'74 mc-8 for a while it took a real long time to get air then everything was fine. Last time I started it I got no air, hooked it up to the shop compressor still no air. I'm pretty sure I have a check valve stuck but I tried looking before it got dark tonight and had no luck. I have a manual in pdf but I can't read that particular schematic. First if anybody knows where to find the check valves that would be great. Second if anybody had any other ideas that would be great to. Thanks!
You say no air. Where are you checking for air pressure, "at the gauge?". The gauge is connected to the accessory tank (the last tank to get air). Where are you connecting the shop air to the bus? Have you tried opening the drain valve on any of the other tanks to see if there is air pressure there? I would start with the ping tank (first tank after the air leaves the compressor). If no air here, problem is in compressor/govenor. If air pressure at ping tank, go to wet tank, etc. Hope this helps, Jack
For the moment I can't run it so I'm just looking for ideas. I was giving air to it through the valve in the engine bay. If I bled it back from that point it seemed as though I just had a line full vs a tank. I need to get my current project done so I can move onto blocks. Any other ideas are welcome, and if anyone has a diagram they could most of the air system that would be great also.
Mine was doing the same thing and it turned out to be the pressure protection valve by the front tanks it was alot of joy getting to it ,took four days to jack it up high enough to crawl underneath it . The valve itself was only about $35.
Bob Chvyman
If you have an air drier, and the purge valve sticks (or freezes) in the purge/air compressor cut-out position, you will get no air past it. The fill line in the engine room is ahead of the air drier, so won't help.
Long steel rod against the bottom part of the air drier in through a wheel well and whack it with the hammer might convince it to move... you'll want to do a tear down to prevent it from re-occurring.
happy coaching!
buswarrior
I got it running this evening so I have some more info. It pumps up very slowly purging every minute or so and faster on high idle. The fill line in the engine bay is running 130psi. The parking brake will disengage, the driving brakes have good feel. The dash gauge shows low and the low air buzzer is going off, until the dash gauge hits 90psi wich takes at least 30 min. The purging continues but I seem to have good air pressure every where except the gauge and the buzzer. I don't get it. But I will assume that the dryer is to blame?
If your gauge and buzzer come off the accessory tank, maybe you should look for a problem there: stuck check valve, leaks in tank, leaks in lines?
The purging cycle is triggered by the governor on the compressor from what i understand. You might want to try that first. It is probably best if you service the drier also. On the semis I run we service the dries about every 300,000 miles or so. Who knows when yours was last done. after all that you might want to look into the pressure protection valve.
Most of these things you can bypass. That would be the best thing for trouble shooting.
More forensics required before intervention.
You need to observe whether the pressures are cycling in the "good" parts of the system in any sort of relationship to this frequent purge of the air drier.
Install suitable fittings in order to use some kind of pressure gauge.
A suggestion: Regular male airline fittings installed in each air tank drain allows both a shop airline or a gauge with a female connector to be quickly and easily installed, both in the driveway, and out on the road during "pneumatically challenged" moments.
If the pressures are cycling up and down by 20 - 25 pounds, so in your case, up and down from 105 to 130, then the governor and air drier are doing what they are intended to do. You really want a reading off the wet tank.... hmmm, in a 1974 MC8.... do you know which tank? My book is temporarily MIA....
Just because the brakes function, that is no indication as to how much air pressure might be behind them.
Next question is: Is the air leaking out? Where, and how heavy?
brake valves, air lines, air suspension, radiator shutters/dampers, air wipers, whatever else you have that has been plumbed or unplumbed, now or in the past, rightly or wrongly, by previous owners or the manufacturer.
I wouldn't be worrying about carving into the system or spending money using the shotgun parts changing approach until you have thoroughly and inexpensively evaluated where and at what pressures the air is moving around inside, or escaping, the system.
happy coaching!
buswarrior
Yeah I'm going to check it out more tonight. I made some blocks so I can get under it and check out the dryer. I like your idea of valves on the tanks! I have no leaks that should be causing any problems other than the frequent purging action. I have never approached anything with the "shotgun parts changing" method. If anything I'm guilty of not buying parts if I can get it to work with out.
