Need HELP understanding my air brake system and compressor! - Diesel Truck Forum -
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post #1 of 5 (permalink) Old 10-17-2018, 04:28 AM Thread Starter
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Need HELP understanding my air brake system and compressor!

Hi guys-

I'm encountering a tricky situation with the air compressor on my 1991 International 4900, with the DT466.

So, a while back my air compressor (for the brakes) stopped working. It just stopped making pressure one day. I bought a replacement and installed it. Well, after about 600 miles of driving it started making a strange sort of high-pitched noise, AND I began to see a lot of evidence of oil in the compressed air, AND it began to slowly lose its capacity to make air (taking longer and longer to fill the tanks). Well it was still under warranty so I brought it back and got a replacement, which I then installed. The compressor is made by Haldex.

Before finishing the installation I checked for coolant flow through the compressor head (check) and also for engine oil flow through the compressor crank (check). Amazingly, however, this brand new compressor is also not building up air pressure, AND it's making the same high-pitched sound that the other one did when it started to fail. I think it's pretty unlikely that it is defective, straight from the factory, so I think there must be something else going on and that's what I'm trying to understand.

I now believe that the high pitched sound is actually coming from the air intake to the compressor. It has a block component which looks like some sort of reed-type valve, and I think the sound is coming from that. I actually don't know which way is the correct way to install that valve block, so I guess that could be part of the problem.

The other component that I think might be at fault is the air dryer / unloader. This is the component that dumps air when the pressure builds up too high. OR.. it could be the governor, which is a small assembly that bolts to the side of the compressor head, contains a spring-loaded valve of some sort, AND sends two hoses off in the direction of the dryer / unloader. As you might be able to tell, I don't know too much about this circuit. But it seems to me that it might be implicated. For instance, if the dryer / unloader OR one of the hoses going to it is clogged, it might be tricking the compressor into not making pressure. To be honest that theory doesn't seem to make a whole lot of sense; I think the compressor should just be making pressure all the time and letting the governor and unloader dump air when needed. But I'm just grasping to understand what's going on here...

OK, thanks for making it this far...
Please let me know your thoughts, if you have some insight about this problem.
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post #2 of 5 (permalink) Old 10-19-2018, 08:02 PM
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sounds like there are a couple issues going on here . 1. the air intake hose going to the compressor ,does this come from the air filter or is it plumbed to the engine intake or intercooler? if coming from the engine intake ,is there oil in this hose,could indicate a failing turbo. 2.the block on the side with the reed valve looking pieces ,I believe they face inward to allow air flow into compressor. 3.the lines going to your air governor,one line is a air signal from the drier to the governor to tell the governor when to unload(when there is 125psi coming back to governor ,the governor will send air to the unloader valves in compressor ) the second line is a pressure signal from the governor to the drier purge valve to spit off ,purge moister. A few things I would check,the compressor discharge line( the line sending air to the drier to fill the tanks) is this a copper line , stainless steel braided or rubber airequip line.check to see in this line is blocked,copper lines sometime pack with carbon,the stainless line will have a teflon liner inside they usually do not block,if it has the fabric this line has a rubber inner liner and cannot handle the heat built up by the compressed air. it is also possible your air drier is blocked and needs a service kit I hope this helps ,Bob
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post #3 of 5 (permalink) Old 10-22-2018, 02:41 AM Thread Starter
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Hey Bob-
Thanks for this reply. Very helpful.
I've spent a good part of the weekend looking at this.
Air compressor has its own dedicated little air filter bolted to the side.
I used fittings to bypass the air dryer; still no pressure build.
I then used fittings to take the governor out of the system; suddenly the compressor made good pressure, quickly! Of course I shut the engine before pressure went too high.
I took the governor apart and it was really dirty and "loose" in there (parts, seals not seeming to fit tightly). I cleaned the heck out of it and reinstalled it, but still no pressure. Will be attempting to get my hands on a new governor tomorrow and am hopeful that will solve it.
I think I'll be servicing the air drier soon as well, as I haven't ever done that in the 7 years of owning the truck.
Thanks again, your explanation was crucial in helping me understand the functions of the different components in this system
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post #4 of 5 (permalink) Old 10-23-2018, 02:39 AM Thread Starter
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New governor (cheap!) did the trick.
Truck now back to normal, luckily!
Thanks again,
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post #5 of 5 (permalink) Old 10-24-2018, 01:20 AM
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Glad to hear it.
Thank for posting the fix for others to learn from.
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