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Author Topic: Air compressor stopped suddenly  (Read 3963 times)
Sam 4106
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« on: July 07, 2013, 08:40:58 AM »

The air compressor on our 8V92TA DDEC II suddenly stopped working. I aired the system with the shop compressor and can hear no leaks back through the engine compressor nor from the air drier. The dash gauge holds steady at 110 PSI. I replaced the air governor since it is easy to change and I have one. Didn't work. I removed the discharge line from the compressor and started the engine after bleeding the pressure down. No air coming out of the discharge. What should I try next? The engine has 99,557 since new and judging by the other things that were done at the time of the engine change, the compressor was new or rebuilt too.

Can I remove a discharge valve and tell if the compressor is even turning? My guess is that the fiber drive disc is worn out and the compressor is not turning. I want to confirm that, if possible, before I remove the compressor. Since the PO didn't leave access through the floor, it is going to be difficult to remove the compressor.

Thanks for any advise you all can give, Sam
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1976 MCI-8TA with 8V92 DDEC II and Allison HT740
Scott Crosby
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« Reply #1 on: July 07, 2013, 08:59:41 AM »

Is the unloader stuck?

Take off the air intake line.  And view the unloader position.
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61 GM Fishbowl TDH 4516 102" 35'
1947 GM PD 3751
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Scott Crosby
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« Reply #2 on: July 07, 2013, 09:04:43 AM »

Air compressor unloader


When the governor sends air at 120psi (or what ever you have your pressure set to) this opens and closes allowing the compression from the pistons in the compressor to either build or not.   Basically it opens a door on top of the head so no pressure can build, when the door closes pressure can then build. 
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61 GM Fishbowl TDH 4516 102" 35'
1947 GM PD 3751
www.busgreasemonkey.com
Sam 4106
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« Reply #3 on: July 07, 2013, 10:11:16 AM »

Scott,

Which part of the video shows the unloaded position, and which would be the stuck position? I have never had a compressor apart so I am totally ignorant about how the unloader works. I have looked at both the maintenance and parts manuals but don't yet understand  how the unloader functions.

Your help is appreciated, Sam
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1976 MCI-8TA with 8V92 DDEC II and Allison HT740
Scott Crosby
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« Reply #4 on: July 07, 2013, 10:46:40 AM »

I tested that with shop air.  The unloader holds the inlet valves up off their seats to stop compression via air from gov cut out.  The beginning of the video is unloaded and I am draining air to get back down to the cut in pressure and then the valves pop back to the loaded (build air) position.   Shop air build and release it will pop up and down.  You don't have to have the engine running to test that system.
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61 GM Fishbowl TDH 4516 102" 35'
1947 GM PD 3751
www.busgreasemonkey.com
Purplewillie
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« Reply #5 on: July 07, 2013, 12:11:43 PM »

I just went thru some of this with my 4905. As long as your shop air compressor has enough pressure, undo the intake air on the front of your compressor so you can see the unloader work as in Scott's video. Start airing up your coach , at whatever your gov is set to for cutout( mine is just over 130 psi) the unloader should pop up.  After they do, drain off some air , and they should go back down so the compressor would pump air again. Mine is about 100+ psi
Mark
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Mark & Char
1976 P8M4905a 8v71 v730
British Columbia Canada
Purplewillie
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« Reply #6 on: July 07, 2013, 12:11:53 PM »

Double post Huh
Please delete
Mark
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Mark & Char
1976 P8M4905a 8v71 v730
British Columbia Canada
luvrbus
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« Reply #7 on: July 07, 2013, 02:02:16 PM »

Sam they don't have a fiber gear you probably sheared the key on the compressor drive would be my guess if it is making no air period they will always produce some air if turning
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Sam 4106
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« Reply #8 on: July 07, 2013, 02:32:10 PM »

Scott,

I did as you suggested and the unloader works as you describe. Then I took out a discharge valve, pulled the fuses feeding the DDEC, and had my wife crank the engine without it starting. There was no movement of the compressor piston. Anything else I should check before I start pulling the compressor tomorrow afternoon after my medical appointments? I'm guessing I will need to drain the antifreeze, pull the compressor, make sure the compressor is reusable, replace the key, replace the compressor, put the antifreeze back in, and see if the compressor pumps air. Anything I've forgotten?

luvrbus,

Have I done any damage to the compressor drive or can I just replace the key? This is turning into an ordeal. Luckily, the bus is in my shop and not on the shoulder of some road.

Thanks, Sam
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1976 MCI-8TA with 8V92 DDEC II and Allison HT740
luvrbus
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« Reply #9 on: July 07, 2013, 02:55:02 PM »

No the key is on the compressor shaft if the compressor turns freely just replace the key and torque the nut be sure and check for water in the head of the compressor that is usually why that key shears fwiw, that is a nice easy compressor to remove on a MCI lol I feel for you Tongue 
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wg4t50
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« Reply #10 on: July 07, 2013, 04:00:59 PM »

Confirm the lol replacing the compressor on the MCI, i think I was speaking in tongue by the time  I got one back in thru the hole in floor.
Dave M
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Central Virginia
Sam 4106
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« Reply #11 on: July 07, 2013, 04:16:21 PM »

I have studied the MCI 8 maintenance manual and can't find a torque spec for the crankshaft nut. Any thoughts? Is the key something special, or can I use one from the hardware store? There was no water in the cylinder I looked in, but I will check the other one too. Will I need to remove the power steering pump to get the compressor out? Any other helpful hints? Anyone want to come and help me with this project? LOL

Thanks everyone, Sam
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1976 MCI-8TA with 8V92 DDEC II and Allison HT740
Dave5Cs
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« Reply #12 on: July 07, 2013, 05:42:13 PM »

No but the PS tank removal make it a bit easy. Brian told about that.

Dave5Cs
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« Reply #13 on: July 07, 2013, 05:55:16 PM »

I am definitely not an expert.  I had a oil leak on my compressor. I didn't remove the cooling lines.  I was able to pull compressor back far enough to be able to build a gasket and replace the compressor.  I have a MC-9 with the poo tank removed.

John
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John Riddle
Wells NV
1984 MC9
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« Reply #14 on: July 08, 2013, 06:10:48 AM »

I tried the easy fix solution.  On my GM I had to drain the coolant because I have hard copper lines plumbed to it.  It's probably best to pull it all the way off to inspect and clean everything.  I would replace the unloader kit at the same time since you are going to have access.  It's only about $20.  I think pulling the compressor on an MCI is just as easy as on a GM but with one extra step - pulling the engine Smiley  Where are you located at, maybe someone can come help - hopefully a skinny but strong, double jointed bus mechanic. 
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61 GM Fishbowl TDH 4516 102" 35'
1947 GM PD 3751
www.busgreasemonkey.com
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