Another weird thing happened. To make things fast I filled the emergency fill for the parking brake, parking brake would not disengage. Hooked air to fill in engine bay gauge moved fairly quickly and stopped at just over 30psi. Started engine to make air and nothing was happening according to the gauge. Checked it a couple minutes later and I had 75psi. Watched my gauge for a while and nothing was happening so I started hitting the brake peddle over and over again it ran the dash gauge down to 25psi before it stabilized. After that it would not make pressure at all, I still had shop air hooked up. So I blocked the bus up and crawled under it and the purge valve is not going off its the pressure protection valve and it is blowing oil and water all over. So...can the purge be blocked making a reservoir of oil and water blocking some of the air ports? A couple hours later I crawled under and pulled the pressure protection valve and it ran a good stream of oil water out.
Chances are you have a plugged ( usually with carbon) discharge line or ping tank. disconnect the air discharge line at the ping tank and start the engine. If their is lots of air coming out go to the next connection until you find the blockage.
Gerry
ok, we have trouble...
Take some pictures or call the components by their proper names or this is just a jumble of BS.
The "purge valve" - is this at the bottom of the air drier or somewhere else?
The real "pressure protection valve" does not have facility to drain anything. It is sealed, with an adjustable cap that screws up or down to change the pressure at which it allows air deeper into the air system.
By "discharge line" which line do you mean?
What do you mean by "ping tank" and where is it?
Trouble shooting from a distance is impossible without common terms.
happy coaching!
buswarrior
The pressure protection valve allows the air pressure to escape if the pressure gets to high ( over 150psi.) usually because the unloaders on the compressor are stuck. this protects the air system. it is usually located near the compressor discharge line in front of or after the ping tank.
The pig tank (if it has one) is placed directly after the compressor discharge line. the ping tank is used to soften the compressor pulses and cool the compressed air coming from the compressor. this reduces the condensation that forms when the air cools.
Quote from: buswarrior on September 09, 2008, 01:34:48 PM
ok, we have trouble... this is just a jumble of BS.
Trouble shooting from a distance is impossible without common terms.
happy coaching!
buswarrior
I will agree and we could have a jumble of BS. I had to look up ping tank and every definition of the term I could find is a tank to cushion the air ride. Usually one tank per corner. So on to my problem and I hope the right terms. From what I can tell the purge valve has not been working on the dryer, about mid way up on the dryer is a fitting that I'm assuming is the pressure protection valve or at least a pressure protection valve for the dryer. Bear with me I'm no air brake specialist. I have been reading the bendix hand book though so I'm getting better :). So the valve in question that I'm calling the pressure protection valve has been going off on a regular basis blowing water and oil all over. I'm assuming a accumulation of crap that the purge valve has not been getting rid of. Yesterday I pulled the pressure protection valve at low pressure and got a good steady stream of goo to come out after I did that I'm getting better pressure, although I did not have time to get into the forensics of it what ever I did helped the problem. So my question is if the purge valve has not been working correctly can the accumulation of goo in the dryer cause my problems?
I had a similar problem a while back on a Kenworth tractor. I went to change the dryer cartridge and found a lot of oil/water/sludge in it. prior to us getting the truck the compressor had gone out and been changed but no one cleaned out the oil it pumped into the system. I had to pull the dryer apart and flush the line from the compressor to the dryer. after all the crap was out the dryer started to function as designed and everything worked again. so in answer, yes, if the dryer is full of goo then it can cause all types of problems.
while the early A2 and A3 dryers are good I was told by a friend in the truck parts business that parts are getting tougher to get and if it failed on the road I may have to spend a couple of days waiting on parts so I replaced mine with a new AD-IP unit with the spin on dissacant cartrige.
good luck
steve
Thats what I thought although I can't get my mind around why. What is the best way to clean and/or check the function of the purge valve off of the governor?
