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Author Topic: Intermittent AIR Problem  (Read 4795 times)
Nellie Wilson
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« on: January 14, 2010, 01:54:33 AM »

Hi Guys -

(This is a copy of my BNO board. If don't spend time there, you might not get the 'inside jokes' - but I'll be happy to explain Smiley)

Back again, and same old problem. I was sailing smooth, not a hitch and happy as a clam. Did Jack's Bussin' 10 (what a great rally!) and - on the way back to his place (for the BYOB thingy) - I stopped for some much needed fuel.

Fired her up and... no air pressure. Had enough to safely move away from the pumps, but nothing (revving,tapping, banging, screaming, begging. cursing) would load the compressor.

Oh, now and then the needle would rise a few pounds but it would always stop short of 'driveable' pressure.

Jack, BK and Tom (White Eagle) came over to assist, but nothing worked. So we filled it with shop air in hopes of nursing it back to Jack's.

Well, halfway there (here) it started making air again. Right up to 160 psi when we shut her down (bleeding it to 90 psi first).

Since being here, she makes air every time we crank her up. And - like Jack says - how do you diagnose a problem when everything is working?

So, consensus of opinion is: Got to be 1) the unloader valve or 2) the governor (and we all know how well I get along with governors  Cry).

Current thinking is: Just replace both to be safe (and keep the "old" ones as spares). Any other thoughts?

The governor is an easy switch (except in White River Junction) but changing the unloader requires some major surgery.

So if there's something we're missing, I'd sure like to know before lighting the ol' blowtorch.

Thanks much,

Nellie Wilson
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« Reply #1 on: January 14, 2010, 02:54:25 AM »

I think you have been given some good advice. Had that happen on my first coach, and took it to the mechanic who replaced both ($240). Now that was a few years ago!!!

Mine was a little different in that I was not sitting at the fuel pumps, I was driving, and watched as my air went down. Buzzers came on, I pulled over and smacked the compressor with a hammer. It began to work again, but air is no play thing on a bus.

Repairs were done very quickly, but I learned how something NOT major can shut you down, if you are not careful. I was more concerned about hammering the spring brakes, and being stuck in the middle lane of I-10 in rush hour!  Shocked
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luvrbus
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« Reply #2 on: January 14, 2010, 04:05:52 AM »

Nillie, if you have a air dryer that is their way of saying I need attention also.


good luck
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gumpy
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« Reply #3 on: January 14, 2010, 04:42:12 AM »

While the dryer may need a rebuild, the problem is probably the governor. It most likely stuck.  It's about $15 to replace. You could probably tear the old one down and rebuild and lube it, but the idea of having a spare is not a bad one.

However there is one other concern. You said your bus built air to 160 lbs. I hope that's a typo, or some metric number, because that's dangerously high and indicates even more problems if correct. Please clarify.

craig
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Craig Shepard
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« Reply #4 on: January 14, 2010, 04:47:06 AM »

Are MCI's different than most I thought the relief valve would pop at 130 # on the tank ? they do on a Eagle



good luck
« Last Edit: January 14, 2010, 04:49:39 AM by luvrbus » Logged

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JackConrad
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« Reply #5 on: January 14, 2010, 04:55:00 AM »

However there is one other concern. You said your bus built air to 160 lbs. I hope that's a typo, or some metric number, because that's dangerously high and indicates even more problems if correct. Please clarify.
craig

BK had tried to re-adjust the governor to get it to start pumping air, which it would not do. On the drive to our place it suddenly started pumping and with the governor at the new setting it hit 160.  Governor is now re-set to 120.  We know compressor will pump up and we hear no obvious air leaks.  Problem seems to be in compressor control (governor and unloader).  On her MC-5, unloader is located directly under a floor framing mamber with about less than 1" clearance.  Jack
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« Reply #6 on: January 14, 2010, 05:18:12 AM »

For those that don't have it here is a good place to go for trouble shooting a air system

http://www.bendixvrv.com/itemDisplay.asp?documen... 


hope I got it right if not just go to the bendix site




good luck
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gumpy
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« Reply #7 on: January 14, 2010, 05:19:37 AM »


BK had tried to re-adjust the governor ...


