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Author Topic: AD-2 Air Dryer  (Read 6775 times)
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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gumpy
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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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pvcces
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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
Suncatcher
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/
Ross
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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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rv_safetyman
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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/
Ross
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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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rv_safetyman
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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/
DrivingMissLazy
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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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