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Old 05-16-2012, 07:49 PM   #29
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So, if the compressor isn't running constantly and I am maintaining a constant 125psi air pressure, is the dryer "spitting" every 20 seconds something that I even need to worry about?
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Old 05-17-2012, 05:32 AM   #30
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So, if the compressor isn't running constantly and I am maintaining a constant 125psi air pressure, is the dryer "spitting" every 20 seconds something that I even need to worry about?
IMHO, no. I've heard many a truck air drier react that way. There's one specific model, often used on Volvo trucks, that's notorious for it. I wish I could recall exactly what it was. I know it's not the Bendix AD-9 though. I think it's an Arvin-Merritor drier.
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Old 05-17-2012, 05:40 AM   #31
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Please chock the wheels before releasing the parking brake and walking around the coach. Safety first always.
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Old 05-17-2012, 08:25 AM   #32
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Is there a way to tell if the compressor is cycling on and off correctly or to tell if it is running constantly? I don't want to burn up my compressor.
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Old 05-17-2012, 04:51 PM   #33
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Is there a way to tell if the compressor is cycling on and off correctly or to tell if it is running constantly? I don't want to burn up my compressor.
If your gauges never exceed 125, the compressor isn't pumping. There are pop-off valves in the tanks that would go off usually at 140 psi should a compressor not unload.

Follow this ..... the air governor gets pressure from the wet tank which is the first tank in line. When the tank pressure reaches the governor setting .... 125 psi generally .... it opens and sends air pressure to the unloaders on the air compressor which opens the valves. The compressor continues to operate normally but it doesn't build pressure because the valves are being held open.

This air pressure signal also goes to the air drier and causes it to spit, to expel any accumulated moisture. There's a valve, springs, heater, etc on the air drier that does this. That valve assembly could be closing after it spits therefore allowing it to spit again.

A long time friend at the Volvo dealership in Lexington, Ky. told me lots of the Arvin-Merritor driers on Volvo trucks do this. No cause for alarm.
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Old 05-17-2012, 07:51 PM   #34
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Thanks for the info. From some of the earlier posts, I was afraid that the dryer popping every twenty seconds meant that the compressor was running continually and was going to burn up.
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Old 05-18-2012, 05:07 AM   #35
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Thanks for the info. From some of the earlier posts, I was afraid that the dryer popping every twenty seconds meant that the compressor was running continually and was going to burn up.
Nope, not at all. You probably should have your air drier serviced though if you haven't in the past few years. A lot of models used on RV's have a big spin-on dessicant filter and it can be changed in about 10 minutes. If indeed it's a Merritor-Wabco, it'll probably look like one of these.

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Old 05-19-2012, 08:42 PM   #36
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Thanks again for the help!
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Old 05-20-2012, 08:41 PM   #37
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One last question...When the dryer releases the air, it gives a puff and then hisses (pretty loud) for about 3-5 seconds before it quits. Most other motorhomes and trucks that I have heard just give a quick puff and that is it. Is the 3-5 second hiss normal or does it indicate some type of issue?
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Old 05-21-2012, 04:42 PM   #38
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One last question...When the dryer releases the air, it gives a puff and then hisses (pretty loud) for about 3-5 seconds before it quits. Most other motorhomes and trucks that I have heard just give a quick puff and that is it. Is the 3-5 second hiss normal or does it indicate some type of issue?
If it keeps leaking out of the air dryer after the first puff it needs rebuilt, which is more then just changing the filter. The desiccant in the filter breaks down and mixes with dust etc. and gets into the valves of the air dryer causing the dryer to keep leaking off till they finally seat.
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Old 05-21-2012, 08:08 PM   #39
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Timex pretty much called it. You may be a couple years too late changing the filter.
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Old 05-23-2012, 11:05 AM   #40
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Thanks for all of the help. I just bought this motorhome a few months ago and have been trying to work out some of the bugs. It seems like the previous owner did not perform much preventative maintenance. Two more questions, is the compressor RPM tied to the engine RPM or it somehow constant? Also, my compressor is a Holset. I ordered a new governor from Freightliner based on the last 6 of my VIN, but I cannot find anything resembling the governor on the compressor itself. Is the governor located on the compressor or would it be in-line somewhere between the compressor and the air dryer?
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Old 05-23-2012, 05:36 PM   #41
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The compressor is gear drived off the engine so their speed increases together. The air governor could be mounted anywhere. Follow the small air line from the head of the compressor as it's attached to the governor.
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Old 05-25-2012, 07:30 AM   #42
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Update: Changed the air dryer filter, the internal coalesent(sp?) filter and the relief valve last night. The internal coalesent filter was in several pieces and it looks like the previous owner never changed any of this or at least not in the last few years. There was dirt and pieces of the broken coalesent filter where the coalesent filter seats in the air dryer. Cleaned all of this junk up and it doesnt look like the valve that you can see in the bottom of this area has any crud in it. Reinstalled everything and got a larger than normal burst of water out of the spitter the first time it cycled. Held my breath but still spitting every 20 seconds. Replaced the governor as well, but same thing. Does that narrow it down to unloader valve in compressor or bottom of air dryer or both? Does this now indicate that the compressor is running loaded all of the time since my pressure NEVER moves from 125 and the air dryer keeps spitting so frequently?
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