GM Leveling Valve Question

richard5933 · September 26, 2018, 04:30:24 AM

We're a week into a two week trip and have been having issues with the ride height valves (leveling valves) on our 4108. It was my understanding that the purpose of the leveling valves on the GM was to maintain a constant body-to-axle distance. When one side was loaded or unloaded, the valves would adjust to keep both sides of the body the same height. The manual shows a measurement taken between the top of the axle and the rubber bumpers.

1) Whenever the road is crowned or we park on a lot which slopes side to side, the bus seems to have a lower ride height on the side of the bus on the downhill side.

2) The ride height seems to want to randomly change and drop on one side, usually the curb side.

We were at US Coach this week for some routine service. While there Bill tried to get things adjusted for us but for some reason things did not cooperate. Even though both rear valves were replaced with new less than a year ago, he replaced one again since it looked like it might be the problem. After much trial and error, he got both sides to stay even after dumping the air and refilling.

Of course, things couldn't be that simple. We pulled into a campground yesterday evening. The lot had a good slope to it, and on the downhill side (curb side) the top of the tires was even with the bottom of the fenders, and on the uphill side (driver side) there was about 2" above the tires. The bus was clearly slanted way more than the pavement under it. Normal is about 3/4" of space above tires/below fenders on each side.)

We pulled into our site which slopes the other way, and for whatever reason the side that had the low ride height in the parking lot was still low. I pulled into the site with the side with the lower ride height on the uphill side of the site, so for now we're sitting level. Good for camping here a few days, but worrisome as we hit the road.

When driving on a road with a decent crown I can feel that the bus is exaggerating the slope. Oftentimes if there is a reverse crown it will exaggerate it the other direction.

Any collective wisdom out there about this? Any one able to explain what's going on and why the leveling valves seem to have a mind of their own? Any secret tips to getting things adjusted properly.

A few facts: Compressor easily brings up the bus to 120psi. Recovery is quick, so compressor appears to be working. The leveling valves both exhaust and raise the sides when the arm is moved manually. I stood in the pit with Bill and watched them work. The adjusting blocks move freely and the mounting points are solid and don't have any wiggle or slop to them.

chessie4905 · September 26, 2018, 05:25:01 AM

Some play or slop in the leveling valve links? Maybe different brands of valves have more or less sensitivity to movement.

thomasinnv · September 26, 2018, 08:50:37 AM

Replace the leveling valve on that side. They have a delay valve built in and they can give trouble. Check the mounting bolts first though, if they are loose it can cause the same issue.

goutoe · September 26, 2018, 12:21:12 PM

Richard I recently replaced the right side leveler valve on my AM General transit I unhooked the linkage on the new valve with the bus level and used a hook to operate the leveler arm on the valve, when I had the bus level I reconnected the linkage, it turned out to be very little adjustment from the old valve. mine was about 6" from the top of the tires to the wheel well, seems to adjust itself to the road crown. >>>>> John.

richard5933 · September 26, 2018, 12:32:09 PM

Both leveling valves were less than a year old when this started. The rear right was just replaced again since we couldn't tell if it was acting stupid. I've got the left rear on the way just in case. I've got an appointment next week at the Interstate shop in Milwaukee for their bus guy to try to get things leveled out.

The problem with the road crown is that the leveling valves seem to accentuate the crown - if the crown slopes down to the right, the right side of the bus goes lower than the left, making the bus tilt even more than the crown would on its own. Same (but opposite) if the crown slopes to the left.

Not sure what else could be involved here, but the bus sure seems to want to roll around a lot going down the road.

luvrbus · September 26, 2018, 01:24:35 PM

Not a GM guy on other buses no free play (3/16) adjustment in the overtravel causes that on 1 or both

chessie4905 · September 26, 2018, 01:30:32 PM

You want to stop the roll around? Install a rear stabilizer bar on the rear. Were standard on 4905's and optional on 4108's. Luke should have a take off from one of his parts coaches.

richard5933 · September 26, 2018, 02:19:37 PM

Quote from: luvrbus on September 26, 2018, 01:24:35 PM
Not a GM guy on other buses no free play (3/8) adjustment in the overtravel causes that on 1 or both

I think I understand what you're talking about - there should be a 3/8" dead spot/free play in the center of the adjustment?

