Calling air suspension experts

bobofthenorth · August 24, 2009, 09:22:41 AM

I just ordered three of these little devils from frenchy-bus parts in Quebec. It appears that two of them are the reason we settle onto the stops in about 8 hours now. As far as I can tell they take a pilot pressure off the airbag that is connected to the ride height valve and fill the other bags from supply air. Does that make sense? They are apparently rebuildable but these look a little long in the tooth so I ordered 3 new ones plus one rebuild kit. The parts book calls them a "tag axle relay" but they are connected to both the drive and tag airbags and there's three of them - two on the road side and one on the curb side. They also appear to have an exhaust port. I'm sure by the time I get done fighting with 30 years of rust to get them out I'll have a much better understanding of what all they are connected to.

DaveG · August 24, 2009, 05:13:58 PM

Wow Bob, 46 views and no replies....when you figure it out please let us know! BUMP!

buswarrior · August 24, 2009, 06:44:36 PM

My SWAG would be that there is one for each side of the drives and one for the tags.

Good choice getting fresh ones, I expect you'll find some part of the insides will look like the outsides, if you can get them to come apart without shattering them.

happy coaching!
buswarrior

bobofthenorth · August 24, 2009, 08:13:11 PM

That was my thinking. I figure with one rebuild kit and 3 cores maybe I can end up with one spare when I'm all done. Although I'm not sure why I think I need a spare because these look like the ones that the frenchmen bolted on in 1981.

Melbo · August 24, 2009, 09:11:19 PM

Bob they look original

I hope you have your favorite Panther Pi$$ to get them loose

Good luck and keep us posted

Melbo

edroelle · August 25, 2009, 07:08:37 AM

Do you have coach leveling? Some of the air suspension components act as relays (electrical) changing low volume into high volume.

It amazes me how complicated Prevost made their leveling systems. They had numerous variations, sometimes changing during a model year. After mastering the front, I have tried to analyze the rear of my bus, unsuccessfully.

I have found some of the aluminum blocks corroded in the bore, and unable to seal reliably. Hopefully, yours will be rebuildable.

Ed Roelle
Flint, MI

Sojourner · August 25, 2009, 07:27:41 AM

Bob...They look like two air control relay valves. My MCI-8 does not have them before leveling valve. Does your Prevost have a remote on the dash or automatic control for them?

Can you scan a schematic & post for me?

Thank you, Gerald

JackConrad · August 25, 2009, 11:17:27 AM

Bob,
Could those valves be for the tag axle lift thing that Prevosts do? Jack

bobofthenorth · August 25, 2009, 05:27:46 PM

Mel - the weasel piss is applied and soaking. I went for the $3 a can stuff instead of the $9 a can stuff reasoning that I could use 3 x as much and still come out even. (mainly I'm just a cheap SOB)

Gerald - no useful schematic. They are identified in the parts book as a "relay" valve with a picture to confirm that is what I am looking at. The schematic I do have is more like a block diagram and it shows the drive axle bags in parallel with a relay valve between them and the tags. That doesn't really match the plumbing in the coach but then I don't completely understand the plumbing either.

Ed - It might be something like that if these things do side to side levelling?

The valves are definitely plumbed as relays between supply air and some of the bags. If it did nothing other than speed up the filling and emptying of 2/3 of the bags I suppose that might counteract some rolling if it happened fast enough.

Jack - no, the tags lift with a simple brake can operating through a bellcrank mechanism. The cans are actuated by an air switch in the cockpit - that system is dead simple.

Gotta make a roadtrip tomorrow - I'm hoping the "relays" will be here Thursday. Then I'll go underneath and bleed on it for a while before reporting back. This project was an excuse to buy a set of flare nut wrenches so it hasn't been a complete waste.

stevet903 · August 25, 2009, 06:42:49 PM

Did they have the level low system in 1981? (the system that lets you level the coach from the drivers seat)

If not, the plumbing is set up to dump the air from the tag axle air bags when the tag is lifted. It connects the tag air bags to the drive air bags and the leveling valve when the tag is down, and then disconnects the tag air bags and dumps the air when the tag is lifted. Hope this helps.
Steve

bobofthenorth · August 25, 2009, 07:18:10 PM

That makes sense Steve. There are 3 of these relays - is it possible that one of them is filling the two rear bags on the drive axle? That appears to be how it is plumbed. (but I flunked plumbing)

stevet903 · August 26, 2009, 06:16:19 PM

If you flunked plumbing then try this diagram on for size:

http://prevostparts.volvo.com/technicalpublications/pdf/d140116c.pdf

The diagrams are not great, but it should help.

If that one is not right choose from this page:

http://prevostparts.volvo.com/technicalpublications/en/pneumatique.asp

Also if you are having start issues, the rear DPDT switch is a known problem - it starts with intermittent starts, then finally fails.

bobofthenorth · August 26, 2009, 08:35:59 PM

OK - I'll bite - howinhell did you know I was having intermittent start problems? I changed that switch 2 days ago but I won't know whether it has solved anything until I have shut the frenchy-bus off at a few fuel pumps and campground checkins and successfully restarted it.

pvcces · August 26, 2009, 10:22:37 PM

Bob, not only that, our rear start switch has started doing the same thing just this last trip. That item has made it into my maintenance notbook, along with a few other things.

Good luck getting your switch replaced.

Tom Caffrey

bevans6 · August 27, 2009, 05:20:12 AM

That is one HECK of a schematic! Man! It looks to me like your little mystery valve whizbangs take the air fed from the accessory tank through the leveling valves and then feed it to the tag bags and cross feed it connecting both sides. The two bags on the drive axle on each side look to be connected directly to their leveling valve, in parallel. Your mystery valve is also tee'd off the main feed from the leveling valve, and from the other side of the MV is fed the tag bag, and from the bottom of the MV is a line to cross-connect to the bottom of the MV's on the other side. I have no clue what they are doing. But it's neat to see a total air system schematic like that!

Brian