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Old 10-23-2015, 06:28 PM   #1
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Compressor cycle.

I recently noticed that my compressor was cycling on and off about every 1 1/2 minute while the coach was idling. This seemed way to often for me so I had the system checked for air leaks. The tech found a leak on the left rear height valve and repaired it and said the cycle was now normal. I timed the cycle and it is now cycling every 2 1/2 minutes.

Is this a normal cycle time? Still seems like to often. What is the normal cycle time with the coach just sitting idling with no systems in use?
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Old 10-23-2015, 06:33 PM   #2
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You did not mention which motor but on my Cummins I had same exact problem compressor cycling every 90 seconds. I had a leak in the rear ride height valve. Replaced the valve and now driving down the highway it may cycle every 20 minutes depending on how rough the road is.

Sounds like you have more leaks or other problems. After compressor shuts off shut engine down and walk around coach listening for any leaks. Do not crawl under though unless you have jack stands for safety.


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Old 10-23-2015, 06:53 PM   #3
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If there is no leaks or movement in the air suspension the compressor shouldn't cycle at all. Once an hour would be normal.

If the shop thinks that 2. 5 minutes is ok, find another shop.

The WET tank, the first of 3 tanks, has no gauge, but has many connections that could leak. If that tank pressure drops, it signals the compressor to pump.

There is a regulator connected to that tank that often leaks. There is also a small 1/4 line running from the wet tank to the compressor governor, that often leaks.

Last but not least, at the connection from the air dryer line, to the wet tank is a check valve. If it leaks, the air comes out the bottom of the air dryer unloader valve.

Google "Air Brakes 2007" for a downloadable manual. It explains the whole system.
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Old 10-23-2015, 09:03 PM   #4
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A Preventive Maintenance question that may focus on your issue-- when was the last time the air dryer was serviced/replaced? It is not uncommon for the check valve/purge valve to fail.

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Old 10-23-2015, 09:25 PM   #5
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Air dryer was replaced 3 months ago. Checked for air leaks.
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Old 10-23-2015, 10:12 PM   #6
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If the problem stated after the air dryer replacement, check the 1/4" line that goes from the air dryer to the compressor governer.

To hear a leak, you need to wait until the air dryer blows off and then quickly listen. You only have 2 .5 minutes until the pressure in that line drops off to zero.

When the compressor is pumping there is no pressure to the unloader valve.
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Old 01-25-2016, 03:44 PM   #7
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Finally found the leak causing my compressor to cycle every few minutes. Turned out to be the air control valve, Cummins part #3770743, located on the turbo up high on the engine. Apparently Cummins has been aware of this leak possibility for some time. It only leaks while the engine is running making it almost impossible to locate easily.
Pricy little part by the way.
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Old 01-27-2016, 02:56 AM   #8
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The best way to find air leaks is with soapy water. Listening sometimes helps but you need dead quiet. I made a double male ended quick connect with a shut-off valve between. I can pressurize the coach from my shop compressor and have plenty of time to look and listen for the leak. Do not forget the line up to the parking brake and air horns
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Old 01-27-2016, 07:00 PM   #9
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Jerry, so now how long (average) is it between cycles while sitting at an idle?
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Old 01-28-2016, 02:10 PM   #10
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Matt,
I have not had the chance to let the coach idle for a long period yet. At the shop it idled for 5-7 minutes with no compressor cycle. I had installed an air pressure gauge in the line where you can attach an external air line in the PS rear compartment. Prior to replacing the air control valve you could watch the pressure drop as the coach idled and the compressor would cycle every 2-3 minutes. After replacing the valve the pressure held steady at 120psi while idling. Sometime in the next few weeks I will let it idle for 30 minutes or so to check, but going by the air gage I think my problem has been resolved.
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Old 01-29-2016, 06:15 AM   #11
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jerry davis, when u talk about the compressor cycling on and off are you talking about when the coach is idling and you here the air blow off at the rear of the coach?
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Old 01-29-2016, 06:31 AM   #12
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When the air blows off that is the end of a compressor cycle. The air dryer is discharging any moisture it collected.

A new cycle starts when the pressure in the wet ( first ) air tank drops below 90 to 95 lbs.

You may not see the pressure drop because there is no gauge in that tank. Check valves prevent the air, from the tanks with gauges, from going back to it.

If sitting still with the engine running, with no brake or suspension movement, the air dryer should not blow off at all.

A few small leaks may cause it to blow off every 1/2 hour or so but if it blows off every few minutes, it needs to be checked.
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Old 01-29-2016, 09:41 AM   #13
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mine blows off about every 5 minutes when setting still at idle
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Old 01-31-2016, 07:40 AM   #14
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DOT states,

Air Leakage Rate (Static check) (Pre Trip Inspection)
With a fully-charged air system (typically 120 psi), turn off the engine, chock the wheels, release (push in) the parking brake button and time the air pressure drop. After the initial pressure drop, the loss rate should be no more than 2 psi in one minute for single*
vehicles.

With a cycle range of 90 to 120 psi, you are loosing 30 psi every 5 minutes.

Thats 3 time more than recommended.

Of course you may not see the gauges drop, if the leak is in the 1 tank that doesn't have a gauge.

It's probably not a real big deal but every time the compressor cycles it puts more wear on it. Some have a 25% duty cycle, meaning they are designed to pump 1/4 of the time they are running.

The air dryer also collects moisture every cycle and may need to be serviced more often.
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