Mechanical rear air suspension.

cadillackid · 05-31-2016, 07:41 AM

My bus is equipped with a mechanical rear air suspension.. it makes for a nice ride.. but it continues to operate after I turn off the bus... and over a period of about an hour or two runs my air pressure down to 65 PSI at which time it looks like some sort of mechanical regulator valve "cuts off" and kills the air to the suspension...

of course this means I have to wait for my air to build back up everytime IO want to drive the bus..

is this normal operation or was there at one time likely some other valve installed?

some may have electronics related to their air suspension, mine is Purely mechanical in nature..

-Christopher

EastCoastCB · 05-31-2016, 07:43 AM

Quote:

Originally Posted by cadillackid

My bus is equipped with a mechanical rear air suspension.. it makes for a nice ride.. but it continues to operate after I turn off the bus... and over a period of about an hour or two runs my air pressure down to 65 PSI at which time it looks like some sort of mechanical regulator valve "cuts off" and kills the air to the suspension...

of course this means I have to wait for my air to build back up everytime IO want to drive the bus..

is this normal operation or was there at one time likely some other valve installed?

some may have electronics related to their air suspension, mine is Purely mechanical in nature..

-Christopher

I've only had one vehicle with air ride and its my bus. Yours sounds like mine though. They air down once you shut off the bus. Mine does so fast.

cadillackid · 05-31-2016, 07:48 AM

does yours air both tanks down to 65 and then stops there? my bus will hold air for days at 60-65 PSI once the air suspension shuts off..

I was out trying to chase down air leaks tinking thats why my tanks were going town and it turns out I could hear a tiny little air "motor" running while I was working under the bus.. its last an hour maybe 90 minutes.. then shuts off and the air shocks go nearly flat then... once i start the bus.. my air alarm is beeping.. and I'll notice the gauges build very slowly for the first few PSI as it is re-filling that suspension.. then the pressure goes up quickly...

-Christopher

NowhereFast · 05-31-2016, 07:59 AM

I've got the same thing, I have to wait about 1-2 minutes for air pressure to come back up after not driving for awhile. It doesn't really matter for me as long as I do get air pressure back. Air brakes plus air suspension on mine

jake_blue · 05-31-2016, 08:36 AM

Most likely there is an air leak somewhere and if indeed there is a 'motor' continuing to run after the engine shuts off then the leak is probably in one of the air bags which is why it's continuing to attempt to level the vehicle. Of course at 60psi an air brake equipped vehicle is now in warning mode because it's apparently not recovering air fast enough to continue providing brake functionality so it is not surprising that the air ride system would then disable to preserve air for brakes. It is however positive that once it bottoms at 60psi it stops, meaning there doesn't seem to be any other air leaks in the system.

My instinct would be to not worry about it because you could spend a lot of time and money and aggravation chasing it down. If it recovers quickly enough after you restart the engine then it can't be a very bad leak.

cadillackid · 05-31-2016, 08:57 AM

thats one thing I havent done is test for air leaks in the air suspension itself.. thats a good idea just to see..

it looks like the cut off valve they installed is adjustable.. I cant find a part number on it to look it up.. but im going to seek it out.. I may just disable the air suspension at 90 PSI if I can set that valve up and not affect the max pressure assigned to the air suspension.. it may also be regulating the pressure to it at 60 and of course I dont want to alter that.. I need to read more before i get the wrenches on it.
-Christopher

EastCoastCB · 05-31-2016, 09:13 AM

Yeah, my pass side rear air bag is garbage. Cracked, dry rotted... Its gotta be changed before going across the country for sure.

cadillackid · 05-31-2016, 09:33 AM

my bags look good.. but looks are decieving! but there is a bit of rust around the control block.. could be a leak there.. i'll spray that all down when i get back home and have 120 PSI in the tanks..

out gettin 6 brand spankin new paws on it now!! goin up from 10R to 11R and looks like its all gonna clear nicely and still have good clearance at the tandems and for steering..