I dont know how to test it but I cleaned mine with brake clean. I took apart everything I could to make sure everything that was supposed to move would after cleaning. the first question would be if there is that much goo in it where did it come from. generally the goo is from oil in the system. while there is always a tiny bit of oil if there is enough to goo up everything then I would think that either the compressor is tired and pushing oil through it or the dryer has been ignored for way to long. I found my AD-IP dryer on ebay for $150 new from a company the sells lost/abandoned freight. for that price it wasnt worth it to me to deal with a 26 year old unit.
steve
If I think about things on a postive level I don't think I have ever heard the purge valve let go. With over 2000 miles of driving under my belt who knows how long the previous owner let it go. I'll see about parts, I have a really good supplier locally. The cleaning of the line could be a challenge. Its about 35ft long and not at all accessible. Sometimes I wonder what MCI was thinking? They go over the top on actual material and then make things hard as hell to service. Maybe some high pressure ethonal? Gotta make that stuff pay for my high food prices somehow.
Q500, if you have all the crap in your system the air dryer needs service bad that is the purpose of it. If you have the AD 1 or Ad 2 change the desiccant filter and clean the valve as per Steve parts are available for them at Napa the desiccant filter are a exchange at Napa for around $35.00 the valve is easy to clean and very seldom give problems .The Bendix web site will show how to service it with all the parts numbers. good luck
Thanks luvrbus that was part of the plan.
Here an attachment of a MCI-8 air system schematic.
About the AD1 or AD2 dyer with purge....Non purging is either line is disconnected from governor, governor inoperative or worn out or it could be piston froze in purging valve area. Usually governor problem.
http://www.bendixvrc.com/itemDisplay.asp?documentID=3604
Bendix D-2 Governor:
http://www.bendixvrc.com/itemDisplay.asp?documentID=2382
There is 2 small air line from governor...one to purge's piston and the other to top half of air chamber of air dyer to tell governor when to stop air build up and purge at the same time.
When it working properly...it will purge when air pressure reach the cut-off point at the governor.
About replacement of your air dyer (Ad-1 or AD-2)....make sure you get the automatic purge type like you have now. I could be wrong but I believe class 8 don't have automatic purge as buses do.
FWIW
Sojourn for Christ, Gerald
Here is a ping tank. The large hose entering the side of the tank is from the compressor. The large hose exiting the top of the tank goes to the wet tank/dryer. The small hose coming off the bottom is the drain and should be drained daily. The quick connect fitting is something we added to allow connecting to our shop air (through a double male fitting). The blue DOT tubing connected through the one way valve is from our aux. air compressor. Jack
Thanks for the replies. So after Jacks picture of the ping tank I have checked that out thoroughly and have no blockages. I may or may not have time to check it out some more today but I'm getting closer to the source of the problem.
I made some progress yesterday. I found out that my governor is not makeing enough pressure to purge the air dryer. I hit the purge line to the dryer with shop air about 10 or 15 times and got a huge puddle of goo to come out. Everything seems to work correctly now. I need to repair or replace the governor and service the dryer thanks for all the help!
quantum500....Here a link to AD-2 dryer from Bendix:
http://www.bendixvrc.com/itemDisplay.asp?documentID=3604
About replacing governor....you may not have compressor's inlet filter or missing or worn out. I would get the update version filter assembly. Poor filtering or none will cause wear to compressor's piston & cylinder wall and wear out the governor. I would be concern about compressor yet until see how oil drain out from ping tank.
This will explain how much oil usage is normal on page 4:
http://www.todaystrucking.com/images/BendixCompressor%20TroubleshootBW1971.pdf
BTW....is the air relay exhaust show oily & wet? Two in rear and one in front.
FWIW
Sojourn for Christ, Gerald
Q500, I would look seriously at the compressor,with that much goo it seams as though the compressor is passing a lot of oil. If the goo gets into the relay valves and further along the brake chambers, you are in for a lot of work and money, let alone a system that at best is marginal.
Good luck, Gerry.
The compressor gets air right out of the main engine air filter. So I don't understand what I could update too. I have in the past drained the ping tank on a regular basis and I haven't gotten much if anything out of it. The relay exhaust does not show any oil residue. I think that the problem I figured out has been going on for a long time. Like I said earlier I have never heard the purge on the dryer go off. I didn't really think about it in the past because I thought the dryer was closer to the compressor which would make it hard to hear in a bus, but as I found out it is very close to the drivers seat and I should be able to hear it most of the time. So in reality it could have been a problem long enough that it finally showed itself in this peculiar way. I did get a chance to take the governor apart tonight and it had a chunk of rtv sealant wedged inside of it between one of the valves. I took it out as best I could but I think the valve is stuck and there is no way to take it part any farther that I can see. So that would explain the lack of purge action, and it still does not purge! Another point to ponder is the region I live in which is very arid so while condensation does happen its not nearly as much as most people would see. I'm sure the compressor isn't perfect but I think if everything is working correctly I shouldn't have any trouble with it. I will keep you posted.
