mc9 kneeling problem

tombuchanan · May 29, 2021, 08:35:32 AM

My 1983 mc9 crusader suddenly started leaking out of the passenger rear air exhaust port port, I believe when I used the kneeling toggle to raise the bus. I let it sit a few days and started it again today. The leak started again as soon as the system reached about half pressure. I tried the kneeling toggle and I noticed the passenger side kept rising until the bus was very high on that side compared to the driver (see pictures). I tried to toggle it the other way but nothing happened. I'm confused as what the kneeling toggle does. I thought it just lowered the door side and then raised back to normal but it doesn't seem to do anything half the times I try it and this time it raised it way to far. Am I not understanding something here or does the air exhaust leak have something to do with this kneeling issue too?

Van · May 29, 2021, 09:16:44 AM

Check the level valves on the axle.

luvrbus · May 29, 2021, 09:52:09 AM

Kneeling has nothing to do with the rear and tag axle

lovetofix · May 29, 2021, 11:16:51 AM

Here are the basics of the air suspension system on our MC9 Crusaders.
There are three leveling valves and accompanying pressure regulators that control the ride height of the bus with air bag suspension. The tag axle has its own system of pressure regulators to manage the pressure to its bags which controls how much weight the tag axle caries but does not affect ride height. All of these systems are supplied air from the auxiliary tank which only gets air once the main air system reaches the set pressure to open the pressure protection valve, usually around 65psi.
The drive axle leveling valves in the stock configuration will always maintain a specific distance between the drive axle and the "frame" regardless of kneeling, tag dump, surface unevenness, etc. As long as the Aux air system has air pressure they will maintain their set height in reference to the axle regardless of any other part of the bus air suspension.
Now for the front. There is a single leveling valve for the front air suspension which is all connected. This is so the suspension is not trying to twist the structure of the bus as you go over uneven surfaces. It maintains a specific distance between the bus "frame" and front axle but it does that from a center reference point so the front axle can ride over uneven surfaces without creating side to side twisting or rocking of the bus.
The front suspension, in original configuration, has an additional system of valves to circumvent the leveling valve and quickly drop the front suspension onto the rubber bumpers on the axle. This lowers the whole front end to bring the height of the stairs down to facilitate passengers entering and exiting. When this "kneeling" function is activated the air supply is shutoff to the leveling valve and the entire front suspension is vented through a QR valve, as soon as the switch is set back to normal the valves revert to their original position and the leveling valve again works to maintain that crucial spacing between the "frame" and axle as long as the auxiliary air system has pressure.

As to your current problem. It is either a failure of the leveling valve or (much more likely) an air bag or air beam failure, i.e. the driver's side of the rear air suspension has a hole in it. The air beams are a big problem and have been covered thoroughly on this board. The reason the passenger side is now higher than normal is because the leveling valve is mounted inboard of the wheels so the other side being lower affects spacing of the outer wheel edge.

tombuchanan · May 29, 2021, 12:18:00 PM

thanks so much for the reply, lovetofix, does the fact that air is blasting out of the passenger rear height control air exhaust port give a clue as to what the problem is? The bags all look to be inflating and I don't hear a leak other than the huge release from that port. the picture i attached shows the port that is exhausting (the yellow looking thing on right).

lovetofix · May 29, 2021, 12:52:08 PM

No problem, Tom. I had issues with mine (bad air beam) and rebuilt the entire system so I thought I would try to share what I learned but I am no expert!
I did not realize that it is the passenger side vent that is exhausting. Just to make sure I have all my info straight, the passenger side is over inflated and venting. Is the drivers side inflating or is it resting on the rubber bumper?
Please make sure to solidly block up the bus under the vertical frame pillar between the wheels before sticking any body parts under the bus!

tombuchanan · May 29, 2021, 03:45:32 PM

yes passenger side rear just in front and inside of duel wheel. i crawled under (scary even with blocks) and it's the exhaust port on the height control assembly. it starts leaking about half way up to pressure and keeps leaking entire time. the time it happened, the bus didn't seem so off level but this time the passenger side was way up

hogi6123 · May 29, 2021, 07:15:05 PM

Maybe your passenger side leveling valve is bad or clogged? It could be allowing air into the bags constantly (it should shut off the air at ride height) but then it also lets air out when it realizes that side is too high.

