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  1. #1
    Bobtail Member
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    Problem Building Air Pressure when cold

    I'm having a weird problem on a 2009 Volvo Truck with a Cummins engine, about 260,000miles. It is having a problem building air. Here's the situation, in the morning, when its cold (I'm in Georgia, so just referring to the truck not being run), I start up the truck (usually I'm low on air pressure, about 70psi or so), it builds up the pressure with no problem and shuts of at 120psi. Now, here's the problem, after I'm about to leave and press in the brakes, the pressure goes down like it always had to under 90psi, but it never starts building up again. If I start driving and use the brakes, it'll keep going down and won't climb. Also, I do not have any audible major air leaks. Now, if I shut off the truck and turn in on again, it'll start building again until 120psi and I'll go thru the cycle and it won't build back up after that initial build. After about 10-15 min or so after the truck warms up, I have no more problems. Pressure goes down under 100psi and I'll hear that small hiss from the governor and the compressor will kick in. Now, here's what I've changed so far: Governor (three of them, new Bendix models), filter on the Air Dryer, and then the complete air dryer with another new filter. What am I missing?


  2. #2
    Light Load Member
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    IN MY _ If I tell ya I gotta killya
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    If you're not hearing the governor kick in, than it's not doing it's job,but you've changed it often enuff to know it's not that. My starting point would be check the line coming off the tank to the governor. It may be clogged,maybe when it's cold out,take both ends loose and try to blow air through it. Or another quick thing is get another line made up and just temporarily run it, see if that fixes it ? Any machine shop that makes air line i.e. Weatherhead, Gates, Aeroquip etc. will make one for pretty reasonable price.

  3. #3
    SHC
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    Spoiled Rotten Brat O/O SHC's Avatar
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    Could also have something to do with the air dryer check valve

  4. #4
    Road Train Member Heavyd's Avatar
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    See if the governor is exhausting the cut out pressure when it supposed to. It could be sticking.

  5. #5
    Light Load Member
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    It might be the air compressor head I have seen a few of them where the unloader stick and does not let it build pressure but once the truck warms up it would be ok. Although none of them would build at first than not they just would not build pressure right from the start. But hey it's something to consider also these were the single cylinder holset compressors.

  6. #6
    Road Train Member blanco's Avatar
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    Interesting to find the cause. Welcome to the site.. I'm here in Suwanee, GA.

  7. #7
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    Since you have pretty much ruled out the governor and air dryer (Exhaust port stuck open) I will not go there. Some things you can check though are the compressor discharge (Carbon in discharge port or line) or inlet lines kinked, collapsing or blocked (Any strainer in inlet line). No low points in the discharge line. If synchro valve equipped (SV-1 for example) check it. (If compressor is turbo fed it will not have one of these). Check your air filter (Sounds ridiculous, but it's not).

    With engine off listen for air at the compressor inlet. Could be the unloader pistons or the compressor discharger valves. You could test to determine which but it's pointless. The compressor drive could be intermittently slipping.

  8. #8
    Bobtail Member
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    It is probably the unloader valve on the compressor head sticking. Next time this happens, take off the main output line off the compressor and the govenor line. Check to see if there is any pressure, if not tap the top of the compressor head lightly. This is quite a common problem with these compressors. You can rebuild them with the unloader kit. Also can be govenor signal line holding air pressure and not exhausting it out, check for kinks in line. But unhooking govenor signal line from compressor should put it into loadindg cycle. Hope this helps.

  9. #9
    Road Train Member pullingtrucker's Avatar
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    I agree with power freak and Koolk that you need to check out the unloader valve. Many people have no clue about this important little valve and the abuse it goes through. More than likely a o-ring on it got a little cut in it or it is coked up causing it to stick a tad.

  10. #10
    Light Load Member
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    I would have to give my vote to the unloader valve as well.
    Sometimes trying to figure out why a truck won't build air is tricky, especially when you can't hear air leaking. There are several components that all cause about the same problem. Usually I would check the main supply line for carbon, check the governor signal line, and then go from there. From the sounds of what you have already replaced that leaves the unloader valve. The rebuild kits are reasonably priced and not to hard to change.
    Good luck!

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