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Author Topic: air compressor cycling every 5 minutes, MC 5C  (Read 3219 times)
lostagain
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« on: July 25, 2011, 07:29:01 PM »

My air comp. cycles way too often while going down the road. Every 3 to 5 minutes.

I cannot find an audible air leak anywhere while parked, with or without the parking brake applied (DD3).

The bus holds air while parked and stays up for 2 to 3 days. No leaks in the air bags.

It only leaks when driving.

So far, I have looked at/overhauled the air compressor discharge valves, the air comp. A2 governor, the AD9 dryer purge valve and the  inversion valve.

So obviously, there is air press. somewhere that is leaking  with bus running.  I don't know where to look next.


Suggestions please.


Sorry, I know this has been addressed before, but the search doesn't yield much.

JC
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JC
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« Reply #1 on: July 25, 2011, 07:40:46 PM »

what about the leveling valves ?
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lostagain
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« Reply #2 on: July 25, 2011, 07:52:57 PM »

If a levelling valve leaked while under way, wouldn't it leak while parked as well? I can't picture the difference.

JC
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JC
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« Reply #3 on: July 25, 2011, 08:34:02 PM »

I don't know what is a high cycle rate would be as one is driving, but as the suspension works over bumps and dips, air is coming and going all of the time.
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« Reply #4 on: July 25, 2011, 09:05:17 PM »

If you have an air/water separator-it is its' way of telling you you need to rebuild it.  The rebuild kits are usually less then $100.00.  Please do it soon-since the next step is for the air/water separator to quite working-which means you wouldn't have any air pressure.  Git-R-Dun!  Good Luck, TomC
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« Reply #5 on: July 26, 2011, 04:33:07 AM »

What are the results of your DOT air loss test and compressor recovery test?  Over a timed one minute period, what is  your air loss sitting still, engine off, no brakes applied, then sitting still engine off brakes applied full on, then engine running, compressor not charging, no brakes applied (including the parking brake), then finally engine running, compressor not charging, full brakes applied?

What I am thinking is that if your air loss is only when the engine is running and the compressor is charging but the bus is standing still then the air dryer could well be an issue.  But if everything works perfectly except when the bus is actually moving, that points  towards suspension.  If you have an air door, it can use a lot of air when the bus is moving if the motion causes an intermittent leak (I had that).

Brian
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« Reply #6 on: July 26, 2011, 07:30:23 AM »

TomC, I have a AD9 air dryer. Is that what you mean? I just put in a new purge valve. I did not change the dessicant cartridge because there is no moisture when I drain the tanks. Should I replace it anyway, not knowing how long the PO had it in there before me?

Brian, I will do the tests you describe today and report the results.

Looking at the door air claw is a good tip. It is only on when the ignition key is turned on, so a leak  there would explain my problem.

I will post later with what I find today.

Thank you,

JC
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JC
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« Reply #7 on: July 26, 2011, 08:23:29 AM »

JC,
Also chock your wheels and release the brakes and see if you have a diaphragm or valve leaking bad while released which you wouldn't notice when parked.
Grin  BK  Grin

(also could be a weak hose collapsing making the compressor think it needs to build air when it doesn't!) 
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« Reply #8 on: July 26, 2011, 09:00:51 AM »

Thanks BK.

When driving, I watch the air press. go down from 125 lbs to 90 lbs in 3 to 4 mins. Then comp. cuts in and builds back up to 125. So I am thinking that the air press. is actually down as indicated and the comp. is doing the right thing.

Just back from the shop with tests results:

1 minute tests:

engine off, no brakes, parking brake off: 0 lbs loss.

engine off, brakes on full, park brake on: goes down about 10 lbs on brake app., then holds steady with no loss of air.

engine iddling at 700 rpm, no brakes, park brake off: 0 lbs loss.

engine iddling, full brakes, park brake on: down 10 lbs or so on initial brake app., then holds steady with no loss of air.

