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Cummins Air Compressor Maintenance

March 27, 2011, 12:42:54 pm

Takes four minutes for my system to get up to blowoff pressure when idling at 1,000 RPM.  Used to take the same time at regular idle 750.  So I have been reading about air compressor maintenance checks in the Cummins Manual.  And searching Cummins Air Compressor on this site--to read about a couple of scary and costly stories....

So, does anyone actually perform maintenance on the air compressor as outlined in the Owner's Manual?  When we bought our used 1996 during 2000 from FOT, there (of course) was no maintenance records to be had; and their pre-buy inspection did not list air compressor maintenance as an item.  With the mileage on the coach presently, this procedure should have been accomplished twice: once before we bought it and once while we have owned it.

It seems this would be a good thing to get done.  And it appears straight-forward mechanically.  Wondering if anyone has done this maintenance and if problems occurred.  Also wondering if any of you engineers would comment about the effects of altitude on air compressor performance; we live and park the motorhome at 8,000 feet.

Andy1

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Re: Cummins Air Compressor Maintenance

Reply #1 – March 27, 2011, 02:09:13 pm

Andy,

What exactly is the Cummins recommended compressor maintenance are you referring to? 

Most air compressors share oil supply with the engine and receive their filtered air from the engine intake manifold.  Clearly if either of these is not true, there would be service needed.

Now air DRYER service is a routine service item.

Brett

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Re: Cummins Air Compressor Maintenance

Reply #2 – March 27, 2011, 02:48:10 pm

Air is thinner at 8000 feet.  I would worry more about how well it maintains the air pressure that building it.  Air compressors are just like engines and do get tired, but usually after a long life.  Now air governors and air dryers are another story, as are air tanks. 

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Re: Cummins Air Compressor Maintenance

Reply #3 – March 27, 2011, 04:04:23 pm

Sure, air is thinner at 8,000 feet.  But with their air intake on the engine's intake manifold, I'll bet you have plenty of boost and therefore plenty of air supply to the compressor. 

The beauty of taking air from the engine's intake manifold is that the main air filter filters is, the turbo boosts PSI and the CAC cools that air back down.

Brett

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Re: Cummins Air Compressor Maintenance

Reply #4 – March 27, 2011, 07:44:18 pm

Also, consider whether there might be newer, larger leaks in the system which would slow the fill rate.

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Re: Cummins Air Compressor Maintenance

Reply #5 – March 27, 2011, 11:09:11 pm

Thank you all for your comments.  I am reminded that the air is thinner at 8,000 feet whenever I hike uphill or chop wood.  And I have figured that my generator can only deliver about 70% of rated power at this altitude.  But the air supply to the air compressor comes off the turbocharged engine air manifold, so should not be much affected by altitude.

One of the Air System Test procedures calls for four minutes or less for the system to build from 5-20# to reach 120-30#'s.  Another Air System Test wants 3 minutes or less to build to 50-90#.  So, I thought this may be an important measure of air compressor condition.  As is recovery and maintaining a useable pressure!  Perhaps, my concern is just a case of "a little knowledge is a dangerous thing" and/or I am looking for trouble in all the wrong places.

Operation and Maintenance Manual C Series Engines, Section 7 - Maintenance Procedure at 48,000 mile/2000 hrs/2 years begins by saying that all checks or inspections listed under daily or previous maintenance intervals must also be performed in addition to these listed ....  Then tells of the Cooling System, Vibration Damper, and Air Compressor inspection/checks.  On page 7-9, under Air Compressor, Inspection is a "NOTE: All air compressors have a small amount of lubricating oil carry over" yada-yada... "will form varnish or carbon deposits."  These deposits will affect the air compressor piston rings and their proper sealing. 
   1. On page 7-10 I am told to disconnect the AC discharge line and inspect/measure the carbon buildup in the line.  If the buildup is excessive, then I am to continue inspecting lines to the first tank until I find the acceptable 1/16 inch buildup.  And to clean or replace the lines that exceed this specification.
   2. On page 7-11 I am told to "Inspect any air driers, spitter valves, pressure relief valves" (governor) ... "for carbon deposits or malfunctioning parts.  Inspect for air leaks."  Three months ago I installed a Haldex repair kit for the air dryer.  I was surprised how loose the large canister was.  But I followed instructions and only hand-tightened the new one.  (Cleaning the inside of the air dryer of gunk and oil while in place on the frame is a chore.)
   3. Page 7-12, Air Compressor Intake Inspection has a bold printed warning about holding down the unloader valve when removing the capscrews.  To remove and inspect the unloader valve/spring assembly, and "If carbon or heavy varnish is present, remove, clean, and inspect the compressor head and valve assembly.  Replace parts as necessary."  There is an O-ring and a rectangular seal that are to be replaced no matter how dirty these parts are.  And they are to be installed with anti-seize compound on the seal and 15W40 engine oil on the O-ring. 

Torque value for the capscrews is 10 ft#.

This carbon buildup in air lines and air compressor components is the inspection of which I had questions.

Andy1

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Re: Cummins Air Compressor Maintenance

Reply #6 – March 28, 2011, 10:38:02 am

If you serviced the air dryer and found no evidence of heavy carbon deposits, that should not be an issue.

The incidence of failure of air compressors on RV applications is very low.  An OTR truck with a million miles on it and 18 wheels worth of air brakes-- a different matter.

Here is a copy of the Air System Check that I wrote for the Caterpillar RV Engine Owners Club:

Air Brake System Pre-trip Check


1. Be on reasonably level ground.  Block wheels so the coach can not move even with the parking brake off.

2. Start engine and run until full air pressure is reached. Listen for the dryer to purge (about 120 PSI).

3. Shut off the engine.

4. Release the parking brake by pushing in the yellow button (make sure you don't roll, and do NOT apply the brake pedal).

5. After the initial pressure drop, the system should not loose more than 2 PSI per minute.

6. Apply the brake pedal firmly (still with the parking brake OFF).

7. After the initial pressure drop, the system should not loose any more than 3 PSI per minute.

8. With the engine off, ignition on and parking brake off, rapidly pump the brake pedal to bleed down the air supply. During this stage you should watch for the warning light and buzzer at about 60 PSI and then the yellow button (parking brake) should pop out at about 30 PSI.

9. Restart the engine and build up air pressure again. While building up pressure, check how long it takes for pressure to go from 85 to 100 PSI at "cruising RPM". It should be less than 45 seconds.




Brett

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