Air System Froze up - Sort Of

[Fredward]

Last month I moved my MC-5 from my unheated building into my heated shop. There must be a check valve that froze closed between the rear tank and the front tanks. The compressor ran continuously, the pop off valve on the rear air tank kept blowing off excess pressure but the bus wouldn't air up. So I added shop air at an air chuck I installed right at the ping tank? (located on rear bulkhead right by where the air compressor discharge line is). Then the bus aired up so I could release the brakes and move it.

With the compressor still running and the pop off still blowing off, service brake applications brought the air pressure down to 60 psi and the DD-3s locked on. I added shop air again and released the brakes and moved the bus the rest of the way into the shop being careful not to apply the service brakes anymore!

The air gauge on the dash never went above 30 psi until adding shop air. The bus does not have a dryer. Looking at the air schematic I can't quite tell but I think the first stop after the compressor and ping tank is the rear (wet?) tank. From there it goes to the left hand front tank ?


So if the rear tank is the first stop, where does the governor connect to detect air pressure for cut out someplace other than at that rear tank and there must be a check valve between that rear tank and where the governor detects cut out pressure.

On a warm day later in January (in Minnesota youask?) I drove it 45 miles without any issues. I also do not have an alchohol jar eitherFred

[bevans6]

On my bus, a later MC-5, the rear tank behind the wall of the rear luggage bay is the dry tank.  They put it there because it's closest to the R8 valve that supplies air to the rear brakes.  There are two tanks in the front axle bay, on it's rear wall.  The driver's side one is the wet tank, the passenger side one is the emergency tank.  They put the wet tank far away from the compressor to give the air some time to cool down and start the condensation recovery process, particularly if you don't have an air dryer.  The high pressure relief valve is on the wet tank and should be releasing air at 150 psi, far higher than you seem to be getting to.  The DD3's should not auto-engage until the air in the emergency tank is below around 30 - 35 psi, which is a lot lower than the 60 psi that you said you saw.  The air pressure in the dry tank, or anywhere else in the system for that matter, should not affect the emergency tank since it's protected by one-way check valves.  There are also check valves and the pressure protection valve between the dry tank and the rest of the system. 

The flow of air is basically compressor, ping tank, (air dryer but you don't have one), wet tank, dry tank, PPV/E-filter, tee off to accessory/suspension, and to the emergency tank.  On my tank the governor reads after the PPV, so also after the check valve on the exit from the dry tank.

Your bus could be set up very differently to mine, but I also know that once MCI had a plan for these things they didn't change the big stuff much, just the details.

Brian

[papatony]

Fred  You didn't say how cold it was,those lines will freeze but they would not until it gets below 30 for an extended time .  there are bleed off values on each tank that should be bleed off each fall.      Tony

[buswarrior]

The smallest lick of moisture on the seal of any valve is sufficient to render the valve inoperative.

The seal simply is stuck, and won't allow the moving parts inside the valve to move, blocking the air flow as intended.

Air systems operated in freezing conditions either need a functioning air drier or brake anti-freeze introduced into the system.

Arctic conditions may call for both, depending on your confidence in the air system maintenance over the last year.

Without one or the other, air system failure is guaranteed in freezing temperatures.

You may put the brake anti-freeze directly into your wet tank via a drain valve attachment in an effort to regain functionality. The trouble is getting things unfrozen, and the alcohol mixed with the moisture. It won't mix with frozen, and can't get through if blocked.

Prevention is everything in this game.

happy coaching!
buswarrior

[Busgeek]

Bus warrior got it right!!!

[Fredward]

Well here in Minnesota, we definitely qualify for colder than heck. I have in in my heated shop so I'll put air system antifreeze in it before moving it back to the unheated side. We'll need it up and operating the last week of March. We're heading to New Orleans for a week. I installed an air chuck so I can add alchohol.

I'm mostly interested in understanding what froze up. I think on my '65 vintage coach, the wet tank is actually behind the rear bulkhead. On later versions they might have made the left hand tank up by the front axle into the wet tank. But I have installed drain valves on all four tanks and the only one that has condensation in it is the one back there behind the rear bulkhead.

Fred

[buswarrior]

It could be anything, anywhere. Not worth worrying about. I would have been surprised if it didn't freeze up coming out of a warm garage.

Once you get the alcohol in the tank, while you are still in the warm garage, cycle the air system a couple of times, pump the brakes, apply and release the parking brake several times, and then by draining the tanks in reverse order, furthest first, in order to draw the alcohol deeper into the system and through all the valving. Blow the air horn too.

And put at least a quart in the tank, and leave it there until spring.

happy coaching!
buswarrior