Recovery from cut-in at 90 lbs to cut-out at 125 lbs at iddle is 2 minutes.

So I suppose it could be in the suspension. How do I diagnose that?

I have to go for the rest of the day now, so I will keep looking tomorrow.

JC
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« Reply #9 on: July 26, 2011, 09:19:53 AM »

All your tests look good except for then compressor recovery time of 2 minutes.  Please try that test again, it must be done at 1000 rpm (high idle is good if you have it, if not guess at 1,000) and it should be in the region of 20 to 30 seconds. 2 minutes is a DOT failure.  My 5C, for comparision, is 18 seconds.  You need to make sure you do it after the suspension is fully up and settled, or the reading will be compromised since the compressor will be trying to fill the suspension, not just raise the pressure in all the tanks.

On the 5C your pressure gauge reads the pressure in the dry tank.  As long as the pressure is above 60 psi to make the protection valve open, it reads the pressure in the whole system including the accessory tanks.

I'm going to guess the leveling valves or a check valve, although it's tough if they only lose air when the bus is moving.  I would start by blocking the bus up and manually exercising the valves to see what they do.  One thing that comes to mind that that there is supposed to be a delay mechanism in them to dampen their response time to sharp bumps.  If there is no delay anymore, that could lead to dumping air with every tiny movement.  They use some kind of viscous liquid to accomplish the delay.

edit - I have a pdf file on the height control valve that I could email to you, it's from a manual but it's clearer than my manual's section on them.

Brian
« Last Edit: July 26, 2011, 10:51:19 AM by bevans6 » Logged

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« Reply #10 on: July 26, 2011, 04:07:30 PM »

***** I just put in a new purge valve.

Sure you put it in correctly?
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« Reply #11 on: July 26, 2011, 08:15:37 PM »

Thanks.

Tomorrow I will redo the recovery test at fast iddle which is about 1000 rpm. I know it pumps faster than at 700 rpm iddle.

Niles500, I suppose one of the O-rings could be kinked. However, my air loss symptoms were the same before renewing the purge valve, and the same since.

I will also wiggle the height control valve rods to see if I get one of them to leak.

Let you know tomorrow.

Thanks again,

JC
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JC
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« Reply #12 on: July 26, 2011, 09:30:48 PM »

The other thing that changes between parked and driving is the shutters...

Confirm that the shutterstat controls are not leaking supply air out via the vent when the temp comes up and it exhausts the air in the shutter circuit.

Should be a short release of the air in the shutter lines, and then that's it.
A constant release will be your air leak.

You can also shut off the air supply at the wall mounted portion of the system and see if that changes anything.

Otherwise, suspension bellows have been well known for having cracks that are in a position that seal when sitting still, but leak every time the bellow moves up and down, exposing the crack.
Leveling valves should not be allowing air consumption worth noting while going down the road.

happy coaching!
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« Reply #13 on: July 27, 2011, 03:49:59 AM »

On my 5C, the shutter-stat is in the left hand vertical coolant line running up from the drivers-side water housing on the engine head.  It has two air lines connected to it, it''s a switch that opens and passes air when the coolant temp is over around 160 degrees.  On the supply side it goes over to a filter with a shut-off valve mounted on the roof of the engine bay just over the passenger side cylinder head, and it's fed from a manifold that is mounted on the rear wall of the engine bay.  On the output side, it feeds the large air cylinders that open and close the baffles below the two main cooling fans above the engine, and it feeds the air motors that open and close the shutters that block off the air coming into the radiators.  I had an issue with the filter housing leaking and disconnected the feed line at the manifold on the wall to block off the whole system.

As BW says, it is a system that will only operate once the bus is driving on the road and gets fully up to temperature.  Even in quite hot weather mine would not operate at an idle, the bus didn't get hot enough and I never idle the bus longer than 10 minutes.  Good idea to check, if yours isn't already blocked off!