Oh, well, that explains it!!  Roll Eyes

Curious why the pressure reliefs didn't blow at 130, though. Something's not right there. I'm pretty sure there are supposed to be some reliefs somewhere in the system.

Rebuild the governor and check on the relief valves. It's quite possible the governor is gummed up and not shuttling properly. If it's stuck in the unload position, the unloader valve will be open continuously. Yeah, it's possible it's the unloader valve, but it's more likely the governor.

Mine did that several years ago. Only a problem when it was cold and I first started up. I could smack it with a screwdriver handle and get it to work. Once it started working it was fine till I shut down and it got cold again. I bought a new one, but then cleaned and lubed the old one and put it back on. No problems since. The new one is my spare, now.
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Craig Shepard
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« Reply #8 on: January 14, 2010, 05:27:22 AM »

At a 160 # it should have sheared the compressor drive key also she got away with one the lucky dog


good luck
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bevans6
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« Reply #9 on: January 14, 2010, 06:03:31 AM »

from the MC-5C manual:  safety valve is set to 150 psi.  it's adjustable, by adjusting the preload on the internal spring that loads the valve ball.  there is a pin that sticks out of the body, you can pull on the end of the pin to unload the ball and force it to exhaust.  If you pull on the pin and it doesn't exhaust, take it apart and clean it, and set it to whatever pressure you desire (standard is 150 psi) by adjusting the preload nut.  There are no  rubber seals to go bad, just steel balls and seats and such.  It's on the wet tank, I do believe.  the dash gauge is probably not all that accurate, but  it wouldn't be impossible to just whip the thing out and test it if you think it's bad, if you had a source of 150psi air.  here's a good one "leakage at the exhaust port should not exceed a 3" soap bubble in three seconds"...   Grin

On the basic problem, I would change the governor, since it's job is to tell the unloader when to unload, and to tell the air dryer when to purge and all of that.  It's the guy in the middle.  You could have had a frozen/blocked signal line from the wet tank to the governor that would have caused exactly your symptoms.  

I hope this helps a bit.  As additional MC-5C air system trivia,  the dash gauge reads from the dry tank while the low air warning buzzer, light, and the generator cutoff switch read from the accessory tank.

cheers, Brian.  You ever want anything from the official MC-5C manual, ping me and I'll look it up for you.  I've read it cover to cover now anyway, but I seem to be trying to slowly memorize it...
« Last Edit: January 14, 2010, 06:08:36 AM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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« Reply #10 on: January 14, 2010, 06:36:03 AM »

I don't understand about the 150 psi  setting there are a lot of Bendix valves in the system that the 150 psi exceeds their working pressure.
And I am almost positive MCI and Eagles use the same Bendix valves


good luck
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Ed Hackenbruch
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« Reply #11 on: January 14, 2010, 06:56:04 AM »

Clifford,  they may use the same valves, but everybody knows that everything is better in an MCI!  lol.   Grin
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« Reply #12 on: January 14, 2010, 06:58:55 AM »

Give me a break Ed or I'll send this rain south to Yuma


good luck
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bevans6
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« Reply #13 on: January 14, 2010, 07:00:38 AM »

I have no idea about the 150 psi and other valves, all I have is page 4-20 in the MCI manual to go on.  My air brake safety course also said the DOT specification for safety blow-off is 150 psi, so the number came up twice.  Maybe they picked 150 psi exactly because that is where the other components reach their maximum operating pressure, so as not to exceed that?

this is the valve in the MCI manual, the ST-1:  http://www.bendixvrc.com/itemDisplay.asp?documentID=4047

the bendix datasheet suggests 150 psi for the pressure setting, with a cut-out pressure of 125 psi on the governor.  I suspect the reasoning is that the safety valve won't reseat until pressure drops to 135 psi, and they want to maintain a 10 psi headroom between those two points. 

Brian
« Last Edit: January 14, 2010, 07:05:30 AM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
Ed Hackenbruch
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« Reply #14 on: January 14, 2010, 07:06:53 AM »

Rain? ...you got rain up there?   Really?  clear skies and 85 degrees here in the Foothills yesterday, and clear skies again this morning.  Only had rain one day since we got here on the 27th of Oct. 
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1968 MCI 5A with 8V71 and Allison MT644 transmission.  Western USA
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