Quantum500, thank for your up date.
About air intake connect to engine's filter is better yet.
About governor and 2 small lines. Being you already found rtv or whatever in governor...it possible have a plugged purge line, try blowing via air gun one line at a time so you won't misplace where the line goes back on. The other line is pressure signal line, it could be plugged with water rusted inside,,,,try to air gun it to see if it clear at the other end.
Maybe purge line is leaking some where along the line. Pressurize it with plug at the other end and hand-held pump with gauge from auto parts store. It should hold pressure steady.
The pressure-signal line is under pressure while air in dryer, you may hear it leaking, if not then it good.
D-2 governor...I would go to any heavy duty truck place or NAPA for replacement. They cost about $20. It well worth it and easier than repairing it plus you know it should work.
Model D-2 is use on many trucks with air brakes.
Reinstall with thin coat of # 3 Permatex onto cleaned compressor surface & governor.
http://www.permatex.com/products/Automotive/automotive_gasketing/gasket_sealants/auto_Permatex_Aviation_Form-A-Gasket_No_3_Sealant_a.htm
MITYVAC VACUUM PUMP AND BRAKE BLEEDING DELUXE KIT
http://yhst-13811118617756.stores.yahoo.net/mty-7000.html?productid=mty-7000&channelid=BCOME
Keep posting update till fix.
Sojourn for Christ, Gerald
Quantum 500,
I haven't posted much, but we have had a somewhat similar situation with our MCI DL3.
If you were thinking about replacing the compressor, I would recommended replacing the governor, rebuilding or replacing you whole air dryer first, then if you are still getting significant oil through replacing the compressor. When our compressor went, we were getting a lot of oil out of our tanks. The only caution is if you replace the dryer before the compressor, and if the compressor is bad and sends a lot of oil through the system, then you just ruined your new dryer (not to mention oil is hard on your brake diaphragms etc.)!!! Also be careful with shop air, if the pressure is greater than your coach is regulated for you can blow your brake diaphragms and other pressure sensitive parts.
We replaced the governor, compressor, dryer, and blew out all the lines, which fixed our problem.
FWIW HTH
God Bless, John
OK so now we know there are a variety of issues. first question, how long have you owned this bus? Have you ever replaced the compressor? If not then I would guess that it was changed prior to your ownership. that is the same problem I had with the Kenworth. Just like mine it is possible the compressor was change or repaired (probably the reason for RTV in the system) and they didnt clean out the system. Like Jerry and the others said, just replace the governor and flush the lines and service the dryer. all this assuming you are sure the compressor is not pumping out oil. the biggest problem with the MCI having the dryer in the front is that there is 35 feet of hose that could have a lot of oil and crap laying in it so you will need to unhook it at each end and flush it good. the hydraulic shops us a line cleaner which is basically a foam plug that is soaked in cleaner and pushed through the line with air pressure.
good luck.
That foam for cleaning is called a poly pig when you are buying one fwiw. how does a air dryer work being 35ft from the compressor I have been told by shops that it should be no more than 10ft from the compressor fact or fiction anyone know for sure
I have been told the same thing but most all MCIs have the dryer around the front axle I believe. someone smarter than me will have to figure out the right answer but I cant believe they would do it all these years if it was a problem. I have an Eagle and mine is behind the drive axle so it is about a six foot hose. seems to work ok there.
steve
Quantum500
I forgot to mention, that you might also want to check your shop air. Sometimes the shop compressors are full of water and other foreign material. I'm not sure if you have had the air hooked up much, but that could explain some of the water in the lines.
God bless,
John
P.S. luverbus we have had some issues with the dryer being 40 feet from the compressor. If the compressor makes much condensation and it sits in that main line the line can freeze easier. In addition the main line to the dryer is copper, which splits when it freezes. MCI also makes a heater boot to go on the output from the dryer to prevent freeze ups in that elbow (of course that's provided that your dryer isn't functioning properly and is letting water through).