-Christopher

cadillackid · 05-31-2016, 09:34 AM

oh and I just had another DUH! moment.. that air suspension is part of the Chassis and not the carpenter so it should be in my Service manuals.. so if I RTFM I probably can figure it out..

-Christopher

Brad_SwiftFur · 05-31-2016, 12:26 PM

I have driven quite a few air-ride trucks over the years. The air suspension on my Volvo "airs up" at around 80 (as does the cab) ... and after shut down, it can be heard adjusting itself for several minutes. The air system on that truck has leaks, it will go down to almost nothing overnight. The compressor works well, so we haven't bothered to diagnose it and with nearly 770K miles it isn't likely to be in the fleet much longer.

I'd say your air suspension is "normal behavior", but if you wanted to try to eliminate the leak, my first suspect would be the "leveling valve" as this is a common failure item.

cadillackid · 05-31-2016, 12:43 PM

Quote:

Originally Posted by Brad_SwiftFur

I have driven quite a few air-ride trucks over the years. The air suspension on my Volvo "airs up" at around 80 (as does the cab) ... and after shut down, it can be heard adjusting itself for several minutes. The air system on that truck has leaks, it will go down to almost nothing overnight. The compressor works well, so we haven't bothered to diagnose it and with nearly 770K miles it isn't likely to be in the fleet much longer.

I'd say your air suspension is "normal behavior", but if you wanted to try to eliminate the leak, my first suspect would be the "leveling valve" as this is a common failure item.

im also beginning to think the levelling valve.. I sprayed down the air bags all around and dont get any air bubbles.. now I have not disconnected one yet and pressure tested them with shop air to trukly know...

the air valve seems to constantly hunt though.. making me think that when it shuts off the supply air it automatically goes to exhaust mode with nothing in between so its alsways going up and down slightly..

my bus has a brand new air compressor and only takes a couple minutes to air up to over 90 so its not a pressing deal.. but just an annoyance more than anything. ..

-Christopher

Jolly Roger · 05-31-2016, 04:55 PM

I have debated trying to install air suspension but after many times trying to find what I want? I will stick with what I have?
Cadillac for an idea if that valve is a problem? Why not add a PRV and guage for each bag with check valves for each?
I know I am trying to stay with the original design to keep the house seperate from the bus hitting the road.
For me adding air bags for suspension meant I wanted that system seperated buy valves and prv's and check valves and gauges for each.
Now I want to deal with my spring suspension and add some air powered feet/jacks/legs at all four suspension point to controll the rock roll effect when parked for a while.
Therefore zone controll of each without questioning which one?

cadillackid · 05-31-2016, 05:24 PM

I cant really install a PRV on each bag as the automatic height adjuster valve handles adding air to the bags... it appears both bags are one circuit..

what I could do is put a solenoid valve right after the regulator valve on the tank that the suspension is connected to.. then I could turn the suspension on or off eithe on my console or automatically with the key...

sure it might use some air when I first start, to pump it up, however it wouldnt use all my air down to 60 PSI...

its really just an annoyance since the suspension does work.. the bags hold air and the bus adjusts properly.. might be best to just say that the time involved waiting for my air alarm to shut off is good letting the engine idle and warm up a couple minutes...

I *AM* going to put a switch on those blasted air shutters though!!.. the darned thinbgs closed up on my today and ran my temp gauge up north of 210 before they re-opened..

its 90 out the shutters never need to close..

-Christopher

Jolly Roger · 05-31-2016, 06:01 PM

I was saying if the automatic adjuster valve is a problem Then make up your own manual adjuster valve ID station and put the air where it is needed? Like a test station? Much less effort and thought than troubleshooting a bad TXV valve?
Kiss method?
You are good in the HVAC stuff.
I have installed many a VRF system with the same (in thought ) problems you are having with your air suspension? Not leaks
The engineers saying what you have can't do that and the factory tech saying well it is (you can probably relate) if your diverter/unloader valve is bad? Then I would probably build my own?
In a compressed air world?
Looks my dash board or somewhere has a lot of gauges? Talked myself into it.