Brian
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« Reply #14 on: July 27, 2011, 06:43:52 AM »

 Thanks for the ideas, I'll check that too. Although that whole shutterstat, blower baffles and shutters system is removed on my bus. I will look at where the air supply to it was cut off and how.

Hopefully I find something today.

JC
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JC
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« Reply #15 on: July 27, 2011, 07:03:05 AM »

Buy you a bottle of Big Blu leak detector it will find it for you but most likely you will find the shut down leaking they do some weird stuff

good luck
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« Reply #16 on: July 27, 2011, 07:21:56 AM »

I found my air line going to the purge valve was leaking. I could not hear it very long after the bus was shut down but found it when I ran my had along it while it was running. I was not purging but constantly blowing when running.
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« Reply #17 on: July 27, 2011, 11:50:24 AM »

Just crawled out from under the bus for lunch.

The comp. recovery time at 1000 rpm is down to 1 1/2 mn from 2 mn at 700 rpm. Not great, but better.

I confirmed that it does not loose air while running, just sitting there. It doesn't leak when shut off either.

I then disconnected the levelling valve rods and exercised them up and down and they work as expected, no leaks there.

There is no shutterstat anywhere that I can see. That was removed when the 6V92 was installed.

I also checked the shut down circuit, as well as the fast idle air lines, etc, : no leaks either, but like I said the air press. doesn't go down just sitting still, only when driving down the road.

So now I am thinking about finding all the various check valves to take a good look at them and replace if necessary.

If there is a bad check valve, wouldn't air go back to the compressor while compressing, therefore explaining the slow recovery time? And off course, a partially open check valve would let air out back to the compressor?

I can't think of another way air would leak only when driving.

JC

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JC
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« Reply #18 on: July 27, 2011, 12:22:30 PM »

A bad discharge valve on a compressor will make the compressor circulate air but you say you rebuilt that, it only has one and is under the head not like the unloader valves


good luck
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« Reply #19 on: July 27, 2011, 12:25:09 PM »

JC,
FWIW I just had one acting up several weeks ago and had trouble figuring out the problem. We replaced the drier cartridge and I rebuilt the purge valve. (and dad was doing other things as well).
When we got done it was cycling worse than before we did all this and we went back and re-did things one at a time. All said & done it turned out I accidentally put the purge valve back together backards and once I re-did it our problem went away.

So we really don't know what the cure actually was because we didn't take it one step at a time, but rather just did a group of things we knew needed done anyway.

But in the end we do know that I cost us more time & aggravation by my own mistake of putting the purge valve back together wrong. (as soon as I switched it around and could actually feel the difference it made in feel I knew what I had done!)
Grin  BK  Grin
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« Reply #20 on: July 27, 2011, 02:50:27 PM »

I was just thinking that if your compressor keeps running it will over presser your relief valve on the air dryer and blow constantly. Check to make sure it is not your relief valve is leaking causing it to relieve and not because it is over pressuring. I also just rebuilt mine. your check valve on the dryer will keep the bus aired up and the valve will quit relieving when the dryer runs out of air which will happen quick. There will be no noisy leak after shut down if that is it.
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« Reply #21 on: July 27, 2011, 05:26:05 PM »

The dryer purge valve I replaced was new. All I had to do was put on the O-rings, grease them with the supplied grease and slide it up and put on the 3 bolts. Pretty hard to screw up, although, knowing me, possible, LOL.

I looked at the check valves on the dry air tank. I can just see them, behind the generator enclosure in the drivers side baggage tank... I could possibly turn a wrench on them maybe 5 degrees, if they are not too tight... So I will leave them alone for now.

Since there is no leaks sitting in the shop running or not, I think I will take the bus for a drive tomorrow, and when it starts loosing air, I can stop and at least spray soapy water on all the air connections I can reach in the engine compartment ( shut down cylinder and lines, fast idle, etc). I could have the door claw engage, then off. Trying to narrow it down as much as possible to see under which conditions it leaks, without the wife and kids, trying to get somewhere. Just replacing parts without knowing what is wrong is not the way to go.