hogi6123 · May 29, 2021, 07:23:13 PM

Quote from: lovetofix on May 29, 2021, 11:16:51 AM
As to your current problem. It is either a failure of the leveling valve or (much more likely) an air bag or air beam failure, i.e. the driver's side of the rear air suspension has a hole in it. The air beams are a big problem and have been covered thoroughly on this board. The reason the passenger side is now higher than normal is because the leveling valve is mounted inboard of the wheels so the other side being lower affects spacing of the outer wheel edge.

I don't think a bad air bag or beam is causing his current problem, in that case he would have a leak from the driver's side and not out of an exhaust port.

lovetofix · May 29, 2021, 08:17:03 PM

Quote from: hogi6123 on May 29, 2021, 07:23:13 PM
I don't think a bad air bag or beam is causing his current problem, in that case he would have a leak from the driver's side and not out of an exhaust port.

When I wrote my first post I had not made the connection in my mind that the side that was exhausting was the same side that was over inflated. I agree that there is likely something wrong with the passenger side leveling valve. I would disconnect the linkage to the valve and move the control lever up and down several times to see if that frees up the valve to work properly. Pushing the lever up past the center detent should inflate the bags and pulling the lever down should exhaust air from the bags through that yellow port. This would be much safer and easier to do after removing the drive wheels on that side. Those valves are rebuildable according to the service manual but everyone I talked to said to just replace them.
Also, I can't tell from the picture of the driver's side if it is at proper ride height or not. It might just be the camera angle but it looks like the tires are almost touching the wheel well. My MC9 does not have the rubber trim on the rear so that may be throwing me off, but when adjusted by the book, mine has 4-5 inches of space above the tires.

tombuchanan · May 30, 2021, 07:07:12 AM

i took the linkage off the passenger side valve and moved the valve back and forth and then put together and started it up. i never got to full air since the exhaust port was still leaking but at about 100 psi the passenger side duel wheel had a 7 inch gap between wheel and bus, the driver side duel wheel had about 4 inches and the single wheel had about 2 inches.

tombuchanan · May 30, 2021, 07:32:41 AM

When i took the linkage off, i took the rubber plug on the end of the control body and noticed that when i twisted pulled the valve up and down, the metal shaft its attached to that goes into the valve did not twist with it. The diagram I'm looking at shows a piston with spring that should engage this shaft to turn.

lovetofix · May 30, 2021, 08:15:46 AM

Quote from: tombuchanan on May 30, 2021, 07:32:41 AM
When i took the linkage off, i took the rubber plug on the end of the control body and noticed that when i twisted pulled the valve up and down, the metal shaft its attached to that goes into the valve did not twist with it. The diagram I'm looking at shows a piston with spring that should engage this shaft to turn.

I think you found your problem! It seems that shaft #23 is stuck in the inflate position.
I do not understand why it would be inflating and exhausting at the same time though, unless there is a safety of some sort built into the valve.

That diagram is correct. There is no direct connect between the valve shaft (#23) and the actuating lever (#32).
It seems either the valve shaft is seized up or the springs and piston (#26) that engage the shaft are stuck in the retracted "over travel position."
You will need to take it apart far enough to verify that the overtravel piston (#26) is moving freely and the springs (#27,28) have not gotten soft. Also verify that shaft #23 can rotate smoothly. You could use a pliers on the end of shaft #23 but don't force it too hard.

tombuchanan · May 30, 2021, 09:49:23 AM

any idea where to find a replacement valve, I've yet to find a good source of parts for this bus.

lovetofix · May 30, 2021, 10:40:20 AM

Quote from: tombuchanan on May 30, 2021, 09:49:23 AM
any idea where to find a replacement valve, I've yet to find a good source of parts for this bus.

One of our best sources for parts, and even help with instructions for installing if needed, will be from Luke and Bill at U.S. Coach in Vineland NJ. Luke had all three of my new leveling valves in stock.