BUS air cycles between 120 & 114

[Prather]

Bus air started cycling between 120 and 114 lbs. Changed governor same problem. I have a AD 2 air dryer, thinking now maybe the dryer is creating my problem. As anyone had a similar situation and what was the fix?

Thanks in advance for your help.

Prather

[gumpy]

Where are you and how cold is it there?

My initial thought was governor, but since you said you replaced it, I'd look at the dryer valve and compressor. Seems like there was a similar problem posted several months ago and the result was a stuck or broken valve stem in the compressor or some such thing.... 

[gumpy]

Yeah, here, check this thread....

http://www.busconversions.com/bbs/index.php?topic=659.0

[Prather]

Thanks for the responce, I will let every one know what it was when I get it fixed.

Prather

[TomC]

I can almost 100% assure you-rebuild the air dryer and the problem will stop.  The short cycle times are the air dryer way of telling you it needs to be rebuilt.  Get it done, since the next step will be for the air dryer to stop working which means no air built up.  It's easy to do your self.  Good Luck, TomC

[gumpy]

Tom,

Can you explain how that works?  The air dryer doesn't regulate the compressor cycles.

I just can't picture how the air dryer could cause it.

Seems more likely a governor or compressor problem.

craig

[Stan]

Consider a weak or broken spring on the unloader valve in the compressor.

[TomC]

Gumpy- on the bottom of the air dryer is the unloader that spits out the moisture everytime the compressor kicks off.  What happens is the unloader valve gets old and weak, also the seal and O rings tired.  So what happens is, the unloader valve doen't stay open with the governor, and prematurely closes causing the compressor to pump again.  The governor of the compressor works through the air dryer to unload the compressor.  So the unloader valve is working the compressor, not the governor.  As found out by replacing the governor and it didn't change anything, rebuild the air dryer, especially the unloader valve, and it will operate correctly again.  Good Luck, TomC

[gumpy]

Nah, I'm not buying it....

The governor is not controlled BY the unloader line; the unloader line is controlled BY the governor. The governor works off system pressure.  When the system pressure reaches 120 psi, the pressure pushes the governor piston, which charges the unloader line. The charged unloader line causes the compressor to unload and stop compressing. The charged unloader line also controls the purge valve on the dryer, causing it to open, and stay open. This expells the water. The valve cannot close while there's pressure on the unloader line, coming from the governor. 

The system pressure has to fall in order for the governor to remove pressure from the unloader line. There's a spring in the governor that regulates the cutin pressure. When the system pressure falls and the spring pushes the piston back, the unloader line is exhausted, and pressure is removed on the compressor unloader line, and also the dryer purge valve control port. This allows the compressor to begin compressing again, and the pressure of the air coming into the dryer closes the exhaust port valve.

A defective purge valve on the dryer shouldn't cause the governor to cycle.

The description Prather posted makes me think that either the governor is not adjusted properly (new governors are not necessarily preset and must be adjusted after installation) (Prather, you did adjust the new governor, right?) , or there's a problem with the unloader valves in the compressor.

That's my theory but I'm not an expert on this, so willing to read any hole shooting arguments against it.

craig

==============


Further thoughts.....

Right after posting this, I reread your post and had started thinking about it some more... Just trying to think through this one. Bear with me...

The unloader control line is connected to the compressor unloader valves, and also to the dryer purge valve control port.

So, if the purge valve is defective, and leaking, it could cause pressure loss in the unloader control line, which would directly affect the compressor unloader valves and could conceivably cause the cycling seen by Prather.  However, there should be a continuous supply of system air coming into the governor, so there would have to be a bigger leat at the dryer than the supply from the governor into the unloader control line, and the leak should be heard as a constant exhaust of air from the purge valve....  In looking at the governor diagram, it appears there's not a large port connecting the system air to the control port, so what you are saying appears to be possible.

Hmmm...  So the purge valve leaks and the pressure drops in the control line that is also tied to the compressor unloader valves, causing it to start compressing again?  Seems like you might experience overpressure since the governor is already at it's high pressure position, but the compressor is still compressing. Either that, or you'd have one heck of a blow by on the dryer which you'd hear and feel as constant air coming out.

Well.... without more information from Prather about air conntinually escaping from the dryer, I'm going to stick with my theory. Not enough symptoms to support yours at this point, but I can see how what you said could occur now. Thanks. That gives me one more little piece of understanding how the air system functions.

[Prather]

Called the Bendix Tech.  He said two things to check were the inlet pressure to the governer, if that wasn't the problem to spray some WD 40 on the unloader valves located in the air supply to the compressor.

Put a gage in line to the air supply to the govener, compressor was working as it should, unloading at 120 lbs started compressing again at 105 lbs.

So I have a air leak somewhere. After shutting off engine and letting air decay off for about 20 minutes, still had 119 lbs on gage at dash and 0 lbs on gage at compressor.  

So I am searching for the air leak, my old ears makes it tough, can't hear it but know it is some where.

Thanks for all the input.

Prather

[gumpy]

Well, that just might be what Tom said.  Check your dryer purge valve. Air it up till it stops compressing, shut it off, and check your purge valve for leaking air.

[RJ]

Using shop air, rather than having that big old Detroit running, might help you hear any air leaks better.

That, and a fifth or sixth grade kid from the neighborhood. . .  or a grandkid.

Not to mention the infamous squirt bottle with soapy water!

HTH. . .

[TomC]

This might sound wierd, but sometimes when you stop the engine, because the compressor is not pumping, the spitter valve will seat and you won't be able to hear a leak.  I found the leaking only when the engine is running, which makes the compressor cycle about every 20 seconds, since the spitter valve is leaking.  Once the engine stops and the spitter closes, at 114 as inticated, no leaking.  Once again-rebuild the air dryer completely and this all will stop.  Good Luck, TomC

[Catskinner!]

I had a very simular thing happen to me on my Eagle just a couple
of weeks ago.  I was driving along and noticed my air down to
about 70lbs..  I got off the hiway and checked to see if I had
blown a hose or air bag and couldnt find anything wrong.  Got
back in the coach and the pressure was back up, so we went
on to a resturant to eat, thinking I need to change the air com-
pressor governor.  When we came out to leave the air would not
build above 30lbs. so I turned on my onboard compressor and
proceded to make it on home with about 100lbs. of pressure.

I changed the governor, Nope this didn't fix it.  Shop pressure
on the main supply line to the air dryer revealed the spitter
valve leaking all the time, which I couldnt hear with the engine
running. Changed out the air dryer and everything is fine.

Goodluck!
Catskinner!
Sonnie & Patti Gray
0/5 Eagle 3406 Cat
Pottsboro, Texas 

[Ncbob]

I'm a bit confused here.  I try to call items names that the Mfr. uses.  But what in the world is a splitter valve?  Could it be the relay valve?  Inquiring minds want to know.

NCbob