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Air compressor seems to be staying on


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After hitting a rough section of highway, I notice my air compressor seems to be running non-stop. After hearing the air blowoff after hitting the 120 psi when the compressor normally shuts off, I think I can hear air leaking, then about 30 seconds later I hear the air blowoff again. When using air while driving (braking), the dash air gauge starts going back up after a few seconds instead of waiting to hit 90 psi. 

When I shut off the coach I don't hear any air leaks, so the air leak I hear when running must be related to the compressor running.

I need to drive 350 miles home in a week and I guess it's probably not good for the compressor to run continuously so I'd like to see if I can get this fixed. But while I'm pretty handy, I'm not sure where the compressor even is on these motors. Is it motor-driven? A stand-alone electric compressor? It's about time to change the air dryer filter too so I guess I need to figure this stuff out.

The motor is a Cummins 8.3C (mechanical). Any tips on where to find the air compressor and pressure shutoff sensor would be appreciated. 

Btw, I'd recommend avoiding the 15 miles of I-81 in NY just north of Binghamton at all cost. Whatever asshole road crew went out there to "fix" the broken asphalt over the concrete expansion joints basically installed speed bumps, I think that's the roughest section of highway I've seen anywhere in the country.

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Sorry to hear about the rough road, sounds terrible.

The compressor is mounted to the passenger's side of the engine, and is gear driven off the engine.

On my 97 Dynasty (C8.3 as well) the air dryer/purge valve is mounted behind the passenger side rear axle. If you crawl under the coach behind the rear mud flap you will find it. Make sure you brace the coach first using jacks or some other mechanical means of keeping it from potentially dropping down on you while you're under it. 

Can you tell if the leaking sound is coming from the front axle area or the rear closer to the engine? You should be able to try fast idling until your air pressure in both tanks hits 125lbs and the air dryer purges, then shutting the motor off and quickly going outside to listen for leaks. 

 

 

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You might want to check the Bendix D2 air pressure regulator . This control the cut in and cut out of the air compressor. 

Bendix 275491RX Reman D2 Governor

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1 minute ago, RoadTripper2084 said:

Sorry to hear about the rough road, sounds terrible.

The compressor is mounted to the passenger's side of the engine, and is gear driven off the engine.

On my 97 Dynasty (C8.3 as well) the air dryer/purge valve is mounted behind the passenger side rear axle. If you crawl under the coach behind the rear mud flap you will find it. Make sure you brace the coach first using jacks or some other mechanical means of keeping it from potentially dropping down on you while you're under it. 

Can you tell if the leaking sound is coming from the front axle area or the rear closer to the engine? You should be able to try fast idling until your air pressure in both tanks hits 125lbs and the air dryer purges, then shutting the motor off and quickly going outside to listen for leaks. 

 

 

The sound of leaking air (and air dryer purge) is coming from somewhere near the rear axle passenger side. When I shut off the motor there is no more leaking. 

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23 minutes ago, planodp said:

You might want to check the Bendix D2 air pressure regulator . This control the cut in and cut out of the air compressor.

I haven't found anything that looks like that. But Monaco was nice enough to put the leveling jack and battery box in the way.

I think the device in the lower right is probably the air dryer, and the air purge seems to come from this area too.

 IMG_8595.thumb.jpg.ec2b22b8f0d106222b13e26837eaa312.jpg

Here's a few more pictures, that's as good as I can get for now. I guess I'll have to get the thing started to find out more. I thought I had the air dryer filter replaced when I bought the coach in 2018 but that canister on top of the control unit seems to be pretty old.

IMG_8607.jpeg

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IMG_8609.jpeg

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IMG_8611.jpeg

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If you find the air compressor, there will be 1/4 air line running to the regulator all so know as air governor.

 

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43 minutes ago, jimc99999 said:

The sound of leakin air (and air dryer purge) is coming from somewhere near the rear axle passenger side. When I shut off the motor there is no more leaking. 

I would suspect the purge valve on the bottom of the air dryer isn't sealing 100%.  Usually you can tell if you run the motor and stand outside near it so you can hear it when it purges, it will continue to leak after the purge, and not end in a crisp Pffssst! sound.  🙂

I have the same model air dryer on mine, do you happen to know what make/model it is?

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13 minutes ago, planodp said:

If you find the air compressor, there will be 1/4 air line running to the regulator all so know as air governor.

 

I did find the Bendix air pressure regulator, right in the back. The compressor is right behind the large hydraulic fluid reservoir so it's difficult to see or get my hands on.

8 minutes ago, RoadTripper2084 said:

I would suspect the purge valve on the bottom of the air dryer isn't sealing 100%.  Usually you can tell if you run the motor and stand outside near it so you can hear it when it purges, it will continue to leak after the purge, and not end in a crisp Pffssst! sound.  🙂

I have the same model air dryer on mine, do you happen to know what make/model it is?

