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Air System Help


KWRB

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I am not a big truck guy, but I have this firetruck i got a few years back that's a local heirloom, that I love. I'm currently having an air system problem that I can't diagnose and I feel like the problem SHOULD be simple.

The air system is pumping up. It seems like it's pumping up reasonably fast (read: doesn't seem to be leaking). It pumps to about 130 psi and then purges through the air dryer. When it does so, I can watch the air pressure drop from 130 to about 80 or so and the purge stops. It pumps right back up to 130 and repeats to purge-pump up cycle about every 30-45 seconds. I replaced the air dryer -didn't fix it. I replaced the air governor -didn't fix it either. I have sprayed all the air fittings I can find with soapy water and listened also, and I can't find a clear leak. I may not have gotten everything, but like I said, I can't hear a leak either.

Any ideas what might be the issue? I can take it to a shop, but like I said, it seems like it *should* be simple...

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6 minutes ago, Farmall1066 said:

I wonder if it’s in your air compressor? Unloader valves maybe? 

That's what im wondering 

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Are you certain the air is coming out of the dryer all this time? That sounds like what a safety valve would do… stay open until the pressure drops appreciably. You can rule out unloader valves, all the governor does is divert rez pressure to the cylinder head to hold the inlet valves open. Unless  you hear air hissing out the compressor inlet there’s nothing wrong with it. Does this thing have a desiccant dryer or a spitter?

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13 hours ago, Binderoid said:

Are you certain the air is coming out of the dryer all this time? That sounds like what a safety valve would do… stay open until the pressure drops appreciably. You can rule out unloader valves, all the governor does is divert rez pressure to the cylinder head to hold the inlet valves open. Unless  you hear air hissing out the compressor inlet there’s nothing wrong with it. Does this thing have a desiccant dryer or a spitter?

I'm going to answer these as best I understand. when it is purging, a fast and loud stream of air comes straight out the bottom of the air dryer. I'm 100% certain about this. Is the safety valve you're referencing located underneath the air dryer? It is a Bendix AD-9 air dryer.

 

 

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It just seems like a governor problem even though you said you already replaced it, try another one.  I’ve had a bad one out of the box once.  
Every time I’ve ever had air governor issues, it’s like you are saying, system builds up pressure, purges, drops to the cut in pressure, rapid air pressure increase, hits the high cutout, and purge, and repeat the process.  All happening in a short period of time. 

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No, I was referring to an ordinary reservoir relief valve. Pull up a generic air schematic online and see if there is supposed to be a check valve downstream from the dryer ( or look on your truck) because an awful lot of air is backflowing; the dry should only blow out what’s in the dryer, not drain the whole system. Has anyone worked on the truck ( other than what you have done) , or did it just start doing this by itself?

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1 hour ago, Binderoid said:

No, I was referring to an ordinary reservoir relief valve. Pull up a generic air schematic online and see if there is supposed to be a check valve downstream from the dryer ( or look on your truck) because an awful lot of air is backflowing; the dry should only blow out what’s in the dryer, not drain the whole system. Has anyone worked on the truck ( other than what you have done) , or did it just start doing this by itself?

It's definitely coming out of the bottom of the air dryer. I'll take a look and see if there's another relief valve somewhere. I have what I think is all of the paperwork that came with it when it was delivered in 1992. I know there is electrical schematics in there. There are probably air diagrams as well.

 

1 hour ago, Farmall1066 said:

It just seems like a governor problem even though you said you already replaced it, try another one.  I’ve had a bad one out of the box once.  
Every time I’ve ever had air governor issues, it’s like you are saying, system builds up pressure, purges, drops to the cut in pressure, rapid air pressure increase, hits the high cutout, and purge, and repeat the process.  All happening in a short period of time. 

I could try this. It was only a $20 part, and the replacement is Chinese so a bad one out of the box is possible...

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I don't think it's the governor, that just directs air to the unloader when pressure is high enough. The unloader is a possible culprit. The signal line to the governor is the only one that doesn't have a check valve. Quick check.....

 

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42 minutes ago, KWRB said:

It's definitely coming out of the bottom of the air dryer.

After the purge of air, does it continue leaking out of the purge valve? If so then I’d say the purge valve is bad right out of the box, or not installed correctly.  

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2 hours ago, ihrondiesel said:

I think @Binderoid is on the right track. I had the same issue on one of my trucks earlier this year.  Turned out that I had to replace a check valve between the air dryer and the air tank. 

I’m not so sure now… the signal line coming from the unl port on the governor is under pressure at the dryer the entire time the compressor is unloaded. So, air coming out of the exhaust on the dryer could be from the governor and not necessarily backflowing from the reservoir. It would be easy to blame the dryer at this point, but he replaced it. Also, why did the old one start doing it to begin with?

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40 minutes ago, Binderoid said:

I’m not so sure now… the signal line coming from the unl port on the governor is under pressure at the dryer the entire time the compressor is unloaded. So, air coming out of the exhaust on the dryer could be from the governor and not necessarily backflowing from the reservoir. It would be easy to blame the dryer at this point, but he replaced it. Also, why did the old one start doing it to begin with?

Are you sure you hooked the signal line to the correct port on the governor? Aren’t there 3 ports on it?

 I fought with one a few years ago. Turned out my signal line was leaking. 
I wouldn’t rule out the check valve either. 

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