Air system pressure fluctuation

[billbenham]

I recently bought a 1999 MH with Freightliner chassis and now I notice the air pressure fluctuating on about a twice-a-minute cycle while driving at highway speed on a relatively smooth road. Drops to 90 then builds back up to 105. Is this normal? I've checked for water in the tanks and there was none. I saw on this forum a note that Freightliner spec for air pressure decay when parked and shut down is 2 PSI per minute. I think I am meeting that spec but not sure.

 

[Gary RV_Wizard]

No. You have an air leak, probably a rather large one. The normal system air pressure is 125 psi but the compressor will let it drop to 90 before it comes on an rebuilds pressure. Yours falls immediately and rapidly to the 90 psi threshold and that's a rapid loss.

The Freightliner spec you mentioned is not relevant to anytime the engine is running (meaning the compressor is operating). And it's even rather extreme for a parked (not running) situation. A typical motorhome would probably lose only 1-2 psi per hour in those conditions.

 

[Utclmjmpr]

One of the easiest places to look at is the air delivery line from the compressor to the dryer,, it will be 6 to 8 feet long and a gentle drop from the compressor, the dryer will be at a lower level,, feel for a hole somewhere in the length while under pressure ,, it will have a stainless wire webbing on it..>>>Dan

 

[billbenham]

Thanks for replies. So far I found a significant leak at the air horn solenoid input connection so I've plugged it off and still apparently have another leak elsewhere. I begged a copy of the air system schematic from the Freightliner dealer. What a maze of lines! And the tanks appear to have internal partitions. There's also a tank in front of the rear axle that's not on the schematic. I'm going to have fun with this.

 

[Gary RV_Wizard]

You should plan on changing out the air dryer cartridge - it's an often neglected component and can cause air flow problems as well as no longer removing water from the air system.

Suspension ride height valves are a common source of leaks. So is the air brake treadle valve. Crawling around in/under with a spray bottle of soapy water will likely find multiple air loss points. Be careful underneath in case air suspension suddenly drops. Use jack stands.

 

[billbenham]

You should plan on changing out the air dryer cartridge - it's an often neglected component and can cause air flow problems as well as no longer removing water from the air system.

Suspension ride height valves are a common source of leaks. So is the air brake treadle valve. Crawling around in/under with a spray bottle of soapy water will likely find multiple air loss points. Be careful underneath in case air suspension suddenly drops. Use jack stands.

Both rear bags leaking. So now that's 3 leaks total so far. Is the cartridge inside the dryer?

 

[Utclmjmpr]

Yes.. You can buy a kit for the rebuild.>>>Dan

 

[Laura & Charles]

No. You have an air leak, probably a rather large one. The normal system air pressure is 125 psi but the compressor will let it drop to 90 before it comes on an rebuilds pressure. Yours falls immediately and rapidly to the 90 psi threshold and that's a rapid loss.

The Freightliner spec you mentioned is not relevant to anytime the engine is running (meaning the compressor is operating). And it's even rather extreme for a parked (not running) situation. A typical motorhome would probably lose only 1-2 psi per hour in those conditions.

So when underway, how often would one expect to see the compressor cycle on?

 

[Gary RV_Wizard]

So when underway, how often would one expect to see the compressor cycle on?

I wouldn't expect to see it at all. If just cruising the highway, you aren't braking and the suspension isn't rocking much, so leaks would be the only drain on the system. In practice, there are often some leaks and some suspension movement on turns, acceleration, etc., so maybe an occasional cycle, but you would probably have to be watching for it. Otherwise it would be rare enough to easily miss.

 

[Laura & Charles]

Uh oh. Ours cycles about every 5 minutes. Has ever since we took delivery. At least it’s not getting noticeably shorter. We’re scheduled into Lazy Days in a few weeks but this sounds like something better handled by a chassis shop. Thanks Gary!

 

[Gary RV_Wizard]

5 minutes isn't bad - probably just an assortment of typically small leaks. Ideally the air system pumps up and holds pressure until some normal function actually uses some air, e.g. brakes, suspension ride height leveling, air horn, whatever. On a smooth flat road, that should be near zero, if curves, dips, stop & go, etc, then air gets used and the compressor cycles as needed. More commonly, though, the dozens of connectors and valves loose a bit of air. The brake treadle valve is sometimes afflicted with wear & tear leaks and maybe age-related leakage too.

Try parking on reasonably level ground with jacks up and engine off. That takes motion-related air use out of the equation. See how long it takes for the pressure to drop.

 

[Lou Schneider]

Uh oh. Ours cycles about every 5 minutes. Has ever since we took delivery. At least it’s not getting noticeably shorter. We’re scheduled into Lazy Days in a few weeks but this sounds like something better handled by a chassis shop. Thanks Gary!

The Federal limit for air system pressure loss is 1 PSI every 2 minutes. If your air compressor kicks in at 100 PSI and cuts out at 125 PSI and the compressor is cycling every 5 minutes, you're losing 5 PSI a minute, or 10 times the allowable leakage.

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[billbenham]

Having replaced both rear air bags and fixed a big leak at the air horn solenoid valve, the cycling is a whole lot less, but when I shut down, the system loses air gradually over a few minutes so still looking for small leaks.

 

[billbenham]

Well now I find some small leaks around my air brake control valve. (Pressure switches I think.) Trying to find out if there's any kits available.

 

[Gary RV_Wizard]

The Federal limit for air system pressure loss is 1 PSI every 2 minutes. If your air compressor kicks in at 100 PSI and cuts out at 125 PSI and the compressor is cycling every 5 minutes, you're losing 5 PSI a minute, or 10 times the allowable leakage.

That's with engine off and park brake released, i.e. what I suggested he measure. It's not the same thing as what Charles described as his 5 minute cycle rate.

 

[Ray-IN]

The air governor turns the compressor on/off. I would bet rebuilding the air dryer unit solves the problem. Is it a Haldex or Meritor unit? There is a backlog for both new units, rebuilding is the quicker route.
The desiccant filter should be replaced every year for an OTR truck, for MH.s every 3-5 years will suffice unless you drive 80-100K miles a year.
The air storage tank in front of the rear axle is a makeup air tank for the rear air springs only. This is for a Spartan MM chassis, but shows that tank, which has a manual water drain via a radiator petcock.

 

[Laura & Charles]

That's with engine off and park brake released, i.e. what I suggested he measure. It's not the same thing as what Charles described as his 5 minute cycle rate.

Thanks to all for the replies,
Gary/Lou: Is there a federal standard for air loss rate while running? (Maybe a stupid question considering Gary’s initial indication that it should be rare event. But then again, if it’s going to lose psi at rest, why wouldn’t it when running?)

 

[Laura & Charles]

5 minutes isn't bad - probably just an assortment of typically small leaks. Ideally the air system pumps up and holds pressure until some normal function actually uses some air, e.g. brakes, suspension ride height leveling, air horn, whatever. On a smooth flat road, that should be near zero, if curves, dips, stop & go, etc, then air gets used and the compressor cycles as needed. More commonly, though, the dozens of connectors and valves loose a bit of air. The brake treadle valve is sometimes afflicted with wear & tear leaks and maybe age-related leakage too.

Try parking on reasonably level ground with jacks up and engine off. That takes motion-related air use out of the equation. See how long it takes for the pressure to drop.

I’ll try that when we next move. The digital gauges won’t work with engine off, but I believe the analogs will.

 

[Larry N.]

The digital gauges won’t work with engine off, but I believe the analogs will.

You could turn on the ignition switch without starting the engine.

 

[Laura & Charles]

You could turn on the ignition switch without starting the engine.

Display just reads Engine Not Started.