cadillackid · 05-31-2016, 06:23 PM

my way more is to figure out if the behaviour is normal.. then if its not to rteplace the bad auto valve if it is in fact leaking air into the bags causing the system to think its raised too high and exhaust air.. an "internal leak" of sorts..

yeah like a TxV that decides its not going to reduce the flow and saturates the evaporator with liquid... been there before more than once...

or the thermal bulb gas leaks out... valve goes to its minimum... Most techs would punch a bunch of freon in.... you or I would probably stick a bunch of temp probes on first to try and figure out what its doing...

clear site glass, low head, low suction... fools a lot of techs out there..

I like dashboards full of gauges.. I'll be adding a few as.. like you..I want to know whats going on inside of things... lots can be learned when you learn the "normal" of your gauges.. and then see one go out of whack.. gives you often a heads up...

-Christopher

Brad_SwiftFur · 05-31-2016, 10:05 PM

Out of all the trucks I have driven, only a couple had gauges for air suspension pressure. The primary reason for these is so drivers can get an idea of how heavily loaded the truck is. With some experience, a driver can get a reasonably close estimate if he will be overweight (either by gross weight, or by axle weights/weight distribution). While you *could* add such a system to a bus (and many builders likely offer it as a factory option), I personally see little need for it, no more weight than most of us carry.

Leveling valves aren't all that expensive, and not all that hard to install. 2-3 bolts, a couple air lines, and the linkage is usually all that's involved. Air bags are pretty tough, for all the abuse they take, they have to be. I've seen 'em fail, but slow leaks are fairly rare.

My own personal experience as to air systems losing air pressure - Suspension leveling valves. Number one failure rate on the trucks I have driven. 7 years with one company, 2 Volvos, 3 valves replaced. Previous company, 60+ air ride trailers, we replaced 2-3 valves a month (of course the trailers were showing some age and heavy use).
Number 2 on this list - Air dryer. 2 replaced so far. They can be rebuilt, but at $95/hour for labor, it comes out cheaper to replace the whole thing. My truck uses an oddball dryer, but many trucks and buses use a "universal" one. Typical symptom of air dryer failure is a constant hissing of air pressure, and air system may not build up past 60 PSI (or sometimes not at all) ... a partial failure can have the system leaking quickly at idle but a strong compressor can keep up at higher RPM's. Some dryers have a serviceable filter which should be done periodically.
Number 3 would be "everything else". Air compressors can fail (and occasionally do), air bags deteriorate with age, tank purge valves can gum up and stick, plastic air lines can get brittle, rubber seals degrade ... all the usual stuff.

Sk8r_Cowgirl · 05-31-2016, 11:59 PM

You're making this way more complicated than it needs to be... the air is going somewhere. Air it up til it purges, shut the engine off and go see what the air is coming out of. If it is an air bag, replace it. If it is a valve, make sure no other valve could cause it to leak. If not, replace it. What kind of bus is it? Air ride systems are pretty simple, seriously. You don't need to use shop air for anything. Just need soapy water, your ears and feel for a breeze coming out of something it shouldn't.

jake_blue · 06-01-2016, 12:37 AM

I had one truck that just due to age and the fact that EVERYTHING was air-actuated and it would purge every last pound of air while it sat overnight. The suspension would be sitting on the stops and it'd have to run for about ten minutes just to build up the air pressure for all the bells and whistles and the brake system. The worst was when I would get unloaded and then try to shift my tandems before the trailer air bags were reinflated... Not enough air for everything to happen at once. Needless to say, every time I took it in for maintenance I included that as a trouble but they kept saying it wasn't one single leaking point but the combined effect of all the connections and interconnected air systems, each leaking a little from age and wear but altogether equating to a truck that wouldn't hold air overnight. They could have just been B.S.ing me in order to get out of doing something relatively laborious but since I couldn't detect a hiss in any one place it was the most reasonable explanation I could come up with myself. So take it for what it's worth but at least you know which system is affecting your problem which should make it easier to pinpoint.

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