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Compresor activated noise, ideas?

February 28, 2015, 06:43:02 pm

We have a relatively high pitched constant noise, perhaps you would characterize it as a whine.  I can hear it easily if the coach is not moving, it is hard to hear if driving.

This noise began about 600 miles ago, first noticed when heading out of town, when building air pressure to drive..  I took it by MOT, got a quick listen and folks said it okay to drive, it may be a check valve, had heard such before.  However since then, I have found  it clearly occurs when the cummins engine air compressor is running.  It begins when the compressor comes on, stops when it pops off.  I have also heard the noise when pressing the brake but that may have simply coincided with the compressor running.  I ran all air loss tests and no problem.

Other information:  The air pressure cycles down from 120 psi down to 90 psi in about two minutes whereas in the past it took about four, depending on road conditions.  Also, when parked with engine running, if there is a cycle down from 120 it would be very long. Perhaps unrelated information.

I tried  Cummins or web literature to find out about that compressor, how it operates and might troubleshoot.  Not found much yet.

Ideas of the cause, seriousness?

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Re: Compresor activated noise, ideas?

Reply #1 – February 28, 2015, 07:09:18 pm

Mike,

That is too wide a range between cut in and cut out PSI.  I would change the air governor.  Pretty inexpensive and easy to change.

Like changing the fuel filters if a performance issue.  It may not be the problem, but it is a cheap first step and could certainly be related to that 30 PSI swing and perhaps the noise.

And that PSI drop over such a short time indicates a pretty good leak unless you are off roading with a LOT of suspension movement/brake application.  So "another first step" (isn't that an oxymoron??) would be to track down that excessive amount of air consumption.

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Re: Compresor activated noise, ideas?

Reply #2 – February 28, 2015, 08:59:38 pm

I would check the discharge line from the compressor they usually are stainless steel braided but the inner lining will deteriorate. There is a lot of heat from the discharge air and they can start to leak when the compressor is running. If it is original It might be time to replace it. You could spray it soap bubble solution and see if you see any bubbles anywhere.Pump down the air pressure with the brake pedal so the compressor will kick in.

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Re: Compresor activated noise, ideas?

Reply #3 – February 28, 2015, 11:23:40 pm

Thanks, two good ideas I think.

Fyi, i did a search and found Bret's outline of hiw to check brakes, you wrote it Brett for a Cat owner a few years back and posted on this forum.

Glad to see neither of you at your first ideas think it tbe compressor, that sounds more diffucult to change out or repair.

Mike

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Re: Compresor activated noise, ideas?

Reply #4 – March 01, 2015, 01:05:05 am

The discharge hose off the compressor are prone to failure over time due to heat,usually they will open and being it is enclosed in a stainless surround the compressed air ,could make a weird whistling type of sound. when the compressor unloads,reaches cutout pressure,it would stop. a small split in it could cause what you are hearing.jmo

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Reply #5 – March 06, 2015, 09:57:46 pm

Mike, we'll be looking to see how you resolve this.
Regards, Dick

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Reply #6 – March 06, 2015, 11:28:28 pm

Dick, I told MOT of Brett and Stump suggestions to check, they agreed.

A friend of mine....not exact knowledgef this system...wonders if compressor head gasket might be leaking.  I wonder about ride height valve.

Wondered if need to take to Bernd but MOT says they can fix either of those

Anyway, appointment is march 17, will certainly let all know outcome.

Mike

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Re: Compresor activated noise, ideas?

Reply #7 – March 13, 2015, 04:57:57 pm

Out to visit with James Stalling a moment, drop off a package for Mark who is getting work done later in March.

I told James a couple of weeks ago, I may want to get the tail lights on our coach like Roger Engdahl had done, told me would be a good safety feature.  James strongly supports the idea, so we talked briefly to let me know when can work us in.

So I mentioned the compressor noise to James and the ideas by Brett and Stump.  He said another option is the check valve on the air compressor.  So I will mention to MOT.  But makes some sense maybe.......the leak down rate is much faster after the compressor gets me up to the 120 psi, then the rate diminishes as a function of pressure drop.  My thinking then became....maybe error?...could point to the check valve.  As the pressure is high, the air is more likely to leak back thru the valve than when the pressure is lower?  Maybe that nonsense, just wrongheaded wondering.

East Texas could use three days of sunshine.

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Re: Compresor activated noise, ideas?

Reply #8 – March 23, 2015, 08:42:29 pm

The Answer....some asked for a report when this was resolved.

It turns out that when two check valves were installed two trips ago, they were the wrong valves.  MOT figured this out by running a separate air line from the back to the front of the coach, and using the new line, pressurized the front tanks.  The noise still persisted.  It was not coming from the line, which by the way they also had found to have and interior coating of dust and wondered if that related.  It was not.  Cleaned the line.

That moved them to taking check valves off, trying air pressure directly onto the valve, held in hand.  The noise was said to be like a train going by.  It was only evident as a loud whine from inside the coach.  The valves were returned to whence they came and MOT asked to try another pair. Hooked those to air pressure. Same result.  The new, next pair also very noisy. 

Upon further work with the supplier, it was determined that a shipment of valves had come in, all the wrong style or whatever you call it.  Wrong model.  SO, got the correct ones, retraced steps to connect all up and problem solved.

I do not know yet, from my testing, if that impacted the air utilization cycle time (from 120 to 90 psi) but I guess it may have.....perhaps the wrong valves let air go back the wrong direction?  Seems odd though, would not have been lost.

Anyway, thanks to MOT.  They spent extra hours on this, only charged for two, saying it only right.  I argued that they had cleaned the line, made trips to test and then secured new valves, then the install and test.  They said the price was fine if I was.

I suspect this very unusual.  But if you should be installing new check valves, be sure you get the correct size, model or whatever identifies these things, regardless how reliable the source. 

Mike

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Re: Compresor activated noise, ideas?

Reply #9 – March 23, 2015, 10:27:20 pm

Thanks for the follow up Mike!

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