com site (remember that you have to register to be on the site – see another thread about Bendix product data sheets).  I started to look for the document, but I am away from my files and on dial-up this evening, so I could only check the title.  I think the document is called advanced troubleshooting for air compressors.  We reviewed it in class and I think it is intended for trucks that pound on the miles each day.  However, it will give you an idea of what is considered excessive and how to measure it.

If we keep going we are going to set some sort of record for the length of a “normal” thread.  However, I think folks have been exposed to some good information.
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Jim Shepherd
Evergreen, CO
’85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
Blog:  http://rvsafetyman.blogspot.com/
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« Reply #38 on: June 04, 2006, 06:54:40 PM »


If we keep going we are going to set some sort of record for the length of a “normal” thread.  However, I think folks have been exposed to some good information.

An interesting point and it is really easy to check.

If you click on any of the headings such as Replies, views, started by  etc. the board will re-sort itself in either numerical or alphabetical order as applicable. Just ignore the first few posts that are sticky. You can even find out all the posts you have made.

Maybe I should make this into a board tip also!

Richard
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Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body. But rather to skid in sideways, chocolate in one hand, a good Reisling in the other, body thoroughly used up, totally worn out and screaming:  WOO HOO, what a ride
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