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Author Topic: Air problems version 2  (Read 1296 times)
Jnbroadbent
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« on: May 05, 2013, 05:38:12 PM »

Started up and moved the bus on my yard about 2-3 works ago, no issues. Parents were in town so I started it up. As I did I walked away from it for 20 mins, came back and the psi was 0. Loud hissing from the front end between the tires. Looks like a filter? Briefly looked at the manual to start troubleshooting but it got too dark. Not sure if it aired up and something let loose or whatnot. Had to cut off fuel to kill the engine

I'll diagnose this week but maybe some input/advice from seasoned guys will put me in the right place. Obviously I will hook up shop air and some soapy water.
« Last Edit: May 05, 2013, 05:40:26 PM by Jnbroadbent » Logged

Jon
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« Reply #1 on: May 05, 2013, 05:53:17 PM »

The air dryer is located there. I would say you have a stuck relief valve. Could use a good cleaning.

John
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John Riddle
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« Reply #2 on: May 06, 2013, 03:19:13 AM »

See if you can locate where between the front tires the hissing is coming from.  The air dryer is mounted on the front wall of the bay, and it's purge valve can certainly stick open.  The wet tank and the emergency brake tanks are mounted on the rear wall of the front axle bay, and the wet tank has a 150 psi relief valve that might have blown and stuck open, in 20 minutes unattended running.  Also mounted on the rear wall of the front axle bay, mounted between the two tanks, are a filter and the pressure protection valve.  It's not likely but possible that one of those has failed.  Last thing that may have happened is a rust hole in something, most likely the wet tank.

Brian
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« Reply #3 on: May 06, 2013, 06:16:55 AM »

See if you can locate where between the front tires the hissing is coming from.  The air dryer is mounted on the front wall of the bay, and it's purge valve can certainly stick open.  The wet tank and the emergency brake tanks are mounted on the rear wall of the front axle bay, and the wet tank has a 150 psi relief valve that might have blown and stuck open, in 20 minutes unattended running.  Also mounted on the rear wall of the front axle bay, mounted between the two tanks, are a filter and the pressure protection valve.  It's not likely but possible that one of those has failed.  Last thing that may have happened is a rust hole in something, most likely the wet tank.

Brian

Brain
Good post I now remember that I had valve on one of my tanks up there fail as you describe. Fuzzy memory for some reason LOL

John
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Ed Hackenbruch
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« Reply #4 on: May 06, 2013, 06:31:05 AM »

 About 5 years ago, (where does the time go!) my pressure protection valve started leaking. Took it apart but it was beyond fixing so i called Luke and got a new one. He said that he had a bunch of them sitting on the shelf and had not sold any for several years but that i was the 4th person in 2 weeks that had bought one from him. When i  put the new one in i added a cable pull drain valve to it as the cause of my other failing was due to moisture over the years. I don't have an air dryer. I also put cable pulls on all of my tanks to make it easier to drain them.   
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Jnbroadbent
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« Reply #5 on: May 06, 2013, 07:09:01 AM »

See if you can locate where between the front tires the hissing is coming from.  The air dryer is mounted on the front wall of the bay, and it's purge valve can certainly stick open.  The wet tank and the emergency brake tanks are mounted on the rear wall of the front axle bay, and the wet tank has a 150 psi relief valve that might have blown and stuck open, in 20 minutes unattended running.  Also mounted on the rear wall of the front axle bay, mounted between the two tanks, are a filter and the pressure protection valve.  It's not likely but possible that one of those has failed.  Last thing that may have happened is a rust hole in something, most likely the wet tank.

Brian

Thanks everyone. I found old posts of yours explaining all the different tank, locations, and purposes. I believe the wet tank was replaced as it looks rather new.

I will give a go at it today.
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Jon
1980 Mc9 w/ veg oil
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Jacksonville Fl
Jnbroadbent
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« Reply #6 on: May 06, 2013, 05:47:06 PM »

Dug a hole behind the front rear tire (where the leak was the most present to my ear) and the leak is most definitely coming from the dryer. If the governor tells the dryer when to purge, is it possible that it's bad? Couldn't really diagnose much today due to some company.