If you read all my previous posts I got the dryer to purge with shop air hooked up to the purge line. I have had the bus for almost a year. The previous owner told me when I bought it that to get the parking brake to release you must first hit the brakes....it doesn't do that anymore. Could it be that this problem has persisted through 2 owners? I have not replaced the compressor, if it does need attention I will probably rebuild it myself if I can find a good deal on parts. My shop air is very clean I do a fair amount of painting and wouldn't risk contamination in my paint. I did find this deal for all you guys with the an old dryer, you can thank me later. ;D http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:BIN&item=380047884121
dont know about MCIs but my Eagle has a brake interlock. you release the brakes then apply 100psi to the service brakes to release them. if yours has this also and it has quit functioning you may want to check with someone to ensure that is wont interfere with any other part of the system. if the compressor has never been changed then there is a good chance it is fading and pumping oil into the system.
good luck.
steve
Quote from: compedgemarine on September 13, 2008, 10:23:05 AM
I have been told the same thing but most all MCIs have the dryer around the front axle I believe. someone smarter than me will have to figure out the right answer but I cant believe they would do it all these years if it was a problem. I have an Eagle and mine is behind the drive axle so it is about a six foot hose. seems to work ok there.
steve
There a reason for AD-2 being in front to allow compressor's line to cooled before dyer to maximize moisture removal via automatic purge.
Read the Bendix Compressor Guide and you will find it suggest at least 15 feet or more.
http://www.todaystrucking.com/images/BendixCompressor%20TroubleshootBW1971.pdf
quantum500...do the free checking first and read the above link about "The Bendix® BASIC test should be the definitive method for judging excessive oil fouling/oil passing." to learn if your compressor is good or bad.
BTW...sign of no oil coating from relay exhaust mean no oil in diaphram. Which no problem but to get dryer AD-2 apart to replace oiled soaked desiccant beads for new one. The instruction is in the Bendix AD1_AD-2 link in my first post.
And replace D-2 governor with open..clear thru no leak lines and I believe will you be fine.
And don't do AD-2 servicing yet until your sure compressor oil consumption is OK.
FWIW
Sojourn for Christ, Gerald
Quote from: compedgemarine on September 13, 2008, 07:11:11 PM
dont know about MCIs but my Eagle has a brake interlock. you release the brakes then apply 100psi to the service brakes to release them. if yours has this also and it has quit functioning you may want to check with someone to ensure that is wont interfere with any other part of the system. if the compressor has never been changed then there is a good chance it is fading and pumping oil into the system.
good luck.
steve
Thats a thought.
Jerry, my AD-2 Bendix service manual under service data #sd-08-2403 page 15 it calls for 6ft min on 2 cylinder compressors and 10ft for 1 cylinder compressors no reference to the max length fwiw
Good grief!
Quantum, you say you've been using the "drain daily" drain in the engine compartment regularly and getting pretty much nothing out of it. The compressor is fine. If it was worn out and pumping oil, you would be getting the first load of oil here, and all over your pants, and you aren't.
Regarding the use of the term "ping tank" MCI calls the small tank after the air compressor a "Discharge Muffler" and it is the first place to catch and drain the moisture from the humidity that has been squeezed out of the intake air.
My air drier contamination theory: Besides simply deteriorating over years of neglect, the air drier may have been hastened to this condition. In an effort to prevent air system freezing, introducing brake alcohol into the air system in the wrong place, (ahead of the air drier) will quickly destroy the desiccant in the air drier, turning it into a mud, which then escapes its containment and plugs up the proper functioning of the purge valve. The normal and small amount of oil that makes it to the air drier may make the mud appear to be heavily oil contaminated. Maybe some previous owner or driver has done this prior to your ownership. Pumping the alcohol in through the drain in the engine room is popularly the way this dastardly deed is accomplished. (Note, if you are a northern busnut, and don't have an air drier, this is an easy and ok way to introduce alcohol to the system) Once ruined, it is only a matter of running time for the purge valve to fail and then over time, the cavity in the air drier fills with a mixture of desiccant mud, oil and moisture. Getting goo out the Safety Valve suggests the cavity has filled somewhat, eh?
