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Author Topic: Air compressor stopped suddenly  (Read 3455 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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« 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'
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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
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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'
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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
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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
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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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« 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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« 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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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
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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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Sam 4106
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« Reply #15 on: July 08, 2013, 09:04:29 AM »

Scott,

I live at La Crescent, MN but my comment about someone helping was made tongue in cheek. I got up early this morning and got the coolant drained and some of the lines off before my medical appointments. I will get back at the job after lunch. It looks like the worst part will be getting to some of the mounting bolts and putting them back will be even harder. I enjoy challenges, so this is just another one. My neighbor is good about giving me a hand when needed so that helps too. I will take your advise about getting a new unloader kit. I just hope I don't need a rebuilt compressor.

Thanks for your help, Sam
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1976 MCI-8TA with 8V92 DDEC II and Allison HT740
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« Reply #16 on: July 08, 2013, 11:05:24 AM »

I just hope I don't need a rebuilt compressor.
Thanks for your help, Sam 

     Clifford's suggestion that absolutely NO air build up points towards a drive problem sure makes sense to me.  I'm hoping that a new key and properly torqued shaft nut will fix it up for you. 
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
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« Reply #17 on: July 08, 2013, 05:14:19 PM »

I got the compressor off and found the problem. It is a drive problem, but not a sheared key. The drive coupling on the compressor is made of a bakelite like material around a steel center, with a key slot, that goes on the crankshaft. The coupling is slipping where the bakelite part was attached to the steel center. It took me about 3 hours to remove the compressor from the engine after I had the coolant drained. One bolt and one coolant line were a bitch to get out. Otherwise not too bad. I will make a couple of modifications to make it easier to put those items back together. I will use two short studs to help align the compressor with the gear housing once it is in place. I called Curt at Central Truck and he has the coupling. Hopefully I can get everything back together so we can take the bus to a Midwest Bus Nuts rally on Friday.

Thanks everyone for your thoughts, Sam
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« Reply #18 on: July 08, 2013, 05:50:28 PM »

Did you find all the pieces of bakelite?  That may not be good inside engine.

John
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Sam 4106
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« Reply #19 on: July 08, 2013, 06:18:56 PM »

John,

The bakelite is intact except where it contacts the steel hub's splines. It seems to be worn smooth as if the internal splines of the bakelite sheared off from the center hub. There is only a small amount of residue inside the bakelite coupler. The engine ran only a short time after the compressor quit pumping air. Its almost time for an oil change anyway so I think I will change it after I get the compressor back on. Or at least have the oil analyzed.

Thanks, Sam
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« Reply #20 on: July 08, 2013, 06:35:24 PM »

I think I read once that those drive gears come with different number of teeth so you may want to research that or make sure the one you get is correct.  I might be wrong but thought it was worth a mention.
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« Reply #21 on: July 08, 2013, 07:52:27 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

Mine had a fiber gear. The compressor seized on I-70 in Kansas a few years ago and broke the gear into a few pieces.

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Craig Shepard
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« Reply #22 on: July 08, 2013, 07:58:36 PM »

The bakelite is intact except where it contacts the steel hub's splines. It seems to be worn smooth as if the internal splines of the bakelite sheared off from the center hub.

Umm, you might want to make sure your compressor still turns before you put a new gear on it and reinstall it. The gears break because the compressor locks up.




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Craig Shepard
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« Reply #23 on: July 08, 2013, 08:06:43 PM »

Yours must be a 71 series Craig most of those had the fiber gear drive the 92's have a steel drive gear with the coated spline coupling you guys from Mn must be tough on compressor drives John Z lost one in Texas a few years back
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« Reply #24 on: July 09, 2013, 02:23:47 PM »

Yours must be a 71 series Craig most of those had the fiber gear drive the 92's have a steel drive gear with the coated spline coupling you guys from Mn must be tough on compressor drives John Z lost one in Texas a few years back

6V92

Pretty sure Sam's is a 92 series, too.

