Air problems

[buswarrior]

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

[Sojourner]

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

[Iver]

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.

[buswarrior]

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

[Sojourner]

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

[Iver]

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.