AD-2 Air Dryer

[Ross]

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.

[Dallas]

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

[NCbob]

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

[NCbob]

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

NCbob

[Ross]

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.

[rv_safetyman]

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

[glenncc]

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

[Ross]

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?   

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.

[pvcces]

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
Suncatcher
Ketchikan, Alaska

[rv_safetyman]

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.

[Ross]

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

[Len Silva]

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

[ChuckMC8]

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-

[Ross]

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....

[Dallas]

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