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Craig Shepard
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« Reply #25 on: July 09, 2013, 03:16:22 PM »

I think we are talking about 2 different things here Craig I call the the drive gear the one that bolts to camshaft 4 bolts sorta of square the coupling goes into never saw a fiber drive on 92 series but anything is possible on these buses   

The old Detroit's in a GM that drive was fiber so if it broke and fell on the gears no harm was done power steering pumps (Vickers) still use fiber flex coupling you find those in oil pans all the time with ground fibers nothing to be alarmed about it's by design
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Sam 4106
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« Reply #26 on: July 09, 2013, 05:29:57 PM »

When I was cleaning parts and bolts, I noticed that the mounting bolt threads have some type of sealant on them. Any suggestions as to what type sealant to use when I reinstall the bolts? I will be putting the compressor back in tomorrow.

Thanks, Sam
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« Reply #27 on: July 09, 2013, 08:22:32 PM »

I think we are talking about 2 different things here Craig I call the the drive gear the one that bolts to camshaft 4 bolts sorta of square the coupling goes into never saw a fiber drive on 92 series but anything is possible on these buses   

The old Detroit's in a GM that drive was fiber so if it broke and fell on the gears no harm was done power steering pumps (Vickers) still use fiber flex coupling you find those in oil pans all the time with ground fibers nothing to be alarmed about it's by design

Ok. I was referring to the gear on the compressor side. That's what broke on mine when the compressor seized.

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Craig Shepard
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« Reply #28 on: July 10, 2013, 09:52:16 AM »

Sam,
I use Loctite Blue medium strength.
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Sam 4106
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« Reply #29 on: July 10, 2013, 10:57:59 AM »

Thanks Lee,

I already had the compressor installed before I read your message. I used a Permatex product as a thread sealer since I didn't know if what was on the threads was a sealer or to prevent the bolts from loosening. The product I used should do both, as would loctite. The label is worn off the tube so I'm not sure exactly what it is, but it is red. I was able to get all but 1 gallon of coolant back into the system, so I must have an air bubble in there somewhere. I bled some air out the petcock on top of the thermostat housing. I will warm the engine up and see if the coolant level drops so I can get the last gallon in. Otherwise putting the compressor back in went well, but took about 5 hours. Good thing I could do the job myself, otherwise it would have gotten expensive. I hope I never have to do that job again.

Thanks Sam
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« Reply #30 on: July 10, 2013, 02:57:28 PM »

   I already had the compressor installed

      Was the input shaft on the compressor turning OK?  (before you re-installed it, of course!)
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
6-cyl, 4-stroke, Leyland O-680 engine

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« Reply #31 on: July 10, 2013, 03:28:49 PM »

Hi Bruce,

Yes the compressor was turning freely, Curt also checked it. I also checked the rods for looseness when I had the crankcase cover off to replace the leaking gasket, they are tight. When I cleaned the compressor I found a rebuild tag. I have no idea what the average life of a compressor is, but I suspect the PO put on a rebuilt one when he put the new engine in about 99,000 miles ago. My air system now has few leaks so the compressor goes about 20 minutes before the pressure drops to 90 PSI and has to work again. Oftener in traffic with a lot of braking.

Thanks, Sam
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« Reply #32 on: July 10, 2013, 11:48:46 PM »

Hi Bruce,

Yes the compressor was turning freely, Curt also checked it. I also checked the rods for looseness when I had the crankcase cover off   

    Good news - hope all is well from here out!   BH   NC    USA
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
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« Reply #33 on: July 11, 2013, 05:46:39 AM »

"I have no idea what the average life of a compressor "

Neither do I.  A cousin bought an old road tractor with well over a million miles.  He didn't know when the compressor was last replaced, but he drove it another 500k.  Said compressor was getting a little weak when he sold the truck.
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Jim Keefauver/1985 Wanderlodge PT36/6V92TA/MT654CR/East Tn.
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« Reply #34 on: July 11, 2013, 06:26:42 AM »

Compressor life all depends on how one is serviced,intake and duty cycles very seldom do people service one Bendix says service the unloader valves at 250,000 miles most just buy a rebuilt compressor when it quits

I under stand not checking the the drive coupling it's a hassle on most buses except the GM's  same thing with a gear driven alternator those are never checked till it's to late
« Last Edit: July 11, 2013, 07:14:05 AM by luvrbus » Logged

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