If you want to, the drier may be successfully returned to functioning with a clean out, desiccant replacement and replacing the purge valve instead of cleaning it is good preventive maintenance. Don't skimp by buying one without the built in heater. You never know when the winds from up here will blow to down there.... And run the fused power line, if you don't have one already.
However, as noted, updating to a more modern air drier has advantages for quick parts acquisition. As for the parts counter guy saying parts are hard to come by for the AD1 and AD2, no, the parts are readily available to order, daily demand for them is down, as they are getting to be quite few and far between out on the road commercially, and he has chosen, for good reason, to not stock stuff that doesn't move.
When choosing a new drier, be sure that you are getting one that is rated for the big air flow requirements of a bus air system. The air drier can only "dry out" so much air on each pumping cycle before it is saturated.
As noted, just go buy a new governor. Again, good preventive maintenance and it isn't worth your time to fool with a rebuild kit.
In air brake circles, the device to prevent a pressurized vessel from damage if due to some failure the pressure climbs beyond the rating of the vessel is called a "Safety Valve". Some slang terms include pop-off valve, blow-off valve. A Pressure Protection valve is the proper name for the device that blocks air from filling the accessory parts of the air system in order to direct the compressor's full efforts at the brake tanks, "protecting" the brake part of the air system from any failure in the accessories.
The Safety Valve may very well have been doing its job properly. If the air drier's condition was blocking the flow of air into the system, the pressure in the line from the compressor to the blockage was quite able to surpass 150 lbs, and the Safety Valve released the excess pressure to protect it from bursting. However, the Safety Valve has very small passages in it, a sealing surface and depending on age, a metal spring, which will now be polluted with stuff which may effect its operation in the future, so again for preventive maintenance, it would be good to purchase a replacement.
By placing the air drier way up there, MCI's air driers enjoy a long service life. Heat in the incoming air charge is hard on the desiccant. MCI is able to get a considerable drop in heat over that distance.
There is no way to have the bus functional and have the airline from the compressor to the air drier filled with enough water to split it under freezing. The Discharge Muffler has been built strong enough to withstand a certain amount of abuse in this area. Remember, MCI is from Winnipeg....
As for cleaning lines, if this line of thought is close to the condition your coach is in, the relatively short line from the air drier to the wet tank, and the wet tank itself, should be the only places that may be contaminated sufficiently to require a little cleaning.
From the air drier, blow toward the wet tank, and see what comes out the drain valve. If you like, feed it a little water, enough to get the level in the tank a little above the drain and see what comes out. Maybe leave it sit for awhile. Don't use any solvents or other chemicals that would be harmful to the internals of your air valves. Rubber, plastic, metal sealing surfaces...
The signal line for the governor goes back to the wet tank, but up high, so I doubt there is any crap in it. With the wet tank drain open, give it a little shot of air before you attach it. The other line goes to the air drier to signal the purge valve, be sure to take it apart at the drier end first and give it a little shot.
Whew!
happy coaching!
buswarrior
Thanks for the detailed message! I appreciate it.
Q500, if you get a excess of water most of the time it is not condensation but a bad compressor.I replace my compressor 4 years and with the new one I had water every day in my tanks that wasn't there before the problem we found out was the head was leaking on the compressor. At the Bendix school in Phoenix the instructor told the class that a AD 2 dryer was the best dryers Bendix ever built the new dryer AD-IS was made because the older units did too good of a job drying the air causing problems with the new auto shift transmissions o-ring failures and fwiw I live in the middel of nowhere and the local NAPA store has had every part I ever needed for my AD-2 dryer
Clifford The Bendix manual tells you to but the air dryer no more than 10 feet from the compressor and down hill for same.
Wayne, what page is that on, I knew it was in the manual and the instructor told us the same ( was awake then) that is why I asked when 35ft was mention but I could not find it
Clifford After looking at the manual I may have to retract my statement. On page 9 of the Air Brake handbook they talk about line size and lengths. 15 feet is as long as they talk about and that is 3/4 line. In my notes of the class I have written 10 feet max. Something must have been said I am no good at making thing up. It does talk about the distance between the compressor and the governor. Thats not an issue with most of us as the govenor is mounted at the compressor. I will have to ask Jim maybe he will remember what they where talking about. They do say the hose must run down hill to the dryer, and they talk abut insulation to proct for freezing.