EDIT: Troubleshooting #22 in the air section of the manual should at least get me in the right direction  Wink
« Last Edit: May 06, 2013, 05:58:24 PM by Jnbroadbent » Logged

Jon
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« Reply #7 on: May 06, 2013, 08:25:21 PM »

Jon,

I'd get my hands and eyes on the leak to tell where its coming from.  That will give you a better idea of what needs repairing.

For about 15$ or less and maybe 30 minutes of work, I'd replace the governor too while you are at it.

Please be safe and block the bus if you are going underneath.  Watch out for your eyes and fingers too!

-Sean


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« Reply #8 on: May 07, 2013, 04:35:55 AM »

The governor has two lines, a sense line and a signal line (at least that's what I call them).  On a later MCI like yours, the sense line comes from the accessory part of the air system, which starts after the Pressure Protection Valve.  After the air system comes up past around 65 psi, the PPV lets air through.  Once it gets to the set pressure of around 120 PSI, the pressure on the sense line overcomes a spring, a shuttle valve inside the governor moves and air is sent to the air dryer purge valve on what I call the signal line.  The air dryer purges and empties the air inside it and in the main feed line from the compressor to the air dryer.  So, while the governor could be bad it really requires air pressure to work at all, and you say you have no air pressure at all.  The purge valve opens when the signal line sends it pressure, and stays open until the signal line takes away pressure and tells it to close.  Regardless, in the absence of pressure on the signal line, it's supposed to close.  There is also a check valve on the output of the air dryer to the wet tank, it's job is to retain air inside the air system while the purge valve is open.  That could have failed also, but from your symptoms it's probably not the main thing going on.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
Jnbroadbent
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« Reply #9 on: May 07, 2013, 04:01:28 PM »

Jon,

I'd get my hands and eyes on the leak to tell where its coming from.  That will give you a better idea of what needs repairing.

For about 15$ or less and maybe 30 minutes of work, I'd replace the governor too while you are at it.

Please be safe and block the bus if you are going underneath.  Watch out for your eyes and fingers too!

-Sean


[url=http://www.herdofturtles.org]www.herdofturtles.org [/URL]
1984 Eagle Model 10S


Shoot might as well! 12.99 at napa,

The governor has two lines, a sense line and a signal line (at least that's what I call them).  On a later MCI like yours, the sense line comes from the accessory part of the air system, which starts after the Pressure Protection Valve.  After the air system comes up past around 65 psi, the PPV lets air through.  Once it gets to the set pressure of around 120 PSI, the pressure on the sense line overcomes a spring, a shuttle valve inside the governor moves and air is sent to the air dryer purge valve on what I call the signal line.  The air dryer purges and empties the air inside it and in the main feed line from the compressor to the air dryer.  So, while the governor could be bad it really requires air pressure to work at all, and you say you have no air pressure at all.  The purge valve opens when the signal line sends it pressure, and stays open until the signal line takes away pressure and tells it to close.  Regardless, in the absence of pressure on the signal line, it's supposed to close.  There is also a check valve on the output of the air dryer to the wet tank, it's job is to retain air inside the air system while the purge valve is open.  That could have failed also, but from your symptoms it's probably not the main thing going on.

Brian


I did verify it was the purge valve but I could quite get to it with the sunlight fading. Tried knocking it with a big stick but no avail. I can kind of push the plunger up but not sure if something is causing it to seize or what not. Tomorrow I will surely get to it

Ran some shop air at 120psi and tapped (eventually banged) on the governor to see if it was stuck and to unstick it. Nada.
« Last Edit: May 07, 2013, 04:49:55 PM by Jnbroadbent » Logged

Jon
1980 Mc9 w/ veg oil
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Jacksonville Fl
Jnbroadbent
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« Reply #10 on: May 12, 2013, 05:32:13 PM »

Replaced gov, waiting for air purge rebuild kit to arrive.

Got under there today, tightened the bolt on the purge valve about 1/2 turn and it aired up 120 psi. Shut it down then went to move it and it's leaking so back to 0 psi! . I just want to move the damn thing!
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Jon
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Jacksonville Fl
Dave5Cs
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« Reply #11 on: May 12, 2013, 07:34:35 PM »

You need an air dryer rebuild. Get the desiccant cartridge also. You will have to take it out anyway to get to the purge valve. I would bet it is wet and plugging the purge open.

Dave5Cs
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