How often should motorhome release air?

[Stump Jumper]

O.K. Had a little extra time tonight, so I tried some of your suggestions. Started the engine and let the air build. Regular air pressure at idle and when under acceleration is about 125psi. Still releasing air about every 20 seconds with a loud puff and then a quieter hiss for about 3-4 seconds after that and then it stops. It is definitely coming from the air dryer near the rear axle. Parked the motorhome and chocked the wheels. Built air pressure, then released the parking brake and turned the motorhome off. No air leaks. Pulled both lanyards on the front air tanks, no water or debris. Any more ideas appreciated!

[Vintage]

Take a look here: ~~~~~School Bus Mechanic: Mercedes MBE Diesel Engine Air Compressor Cycles Too Often
Same type of system we have.

[Craig_R]

Stump Jumper is this a Meritor Wabco Air Dryer?

If it is, here is a trouble shooting guide and parts breakdown of the air dryer. Check out the second item on the third page. Sounds like what you are describing.

http://www.meritorwabco.com/MeritorW...nt/tp97101.pdf

[Stump Jumper]

It is a Midland Pure Air Plus.

[rgreen45]

If you were leaking down enough to bring on the compressor, then pump up to max every 20 seconds .... you'd have a massive leak plus a super compressor. You may have a small leak in the "unload" line on the governor which spits the drier.

I've dealt with heavy trucks for 40 years and I don't think you have a problem that warrants concern.

[Sonnydale]

Mine does the same thing and I thought this was normal, so I just went to check and what do you know, I have an air leak. I went under the coach with the engine off. (I let the pressure build to 125 and then turned it off)
I found the right rear air bag is leaking. I felt around it and it is the bag itself that is leaking and they are somewhat cracked so I will have to replace them.
I emailed FCC for the part numbers, but does anyone know the numbers for the front and rear air bags on a 2001 37ft XC chassis with a 5.9 ISB? They are Firestone Brand.

Thanks

[rgreen45]

Originally Posted by Sonnydale

I found the right rear air bag is leaking. I felt around it and it is the bag itself that is leaking and they are somewhat cracked so I will have to replace them.

Yep. That would do it.

The air governor, which unloads the compressor and trips the spitter, gets it's signal from the wet tank. This is the first tank that the main line from the compressor gets to, is primarily for collecting excess moisture, and is usually a small tank or even a small section of another tank. A leak on it or what little is directly connected to it (such as air bags) could cause short cycling of the compressor.

The other two tanks (usually called green & red on trucks, often called front & rear on MH's) have way more volume. Should something associated with either of those circuits be leaking it would take the compressor much longer to rebuild the air to the spitting (cutoff) point.

[rvjimmy]

Sonnydale just go to a FL dealer and give the parts dept. your serial # and they will tell you what you need. You can stop at one anywhere and then buy local if you want. Jim

[Sonnydale]

Originally Posted by rvjimmy

Sonnydale just go to a FL dealer and give the parts dept. your serial # and they will tell you what you need. You can stop at one anywhere and then buy local if you want. Jim

Thanks Jim, FCC came back with the numbers. They are around $200 a piece online. Cheaper the FW, but still pricey.

[Stump Jumper]

Well, the mystery deepens. I bled all of the air out of the system and then started up the motorhome. Took about 2 1/2 minutes for the air to go from almost 0 psi to 125 psi. Crawled all around the underside of the motorhome with the engine running with the parking brake on and then with the parking brake off with the wheels chocked. No leaks. Turned off the motorhome and crawled underneath with the brake on and then off. No leaks. Let the motorhome sit with the bags inflated and lost maybe 1lb of air in 5 minutes.
The one thing that I noticed that I thought was strange was that the air dryer continued to puff, even when I was releasing the air from the suspension. I appreciate the advice so far, any other ideas?

[rgreen45]

Originally Posted by Stump Jumper

The one thing that I noticed that I thought was strange was that the air dryer continued to puff, even when I was releasing the air from the suspension. I appreciate the advice so far, any other ideas?

My first suspect would be a bad air governor. The "unload" port from the governor when it holds the valves open in the air compressor may have a leak. Otherwise, you may need an "unloader" kit put into your air compressor as that's the second suspect. Air governor should be about $25.00. Replacing the unloader valves ..... a lot more.

[Stump Jumper]

I'll try to get an air governor and see if I can get it installed tonight. Hopefully, that will do the trick. How big of a job is the replacment of the air governor?

[Stump Jumper]

Oh, one more thing. My air guages NEVER drop from 125psi. I assume this points to the fact that the compressor is running constantly instead of cycling, correct?

[rgreen45]

Originally Posted by Stump Jumper

Oh, one more thing. My air guages NEVER drop from 125psi. I assume this points to the fact that the compressor is running constantly instead of cycling, correct?

Don't buy a governor yet. The gauges read the main tanks and not the wet tank. The wet tank is the one that starts I stops the compressor. It'll be the first reservoir the main air line goes to when it leaves the air drier. It may be a separate small tank or a section of a bigger tank. Check very closely for a leak there. Also, your spitter on the drier may be bleeding of the pressure and causing itself to spit again....and no, the comprressor isn't running constantly.