When we compress air, its temperature is raised.
This hot air, on leaving the compressor, is hotter than the desiccant in the air drier can withstand.
The minimum lengths of pipe from the compressor to the air drier are specified so that the compressed air will shed enough of that heat to the surrounding air, via the metal piping, before entering the air drier.
Feeding the air drier air that is hotter than it is rated for will lead to rapid deterioration of the desiccant, and it won't do its job properly, allowing more moisture through into the system.
Depending on application, usually the exposed underbelly of a truck, the metal piping will require some insulation after a certain length of exposed pipe in order prevent so much heat from being removed that the temperature in the pipe as it approaches the drier is unable to get above freezing while the compressor is running. Remember that the airline is completely exposed under a truck.
When the compressor isn't running, the pipe can get as cold as it wants. The problem happens once the compressor starts running, the first bit of moisture laden air may start to freeze inside the pipe, but the accompanying warmth quickly moves down the pipe, warming it and thaws out whatever moisture that initially froze on the walls of the pipe and the rest of the warm moisture gets through. If the pipe is too long and too exposed, the warmth is lost before it gets all the way to the drier, and an ice plug begins to form somewhere ahead of the drier
The relatively sheltered routing of the plumbing in our buses means its easier for it to warm itself over the length of run, and only leaves the last little bit of exposed pipe up front above the axle, where the pipe exits the utility tunnel a potential problem, which is dealt with by the insulation, if it is still intact. Southern buses may have had the insulation discarded long ago, the first time a mechanic had to do anything with that pipe or its fittings. Also remember the other pipes that run in the utility tunnel along with the airline, in particular the large pipes supplying the heater cores. A long length of that run is in no danger of freezing!
A big reason for the Discharge Muffler and its drain in the engine room of the MCI's is to deal with the potential droop in the downhill requirement for the airline from the compressor to the air drier.
Always good to check the documentation. What an Instructor says might have been misstated, or what we think we hear an Instructor say may have been misheard. I can say that, I've done both!
happy coaching!
buswarrior
A quote from Bendix foot note:
To counter above normal temperatures at the air dryer inlet, (and resultant oil-vapor passing
upstream in the air system) replace the discharge line with one of a larger diameter and/
or longer length. This helps reduce the air's temperature. If sufficient cooling occurs, the
oil-vapor condenses and can be removed by the air dryer. Discharge line upgrades are not
covered under warranty. Note: To help prevent discharge line freeze-ups, shorter discharge
line lengths or insulation may be required in cold climates. (See Bendix Bulletins TCH-08-21
and TCH-08-22, included in Appendix B, for more information.)
I search for Bendix Bulletins TCH-08-21 and TCH-08-22 but no dice.
However, I did send e-mail to clarify about 30 feet with AD-2 w/purge.
MCI-8 &9 discharge line is level with floor frame channel all the way to the front of first baggage compartment and down to air dryer.
Soon as I get e-mail from Bendix support, I will post it.
Sojourn for Christ, Gerald
I have been following this thread closely because I now have a problem with my M C 9 building air. I have tried to read all the suggestions and recommendations.
Up until 2 days ago the system was working basically fine. The air seemed to take longer to build than I liked but usually within proper limits.
But then after a short drive and then stopping, and then starting the engine again, the compressor would not build air at all. I pumped the brakes down to around 60 lbs. but the compressor would not load.
I replaced the governor, restarted the engine and the air built up very slowly to where the dryer would purge. I then pumped the brakes down and the compressor would not load.
I turned off the engine, pumped the brakes so the air was down to zero on the gauge, started the engine, and the gauge would not move off zero.
I blew shop air through the governor reservoir line up to the front of the bus and the air would build in the wet tank and then blow back through the line when I unhooked the line. I assumed this line was clear.
I hooked shop air to the fill in the engine compartment and filled the system to around 100 lbs. (limit on my small compressor).
So, is my compressor shot?? What other test can be done to check compressor? Unhook pressure line from compressor, start engine, and check for air coming out???
Thanks, Iver.