I could have my son step on the brakes while I watch the R8 relay valve, and the quick release valves.

I'll take some blocks with me so I can look underneath as well: spray, spray, LOL.

I got to get this resolved,

JC
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JC
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« Reply #22 on: July 27, 2011, 05:37:16 PM »

I am not talking about the purge valve. Check the relief valve. That's what keeps your dryer from over pressuring. Mine hung vertical so the water would set on the ball until it rusted out and started leaking. It should look like a whistle hanging off the side of the dryer.
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« Reply #23 on: July 27, 2011, 05:39:22 PM »

Thanks Eddiepotts, good tip, I'll look into it tomorrow too.

JC
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JC
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« Reply #24 on: July 27, 2011, 05:43:38 PM »

Here is the link to my relief valve problem. http://www.busconversions.com/bbs/index.php?topic=19746.msg215128#msg215128
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« Reply #25 on: July 27, 2011, 06:23:53 PM »

Hey JC one test it looks like you haven't done is, block wheels - release parking brake - apply service brake- check for leaks. the anti compounding valve may be not letting air to the service brakes. you may have a service brake maxi diaphram leaking.  Gerry
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« Reply #26 on: July 27, 2011, 07:02:41 PM »

Gerry, I don't see an anti compounding valve in the maint. manual diagram. I have DD3 brake chambers on the drive axle. Is that why?

I don't have the feeling that the leak is related to the brakes: going down the freeway without brake applications, the air press. goes from 125 to 90 psi in 3 to 5 minutes. But what do I know?

JC
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JC
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« Reply #27 on: July 27, 2011, 09:07:47 PM »

Jc

When I replaced the Allison 740 transmission, Cullen Diesel which did the work said they blocked off a air line that ran to a skinner valve on the trany from  the compressor . They said  it wasn't needed and it was leaking transmission fluid. I don't know if you have the same set up but it might be worth checking. Maybe if you have that valve it might leak air when you put it in drive.

Gary
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« Reply #28 on: July 27, 2011, 11:15:06 PM »

sorry JC didn't realize you had DD3's. Air throttle?
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« Reply #29 on: July 28, 2011, 07:21:32 AM »

  Just replace the screw in check valve in/on the AD9 drier. Leaking back thru  olebusman
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« Reply #30 on: July 28, 2011, 10:03:06 AM »

YEA, I FOUND IT Smiley, Almost...

It is in the air door claw, somewhere between the red valve under the dash and the claw itself at the door.

I just come back from a drive, and the air pressure stays up forever with the red valve off. But leaks down in 3 minutes with it up. So after coffee, it's back to the bus with the soap bubbles. I hope the leak is somewhere accessible, because the PO had the stairs and the wall on both sides of it covered in ceramic tiles, blocking access to the claw, below the pass chair. So if I am lucky, it will be under the dash or in the spare tire comp. Will see.

Later,

JC
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« Reply #31 on: July 28, 2011, 10:47:30 AM »

Good for you, the logical progression of alternatives.  I mentioned the air door possibility earlier, because I had the exact same issue though not as bad as yours.  I had to flip the red cut-off switch as soon as I parked too.

Brian
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« Reply #32 on: July 28, 2011, 11:08:24 AM »

So simple!! Once you find it, LOL.

As soon as I bent over to crawl into the spare tire compartment, I could hear it hissing. Off course I've been in there before, but the door claw was never pressurized. And you can't hear it from above while driving.

It is the line from the door air cylinder quick release valve to the air door cylinder itself. Big hissing hole in the middle of it. I didn't need soap bubbles to find it! So off to town after lunch to have a new one made up. It will be a challenge threading the new hose fitting onto the back of the cylinder, really tight access through the hole alongside the claw, but doable.

I feel great about my bus again.

A big thank you to all of you that contributed with tips and ideas and moral support.

This board is so great.

JC
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JC
Invermere, BC
1977 MC5C, 6V92/HT740
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