The purge valve looks easy enough to replace, assuming it's available anymore. I can't see any make/model info, I can try to get a picture from a couple more angles.

I'm going to start it up and see if there's a continuous air leak from the purge valve. The purge sounds normal though, and even if it leaks, it seems like pressure should drop to 90 psi and kick the compressor on. The purge is controlled by the regulator, right? 

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12 minutes ago, jimc99999 said:

The purge valve looks easy enough to replace, assuming it's available anymore. I can't see any make/model info, I can try to get a picture from a couple more angles.

I'm going to start it up and see if there's a continuous air leak from the purge valve. The purge sounds normal though, and even if it leaks, it seems like pressure should drop to 90 psi and kick the compressor on. The purge is controlled by the regulator, right? 

Yes, the regulator disengages the compressor when 125lbs is reached, and triggers the purge on the air dryer.

You were seeing continuous cycling while driving between 125lbs and 90lbs, correct?  As if the compressor would fill to 125lbs and be disengaged, but then the air would rapidly leak down to 90lbsish and then the compressor would kick back on to refill to 125lbs, etc.  ?

 

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Posted (edited)
3 minutes ago, RoadTripper2084 said:

Yes, the regulator disengages the compressor when 125lbs is reached, and triggers the purge on the air dryer.

You were seeing continuous cycling while driving between 125lbs and 90lbs, correct?  As if the compressor would fill to 125lbs and be disengaged, but then the air would rapidly leak down to 90lbsish and then the compressor would kick back on to refill to 125lbs, etc.  ?

 

No, the compressor seemed to run continously. The air purge would happen about every 30 seconds while pressure stayed at 120psi. If I used any air (like pressing the brake pedal a couple times), the air pressure would immediately start increasing, even from 110 psi, back up to 120 and the air would purge again, and start purging pretty often while air pressure was at 120.

Edit: while the motor was running I could hear air leaking from the vicinity of the air dryer. When I shut off the motor there was no more sound of air leaking.

Edited by jimc99999
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Ah, sorry I missed that detail.

If your air pressure governor wasn't cutting out at 125lbs you would hear the overpressure valves on the wet tank (1/2 of the front tank) releasing at 150lbs. You would also see 150lbs or thereabouts on your air pressure guage, which you are not.

It still seems likely that the air dryer purge valve is sticking, though it could also be the rear brake relay valve in that area, it also has a large purge valve on the bottom of it.  My front brake relay dump valve recently stuck open and caused rapid air loss, resulting in continuous cycling between 90-125lbs and eventually made it so my compressor was unable to keep up to the leak at all. But in my case the leak was easily heard when the coach was shut off. I'm just waiting for new o-rings for it and will re-install, though I'm certain the debris that was causing the leak is now gone.

Note that I'm far from an expert on this stuff, just another guy learning as I go. 

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The air compressor runs when the engine is running, it’s mechanical driven and it is either in bypass mode (exhausting excess air pressure) or engaging the system to keep the pressure at required minimum pressure. 

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5 minutes ago, RoadTripper2084 said:

Ah, sorry I missed that detail.

If your air pressure governor wasn't cutting out at 125lbs you would hear the overpressure valves on the wet tank (1/2 of the front tank) releasing at 150lbs. You would also see 150lbs or thereabouts on your air pressure guage, which you are not.

It still seems likely that the air dryer purge valve is sticking, though it could also be the rear brake relay valve in that area, it also has a large purge valve on the bottom of it.  My front brake relay dump valve recently stuck open and caused rapid air loss, resulting in continuous cycling between 90-125lbs and eventually made it so my compressor was unable to keep up to the leak at all. But in my case the leak was easily heard when the coach was shut off. I'm just waiting for new o-rings for it and will re-install, though I'm certain the debris that was causing the leak is now gone.

Note that I'm far from an expert on this stuff, just another guy learning as I go. 

I had a rear brake relay valve start to leak last year on a cross-country trip, and had to have that replaced. Like you said, you could hear it leaking after the motor was turned off. 

The purge valve does leak for about 10 seconds after purging, before shutting off. Pressure stays at 120psi on the gauge, both needles the same. While idling, about every 30-40 seconds the dryer purges, the purge valve leaks a little for about 10 seconds, and then stops leaking. Pressure stays at 120 the whole time. If I use air by pressing the brake pedal a few times, then pressure rebuilds from wherever, not just 90 psi, hits 120, purges and starts the rapid purge cycling again.

1 minute ago, Rick A said:

The air compressor runs when the engine is running, it’s mechanical driven and it is either in bypass mode (exhausting excess air pressure) or engaging the system to keep the pressure at required minimum pressure. 

Hmm...if it was engaged full time, seems like either the purge valve would be venting full time or I'd be seeing pressures higher than 120. So it must be bypassing at least part of the time. Does the regulator control the low pressure switch from bypass to engaged? Or does the regulator only control the high pressure switch to bypass with dryer purge?

