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Old 11-10-2020, 10:10 AM   #1
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Whistling Air Dryer on Cat C9

Hello Friends,

On our 2007 Revolution LE 40E we have a Cat C9 diesel. Love the engine and its one year pre emissions.

My wife was following me from the fuel station yesterday and she said she could hear a very loud whistling coming from the back of the coach. Load enough she could hear it driving down the highway. I thought she was imagining it and disregarded it. When I parked the coach at home and shut it off, the whistling was very loud and coming from engine bay. When I checked, it was coming from the Air Dryer. It slowly died down and then stopped in a minute but it definitely was coming from the air dryer. I could not find any air coming out when this was happening. This is your basic Meritor Air Dryer. I changed the filter this Spring as well if that matters.

Any ideas?
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Old 11-10-2020, 06:22 PM   #2
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Originally Posted by Stonehands View Post
Hello Friends,

On our 2007 Revolution LE 40E we have a Cat C9 diesel. Love the engine and its one year pre emissions.

My wife was following me from the fuel station yesterday and she said she could hear a very loud whistling coming from the back of the coach. Load enough she could hear it driving down the highway. I thought she was imagining it and disregarded it. When I parked the coach at home and shut it off, the whistling was very loud and coming from engine bay. When I checked, it was coming from the Air Dryer. It slowly died down and then stopped in a minute but it definitely was coming from the air dryer. I could not find any air coming out when this was happening. This is your basic Meritor Air Dryer. I changed the filter this Spring as well if that matters.

Any ideas?
Loose filter?
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Old 11-10-2020, 06:33 PM   #3
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They should have a pop off blow down valve that may not be closing off. Believe you can buy them for about $20.
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Old 11-10-2020, 07:23 PM   #4
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There is a emergency pop off valve, it opens if the air governor doesn't unload the air compressor to protect the system. Its usually mounted up higher and on the side of the air compressor. There is also a pop off valve on the bottom of the air dryer, it should pop off every time the air governor reaches the correct tank pressure. It pops off to dump the moisture captured by air dryer. The emergency pop off should never be opening. The dump pop off, should immediately shut off. Safe Travels
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Old 11-11-2020, 01:50 PM   #5
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Originally Posted by kenwyatt View Post
They should have a pop off blow down valve that may not be closing off. Believe you can buy them for about $20.
Thank you .
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Old 11-11-2020, 01:52 PM   #6
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Originally Posted by Cat View Post
There is a emergency pop off valve, it opens if the air governor doesn't unload the air compressor to protect the system. Its usually mounted up higher and on the side of the air compressor. There is also a pop off valve on the bottom of the air dryer, it should pop off every time the air governor reaches the correct tank pressure. It pops off to dump the moisture captured by air dryer. The emergency pop off should never be opening. The dump pop off, should immediately shut off. Safe Travels
Thank you. It sounded like it was at the air dryer so I will check out the pop off valve on the bottom of the unit. Might just not be closing 100% and if open even a whisker it would shriek pretty loudly.

Thank you.
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Old 11-21-2020, 05:32 AM   #7
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Originally Posted by Cat View Post
There is a emergency pop off valve, it opens if the air governor doesn't unload the air compressor to protect the system. Its usually mounted up higher and on the side of the air compressor. There is also a pop off valve on the bottom of the air dryer, it should pop off every time the air governor reaches the correct tank pressure. It pops off to dump the moisture captured by air dryer. The emergency pop off should never be opening. The dump pop off, should immediately shut off. Safe Travels
This happened on mine on the interstate, I got a low air warning and the air bags dropped down. I pulled over, heard the noise and just pecked the pop off with a hammer, system aired right up and I was able to get home to find out why I had too much pressure.
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