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Air dryer purging every 10-20 secs on my superliner


scottwetzel
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un do the 1/4 in line that trips the purge valve and see if this fixes the problem! it could be the air governor is having a spasm the other way to check the dryer is take shop air and blow into the trip line if dryer trips each time air is applied it's working right. and the problem is elsewhere in the air system.

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there are two air lines that go to the dryer, one is the unloader line, its stamed on the dryer. When the air dryer builds to 120 nominal psi the piston in the governor shifts closing the unloader line and opening the purge line, springs on the unloader valves in the head of the air dryer are overcome with air pressure and are held open. If an air dryer purges way too often the problem 9 times out of 10 is a leak in the unloader side of this system, be it a leak in the 1/4" line, the governor itself or the unloader valves in the head of the compressor. In a perfect system when you reach 120psi the unloader system holds this pressure until you use enough system air (braking, air horn or aux functions) to reach your cut in pressure (approx 90psi) when the sytem will re charge to cut out pressure (115-125psi) at that point the system will again close the valve and purge the system. Long story short, if its purging more than your using air, you have a leak in the unloader side about 90% of the time.

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"Any Society that would give up a little LIBERTY to gain a little SECURITY will Deserve Neither and LOSE BOTH" -Benjamin Franklin

"If your gonna be STUPID, you gotta be TOUGH"

"You cant always get what you want, but if you try sometimes you get what you need"

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there are two air lines that go to the dryer, one is the unloader line, its stamed on the dryer. When the air dryer builds to 120 nominal psi the piston in the governor shifts closing the unloader line and opening the purge line, springs on the unloader valves in the head of the air dryer are overcome with air pressure and are held open. If an air dryer purges way too often the problem 9 times out of 10 is a leak in the unloader side of this system, be it a leak in the 1/4" line, the governor itself or the unloader valves in the head of the compressor. In a perfect system when you reach 120psi the unloader system holds this pressure until you use enough system air (braking, air horn or aux functions) to reach your cut in pressure (approx 90psi) when the sytem will re charge to cut out pressure (115-125psi) at that point the system will again close the valve and purge the system. Long story short, if its purging more than your using air, you have a leak in the unloader side about 90% of the time.

So your saying the governor or the compressor is possible bad?

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What is your pressure gauge figures when it's purging?

I have the same problem, the pressure comes up to 145-150 (must be held about 120) and then the dryer starts to "pffff".

If I apply brakes to drop the psi down it doesnt.

Looks like all the discribed above, I just don't get myself to resolving but it's a hobby truck.

Никогда не бывает слишком много грузовиков! leversole 11.2012

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What is your pressure gauge figures when it's purging?

I have the same problem, the pressure comes up to 145-150 (must be held about 120) and then the dryer starts to "pffff".

If I apply brakes to drop the psi down it doesnt.

Looks like all the discribed above, I just don't get myself to resolving but it's a hobby truck.

My pressure goes to 120-125 then stops and purges but never really drops.

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We've rebuilt the compressor on the vac a few times, after the second rebuild it would have been more cost effective to just replace. After parts and labour twice, a new unit would have been cheaper

Lack of planning on your part does not constitute an emergency on my part....

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try a $25.00 governor first, if you don't see your air pressure gauge move it sounds more like a gov. issue as said before the comp. should build air to 120-130 psi then the air dryer should pop, air should be able to be depleted to 90-110 then the comp should again start building air and the cycle repeats

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Let us know what you find, and thanks for the great explanation of how it works...Mine is doing the same thing...replaced the governer, made no difference (the governer on mine bolts t teh compressor), My air pressure never drops below 125 and the dryers pffts about every minute, I thought since the pressure never drops, the unloader valve was stuck, BUT, as I am typing this, if the unloader was stuck it would build until the pop-off valve went correct? May have to rethink this...

Any thoughts?

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So your saying the governor or the compressor is possible bad?

Its possible but its more likely an air leak on the unloader side, get a spray bottle with a little dish soap mixed with water and with the truck running go spray everything down from te governor to the tank on the unloader line, I'll bet somewhere you'll see bubbles. If you build up to pressure and it stays up on the gauge but continues to purge there is a leaky line or o ring on the unloader side of the system, this won't show on the gauge but unless it holds pressure it will start the process all over again.

"Any Society that would give up a little LIBERTY to gain a little SECURITY will Deserve Neither and LOSE BOTH" -Benjamin Franklin

"If your gonna be STUPID, you gotta be TOUGH"

"You cant always get what you want, but if you try sometimes you get what you need"

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What is your pressure gauge figures when it's purging?

