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Author Topic: air compressor cycling every 5 minutes, MC 5C  (Read 3303 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
Invermere, BC
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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
Invermere, BC
1977 MC5C, 6V92/HT740
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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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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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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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lostagain
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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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lostagain
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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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JC
Invermere, BC
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bevans6
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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

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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niles500
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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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lostagain
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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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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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1972 NTM MK-4 B/SR
lostagain
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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
Invermere, BC
1977 MC5C, 6V92/HT740
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