I'm lazy, and hate taking anything apart until every possible simple diagnosis has been tried.
Are you absolutely positively sure that there are NO air leaks?
At these levels, you need a helper, since the time it takes to get back there with a big leaker, it will be silent before you get to the back from the front.
Any oil in the engine room drain? No oil is a good sign.
If it periodically makes air pressure, you can rule out a drive problem with the compressor.
Put an air gauge on the drain in the engine room and see what pressure you are getting there. This will show whether there is a blockage upstream somewhere, as there will be strong pressure here if it is restricted. Same as the earlier postings with the corrupted air drier. You will get zero pressure here when the compressor is cut out and the drier purged, so you can't jump to the conclusion things are broken right away with a zero reading.
Do the same thing with pumping the brakes down and then watching the air gauge down the back to see what the compressor is trying to do. If the wet tank is not seeing the pressure to cut out, the signal line to the governor isn't getting to that pressure, and won't cut out the compressor, so it will keep running until the wet tank sees the pressure sufficient to cut out. The wet tank is blind to you, without adding a pressure gauge to the drain on it.
A new governor hasn't changed anything from the sounds of it, so perhaps the activating mechanism in the compressor is misbehaving, sticking, worn, partially broken, ???. The governor is signalling, but the compressor is not always responding by cutting in when the governor acts on it to do so.
Maybe do a bench test on that new governor, since pulling it off and making sure you don't have a bad coincidence of a failed or improperly set new unit, is easier than pulling a good compressor off...remember, LAZY....
Fooling with the compressor comes after absolutely confirming that in fact it is the culprit, and there is no blockage, leak or lazy valving elsewhere causing the cussing.
If you determine the compressor is not reliably cutting in, remove and trade it in for a rebuild, and bolt in the fresh one and you are on the way this afternoon. Unless you have the background, or SHE doesn't mind being sidelined again because you rebuilt it wrong, don't bother with rebuild kits. Nobody who is making money with a truck or coach is rebuilding on the shop floor anymore, so why would we?
Bus conversions are notorious for the internals of the air system to get sticky, plug and jam up due to old stuff and LACK OF USE. Without the parts moving regularly, lubricants dry, metal bits corrode, rubber sticks like glue and otherwise stay where they are, and the air pressure or springs that make them move are not strong enough to overcome the conditions the valving now finds itself in.
Commercial duty cycles and the maintenance manuals did not for the most part ever consider the infrequent duty cycle or condition that we try to operate our coaches in.
A systematic replacement of all air system valving over some budgetary time frame would be an excellent strategy for a reliable bus conversion air system, since we have no way of knowing what abuse, contamination, ignored replacement schedules and lack of maintenance preceded our ownership.
happy coaching!
buswarrior
Iver
If no air....with engine running, check for air out of ping or muffler tank manual drain valve.....none.... remove discharge line from compressor and leave it off until finish checking the following and look for plugged carbon build-up and clean until it cleared....see if it pumping air good....none ....remove governor from compressor.... still no pumping air good ....check intake for collapses hose or any restriction.
Let us know what you find!
FWIW
Sojourn for Christ, Gerald
Thanks guys,
I removed and cleaned the engine compartment air drain line. Slight bit of oily crud but basically fairly clean.
I installed an air pressure gauge on the drain line as suggested and used shop air to fill the system.
The air stays up overnight and probably loses around 40lbs. by the next day.
I removed the large output line on the compressor head and checked for obstructions but it looked quite clean.
That is all the time I had today so tomorrow I will check the input line. Then I will run the engine to check my new gauge on the drain line. Then I will remove the large output line and run the engine to see if I get any air output.
Now, if all that fails to show a problem, I am assuming I will have to replace the compressor.
I see removing the input line, the output line, two small lines to the governor, two oil lines, and two water lines.
It looks like four bolts to remove.
Does that mean I have to drain all the coolant from the rads and lines down to engine block??? I have one shut off valve coming from the coolant additive/filter on one side of the crossover pipe but no shutoff on the other side. I will close the gate valves to the front .
Also, I guess a rebuilt compressor does not have the gear installed? Do I have to pull the gear off the old one?
Anything else to consider when removing?
Thanks again, Iver.