I have the same problem, the pressure comes up to 145-150 (must be held about 120) and then the dryer starts to "pffff".

If I apply brakes to drop the psi down it doesnt.

Looks like all the discribed above, I just don't get myself to resolving but it's a hobby truck.

Vlad

that system pressure is too high... you can adjust the govenor pressure setting... under the cap is a set screw and locknut, try a small adjustment, pump down the air and build it up again. setting should cut out no higher than 125psi, the cut-in pressure is usually 25psi lower if your D2 governor is working correctly. the "pffff" you hear may be the safety valve and not the dryer.

BC Mack

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Its possible but its more likely an air leak on the unloader side, get a spray bottle with a little dish soap mixed with water and with the truck running go spray everything down from te governor to the tank on the unloader line, I'll bet somewhere you'll see bubbles. If you build up to pressure and it stays up on the gauge but continues to purge there is a leaky line or o ring on the unloader side of the system, this won't show on the gauge but unless it holds pressure it will start the process all over again.

If it were leaking wouldn't I see a air pressure loss over night?

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Its possible but its more likely an air leak on the unloader side, get a spray bottle with a little dish soap mixed with water and with the truck running go spray everything down from te governor to the tank on the unloader line, I'll bet somewhere you'll see bubbles. If you build up to pressure and it stays up on the gauge but continues to purge there is a leaky line or o ring on the unloader side of the system, this won't show on the gauge but unless it holds pressure it will start the process all over again.

not always

"Any Society that would give up a little LIBERTY to gain a little SECURITY will Deserve Neither and LOSE BOTH" -Benjamin Franklin

"If your gonna be STUPID, you gotta be TOUGH"

"You cant always get what you want, but if you try sometimes you get what you need"

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  • 2 weeks later...

My compressor is a bendix tru flow 501. Not sure what the kit part number was. I went to the mack dealership gave them my vin and asked for a unloaded kit. Pretty simple to install even came with instructions. I recommend doing this first. The seals are small and like 84superdog said they are most likely missing or torn.

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According to the Bendix Quick Reference Catalog, the unloader kit for a Tu-flo 501 compressor is Bendix #279615N. I believe the Bendix "prefix" is 745. So that would make the Mack part number 745 279615N.

Leslie - what is the model of air compressor on your Superdog?

Ken

HOF City, PRR Country, and Charter member of the "Mack Pack"

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Always, always, check the unloader side of the system first for this issue, if its not an unloader line leak its the unloader seals 95% of the time. Changing governors wont fix a recurring dryer purge, changing compressors only fixes the issue if the unloader seals are bad and its a whole lot cheaper and easier to replace the seals than the whole compressor. I think Ive fixed this same issue a hundred times maybe more, its a very common problem, I used to keep a squirt bottle with soapy water and an unloader rebuild kit on my toolbox when I worked at Howard Baer it was so common. Glad you got it fixed before you spent big money on a compressor.

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"Any Society that would give up a little LIBERTY to gain a little SECURITY will Deserve Neither and LOSE BOTH" -Benjamin Franklin

"If your gonna be STUPID, you gotta be TOUGH"

"You cant always get what you want, but if you try sometimes you get what you need"

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Another thing that make the drier there purge too often is if the check valve in the drier is leaking draining the wet tank.On the mack like all trucks the air governor gets its air supply from the wet tank.So if the check valve on the drier is leaking back thru the open purge valve in the dreir the pressure drops on the wet tank and then the governor will engage the comp to charge up the wet tank again so it can lose pressure too soon and same old thing over and over. The 88 mack came out with a AD4 which had a plactic flpper check valve inside but the two wire AD9 which will go in the same brackets will have a out side screw in valve on the bottom which can be changed with out a tear down.

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glenn akers

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Always, always, check the unloader side of the system first for this issue, if its not an unloader line leak its the unloader seals 95% of the time. Changing governors wont fix a recurring dryer purge, changing compressors only fixes the issue if the unloader seals are bad and its a whole lot cheaper and easier to replace the seals than the whole compressor. I think Ive fixed this same issue a hundred times maybe more, its a very common problem, I used to keep a squirt bottle with soapy water and an unloader rebuild kit on my toolbox when I worked at Howard Baer it was so common. Glad you got it fixed before you spent big money on a compressor.

So many people including techs automatically think it needs a compressor. It's crazy!!! I kept telling them how could it need a compressor if the compressor is pumping the system up?? It comes down to when someone else is spending your money it automatically needs a compressor!

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