Dryer purging ever 30 seconds

Discussion in 'Trucks [ Eighteen Wheelers ]' started by bkellerm, Nov 18, 2009.

  1. bkellerm

    bkellerm Bobtail Member

    Nov 18, 2009
    I have a 1997 cummins 8.3 with a holset compressor and AD-9 dryer. The dryer is purging about every 30-45 seconds. It does this whether I am parked or driving. I know I do not have any air leaks as I only lose about 5 PSI over 24 hours.

    A couple of other details:
    -The pressure in the tanks seem to stay steady at about 120psi. It is like the the compressor cut-in is too ealry (or could it never be cutting out?)
    -I just replaced the desiccant cartridge but didnt rebuilt the purge valve.
    -About 10 seconds before the dryer purges there is kind of a "soft" purge that lasts for about 1-2 seconds.

    Any help or advice of what to look for would be greatly appreciated. I am hoping that it is not the unloader valve in the compressor. If it is, how hard are they to change or rebuild? I think it is a SS296 compressor.
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  3. Heavyd

    Heavyd Road Train Member

    Feb 4, 2009
    Your intuition is probably right. I think it would be the unloaders. When the system reaches 120psi the governor allows system air to the air dryer to signal the purge and allows air to the unloaders. The unloaders will just hold the check valves off their seat in the compressor head to stop the air from being pushed out of the compressor. The air compressor head gets pretty hot and the orings that seal the unloader pistons take a beating and get carboned up. They will leak big time and this is why the governor is always going from loaded to unloaded in 30 secs. To test this you would need to apply shop air to the truck while it is not running so the governor is in cutout mode and is allowing air to the unloaders. If they are leaking you can usually hear or feel it blowing back out the intake of the compressor.
    Good luck!
    NCTru919 Thanks this.
  4. Turbo45

    Turbo45 Bobtail Member

    Nov 16, 2009
    also check the small air lines going to the governer for leakage
    use soapy water and spray it on the hoses
    quite offten the line will "bleed" you wont be able to hear it but you can see it with the soapy water
    NCTru919 and Rajiv Thank this.
  5. vegetto

    vegetto Bobtail Member

    Dec 14, 2006
    if you have an air leak from plumbing out of the governor, like say to a fan hub, then these kind of leaks can only be heard with engine running. be sure its nothing like that. I just had a customer the other day that had this same problem, he replaced the governor and air dryer before bringing it to us. The problem? union connector for the fan hub air line was leaking.
    mihalchev, NCTru919 and blacklabel Thank this.
  6. 112racing

    112racing Road Train Member

    Nov 30, 2008
    pocono's, pa
    when was the last time you serviced the air drier ?

    plugged air drier cartridge will cause that

    the govenor can also be the cause but i'd change the drier cartridge first should be done at least once a year anyway.............winters commin
  7. Heavyd

    Heavyd Road Train Member

    Feb 4, 2009
    Air dryer service time is now, but I wouldn't start throwing parts at it with hopes of fixing the problem. Make sure you don't have any leaks first.
  8. Brickhauler

    Brickhauler Medium Load Member

    Oct 1, 2008
    Elizabeth CO
    Mine was doing that about every 2 mins. We thought it was the unloader in the comperssor and it turned out to be the purge valve in the dryer had a bad o-ring on it.
    NCTru919 Thanks this.
  9. kklogger

    kklogger Bobtail Member

    May 16, 2009
    My dryer was purging every minute or so. Changed the small air lines going from the governor to the air dryer and to the the air tank.The lines looked decent but were baked and brittle and leaked air.Solved my problem right away. Dont know how many times I rebuilt my air dryer before this but this fixed it. Also unloader valves are easy to change.
    NCTru919 Thanks this.
  10. down south

    down south Bobtail Member

    Nov 4, 2016
    I have a AD SD dryer been changed 4 times with new dryer. purge valves signal line changed unloader valve and pressure relief valve changed no air leaks no oil in tanks water in tanks normal any ideas
  11. ryanbob1

    ryanbob1 Light Load Member

    Sep 9, 2016
    just put a kit in the air dyer or not worry about it just dumps the air anyways it better if it purges anyway cause it will purge the water and #### out of it
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