System Air pressure while driving

[Mit12]

On my 2008 monaco diplomat 40 PDQ while driving down the road the air pressure goes from 99psi to 120 just about every 3 to 5 minutes. I do know what is normal because I have never watched it before. I parked the coach and dumped the air bags the air pressure was 65 and 7 days later it is at just below 65 psi on the gauge. So I don't seem to have any significant leaks.

[Argosy]

You've got a big leak built into the system, the purge valve on the air dryer. Start it up and watch the air gauge, when it gets to 125 you should hear a blast of air from underneath and see the pressure drop.


If you don't know when the air dryer was last serviced, it's time. They're a critical part of the brake system.

[twinboat]

The air dryer is before the tanks/gauges and has a check valve that stops any air from the tanks to back flow.

When the air dryer purges, the gauges should not drop but should stop moving and hold steady.

Find the DOT air system test and perform that. If it passes, you should be OK. If it don't pass, something is leaking.

[TGAWLAK]

As you are driving the air bags will deflate and inflate due to the road conditions. The leveling valves are trying to keep the rig level so you will see the air pressure guage move as the bags need air and the engine driven air compressor will keep the air tanks in the correct range of air pressure

[d23haynes57]

If you have a Cummins with the pneumatic operated variable gate turbo some air is also used to operate that. Especially with a tag it is common to see the rear air change more often that the front.

[Max Headroom]

Originally Posted by TGAWLAK

As you are driving the air bags will deflate and inflate due to the road conditions. The leveling valves are trying to keep the rig level so you will see the air pressure guage move as the bags need air and the engine driven air compressor will keep the air tanks in the correct range of air pressure

This is what I was going to say. The coach rocking back and forth on the airbags can use some air. Air throttle, pac activator, leaky stepwell cover, etc, it can add up.

[C.Martin]

So, the psi range is acceptable…and is set by the Governor.

If you are making a lot of brake applications…it is normal to have more frequent compressor cycles as you use the system…but if you are cruising down the highway and NOT braking, and still seeing short cycles there is something else going on.

The airbags can vent excess air overboard. They do not really level on the basic system. They control ride height. There are three servos valves which have rods connected that articulate. If the air bags are under inflated, air will be allowed to fill them, if too high, they can vent…but average bumps, etc…are absorbed by the ping tanks.

So, a leak might be causing this…

Ours did the same, until it became acute. I found the Purge valve on the Air Dryer was constantly discharging. Normal operations…it should vent ONLY when the Governor reaches cutout pressure…it dumps air thru the cartridge to regenerate the filter by expelling moisture and contaminants…quick blast followed by a slower release for a number of seconds…but then it should stop. I tried removing the purge valve, cleaning and relubricating, and reinstalling. Twice…no help. So, I was in a hurry so I replaced the entire dryer to get back on the road. Later, I cleaned the purge valve, Governor, turbomcutout valve and check valve. It works perfectly again…but ai didn’t have time to do all that when it decided to quit working on a trip.

There are a lot of lines, and it could just be a leak there. Also check the airbags for seams cracking and leaking too.

[followingsea]

Originally Posted by Mit12

....while driving down the road the air pressure goes from 99psi to 120 just about every 3 to 5 minutes. .....

A pressure protection valve failed causing a similar indication.

I could barely hear a hissing sound from the front of my MH with the engine running. With the engine off I could hear it better but it would stop before I could get under the MH. I had to crawl under with the engine running and feel around until I found it.

The part was under $20.

[Mit12]

Today I went out to the garage and started up the coach and let it run until the air reached 120. I shut off the coach. My plan is to see how long it takes to set back down on the leveling jacks. They have about 2 inches off the ground. I can not hear any leaks. I will update.

[Mit12]

So I started the coach and let it air up to 120 psi. Shut the engine off. It has been on the airbags for close to 24 hours and the coach is still on the air bags holding the jacks about 2 inches off the ground. I think that I can rule out any significant leaks. I am wondering if it could be the extremely worn out shocks causing this issue? I know that the coach leans in light wind and will porpoise over bumps. Would this cause the air to drop more than usual? It is on my list to replace all 8 original shocks with Koni shocks before my next trip.
I appreciate your input.

[Timaz]

Shock absorbers don’t control ride height and make no difference in your ride height, whether they’re good or bad

[twinboat]

Shocks control rebound when going over bumps. Rebound can cause the ride height valves to cycle, and that can use some air as the bags fill and dump slightly.

[Mit12]

I am at a loss here as to what would cause the pressure fluctuations. The coach has been sitting for 48 hours and it is still on the air bags holding the coach off the leveling jacks by 2 inches. I would think that I have ruled any significant leaks that would cause the pressure fluctuations. There are two pressure senders and gauge readings and they both are changing at the same rate so that rules out a gauge or wiring issue.

[twinboat]

Originally Posted by Mit12

I am at a loss here as to what would cause the pressure fluctuations. The coach has been sitting for 48 hours and it is still on the air bags holding the coach off the leveling jacks by 2 inches. I would think that I have ruled any significant leaks that would cause the pressure fluctuations. There are two pressure senders and gauge readings and they both are changing at the same rate so that rules out a gauge or wiring issue.

The ride height valves are not moving during your test. Try rocking it side to side, ypu may need help.

One other test is to release the parking brake ( push button in ) and see if it still hold air pressure.