AD-2 Air Dryer

rv_safetyman · 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)

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.

rv_safetyman · May 30, 2006, 09:22:33 AM

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

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.

gumpy · May 30, 2006, 10:22:16 AM

Quote from: rv_safetyman on May 30, 2006, 08:52:53 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)

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

Ross · May 30, 2006, 10:40:16 AM

Quote from: gumpy on May 30, 2006, 10:22:16 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.

rv_safetyman · 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.

Ross · May 30, 2006, 03:05:41 PM

Quote from: rv_safetyman 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.

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.

NCbob · 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.

NCbob

Ross · 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.

rv_safetyman · 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.

gumpy · 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

Ross · June 02, 2006, 05:26:32 AM

Quote from: gumpy 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

Exactly...If the governor had failed, I would have seen an overpressure on the guages and the overpressure valve would have popped.

gumpy · 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.

Ross · June 02, 2006, 06:08:27 AM

Quote from: gumpy 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.

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.

gumpy · 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.

rv_safetyman · 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.