Can you explain a bit more about this so I can explain to the guy at the shop who will be climbing under?

Thanks.

luvrbus · September 26, 2018, 02:48:35 PM

They should be adjusted where you have 3/16 of inch of free play up or down when the set height is achieved they work that way on other buses.I posted 3/8 th it should be 3/16 sorry I will correct that

Geoff · September 26, 2018, 03:04:13 PM

The GM RTS has a built in lower left side to compensate for road crown. Don't know about other GM coaches. If you're squirrelly on the road it could be from weak sidewalls on your tires.

chessie4905 · September 26, 2018, 03:45:32 PM

4108's were known for more side rocking than other models. Also, have you considered new shocks? 35 years old.

richard5933 · September 26, 2018, 03:52:39 PM

Quote from: Geoff on September 26, 2018, 03:04:13 PM
The GM RTS has a built in lower left side to compensate for road crown. Don't know about other GM coaches. If you're squirrelly on the road it could be from weak sidewalls on your tires.

We're running Firestone FS400 with an 'L' load rating. They are aired up to about 10% over the chart amount for the actual weight. Unless there is some hidden damage to the otherwise great-looking fairly-new tires I don't think that's it.

Quote from: chessie4905 on September 26, 2018, 01:30:32 PM
You want to stop the roll around? Install a rear stabilizer bar on the rear. Were standard on 4905's and optional on 4108's. Luke should have a take off from one of his parts coaches.

I have the parts book for the P8M4108A & 4905A in front of me, but I don't see a rear stabilizer bar on any of the charts. There is one shown for the front, but not the rear. Was that something the earlier models had which was discontinued? If so, would it fit the setup on a 4108A? Any more information about this?

Quote from: chessie4905 on September 26, 2018, 03:45:32 PM
4108's were known for more side rocking than other models. Also, have you considered new shocks? 35 years old.

Actually was thinking of that while starring at the undercarriage yesterday. They appear to function well, at least based on my understanding of how things should be. That said, are the shocks something that can be sourced by the shop at home in Milwaukee? Do you have a modern part number for these? Or, is this something that should be gotten through Luke?

chessie4905 · September 26, 2018, 07:17:15 PM

Underneath, the rear of 4905 and 4108 are identical. It even states it was optional in service manual under rear suspension.

dtcerrato · September 26, 2018, 07:31:48 PM

The fact that you were in the pit with Bill running through some test cylcles confounds me why they didn't show up then. Let's throw a couple more variables into possible leveling valve issues. From past experience on our GM 4104 (retrofitted with 4106 leveling valves) the following items can make a leveling valve work weird or for a better choice of words inconsistent. 1 - dirty or partially blocked filters. On our air input line to the leveling valves are cintered brass strainers (filters) inside a small enclosure that resembles a pregnant coupling. On some set ups there is also a filter on both sides of the leveling valve to also filter the air which returns through the leveling valve from the air beams to be exhausted. In our case we don't have the 2nd filter mentioned, instead there is a copper tube approx 2-3" soldered into the bushing threaded into the bottom of the air beams for the plumbing to attach to. This little vertcal copper tube prevents oil, debris, & in some cases water that lay in the bottom of the air beams from entering into the plumbing during air discharge of that beam. Wonder which set up you have? As Jon stated water will haunt a leveling valve but so will oil or debris. 2 - the GM "check valve" officially called the leveling "adapter" can be malfunctioning, partially. or intermittently fully clogged or constantly open. Even running fairly new or recently changed out leveling valves - unless the system environment was totally checked out prior to installing the new leveling valve - presto same problem can prop up. Hope that all makes sense.

Lin · September 26, 2018, 10:12:52 PM

I have seen it recommended to adjust the ride height valve linkage by measuring the bus chassis to the ground. I find it easier to balance everything by measuring the extension of the air bags when on level ground. I think that my 5a is about right at 7 inches of bag extension. I may be off since I have not checked it in a while. I wonder if all of your bags would have the same extension on level ground. If not, you could adjust the linkage accordingly.

Our original ride height linkage had too much play because the grommets on both sides had rotted out.