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Nevermind, you already answered my question. 

9 minutes ago, jimc99999 said:

Does the regulator control the low pressure switch from bypass to engaged? Or does the regulator only control the high pressure switch to bypass with dryer purge?

Both. There is an air line from the wet tank at the front to the governor, which it uses to sense the air pressure. Fills to cutoff pressure (generally 125lbs or so), and then disengages compressor (and triggers the air dryer purge valve), then when it sense a pressure drop of 20-30psi it re-engages the compressor.

Screen Shot 2022-06-22 at 10.41.33 AM.png

Edited by RoadTripper2084
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Does your air drop down after shutting down? I would lose air in about a 1/2 hour shutdown. I would have to wait for the air to build up before going on. Found a broken 1/4 in line up by the engine valve cover. I got a splice at napa and I was back on the road.  If you chance the Bendix D2 air pressure regulator be sure you don't plug unused holes. I made that mistake. Its easy to do. 

Good luck.   

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With your park brake off and wheels chocked is there a leak you can hear when walking around the unit. From your description it sounds like a brake pot could be leaking. Note you need to have air pressure built up to do this simple check

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22 hours ago, Bob Schmeckpeper said:

   If you chance the Bendix D2 air pressure regulator be sure you don't plug unused holes. I made that mistake. Its easy to do. 

Good luck.   

I'm thinking you mean 'do' plug unused holes.

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12 minutes ago, 96 EVO said:

I'm thinking you mean 'do' plug unused holes.

I've ordered a regulator and I'll just install the new one in the same configuration as the old one. Whatever holes are plugged on the old one will be plugged on the new one.

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40 minutes ago, 96 EVO said:

I'm thinking you mean 'do' plug unused holes.

No, be very careful that you put everything back the way you found them.  I  made the mistake of plugging all the holes.  I am now the proud owner of 3 of those valves.  Thinking they were bad. Finally looked at my old one an located the hole that was never plugged. Opened that hole back and fixed my problem.  

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While your description of whats happening sounds like a failing governor, I wouldn't rule out a faulty dryer!

May want to just replace both, before it takes out your compressor.

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Posted (edited)
10 minutes ago, 96 EVO said:

Your air dryer doesn't look like it has been serviced in a long time!

I know. I thought I had the air dryer filter replaced when I bought the coach but if that's a removable filter on top of the air dryer it's obviously more than 4 years old.

I do check the tank drains periodically and there's never any moisture expelled. 

Edited by jimc99999
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1 minute ago, Bob Schmeckpeper said:

No, be very careful that you put everything back the way you found them.  I  made the mistake of plugging all the holes.  I am now the proud owner of 3 of those valves.  Thinking they were bad. Finally looked at my old one an located the hole that was never plugged. Opened that hole back and fixed my problem.  

Is your governor bolted directly to the compressor?

3 minutes ago, jimc99999 said:

 

I do check the tank drains periodically and there's never any moisture expelled. 

Thats good!

Too many DP owners don't understand the importance of doing this!

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I replaced the governor, it didn't change anything.  

According to this video, if the check valve in the dryer is leaking, air will bleed back to the compressor, dropping the pressure in the wet tank, and wet tank pressure isn't shown on the dash gauges, but does control the governor.

 

I can't easily access the compressor end of the supply line but I loosened a line on the supply side of the dryer and there was a lot of pressure behind it. I'm not sure where that small line goes from that T adapter on the supply side, seems like all air should go through the dryer before being used. But loosening that line with the motor off, there was a lot of pressure. So I suspect that like in the video, the check valve on the dryer is bad (you can see it between on the dryer where the output line is connected). I don't have the right wrenches to loosen the supply line. I tried a channel-lock plier but I'm not home yet so I was afraid to pull hard enough to break something.

IMG_8655.thumb.jpeg.763161ac18403cf810df5972228337e4.jpeg

 

I talked to a shop about servicing the air dryer, they said that given shop labor and service kit prices, it's generally cheaper to just replace the dryer. The dryer looks close (but not quite identical) to the Wabco System Saver 1200 Plus. 

https://www.anythingtruck.com/category/htp-brake-air-dryers.html

On mine I see the check valve similar to the Bendix AD-9 the guy repaired in the video, but mine is not a Bendix AD-9. And while it looks similar to the Wabco System Saver 1200 Plus, that one seems to have an internal check valve. 

Does anyone know what air dryer was used on these late 90s coaches, and whether it's possible to get parts?

I can probably manage replacing it myself as long as I don't have to manufacture a mounting bracket, and I guess even if I have to have a bracket made it's probably cheaper to handle all that myself with a machine shop than have a truck shop do it. The air dryer itself looks pretty simple to hook up (supply, output, and control air lines).  I'll have to double-check if the one I have has a heater. 

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