Bendix AD IS Purge air leak

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Rockslyde, Sep 24, 2021.

  1. Rockslyde

    Rockslyde Bobtail Member

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    I have a 2014 Peterbilt 579, Mx13 motor. The air dryer is a Bendix AD IS. When the air pressure goes a little below 100 psi, the compressor kicks in. While the air pressure is building, there is no leak, as soon as the dryer purges it starts leaking from the purge valve again and continues to leak until the compressor kicks in again. (about 4 or 5 minutes) If I turn the engine off while the dryer is leaking , it stops leaking. So it only leaks while the engine is running. I changed the purge valve and desicant cannister this morning, no luck. So, eager to get it back on the road I went and bought a new dryer. Swapped it out, still leaking the same way.
    I've been researching it, and it's been suggested that the unloader valve in the compressor might be the culprit. I,m not sure what it looks like or where its located on the compressor. The compressor is located on the driver side near the firewall and its pretty tight on space there. Its a Wabco 1977704PRX, It's fairly new, replaced about a year ago. Can anyone help me out on this?
     
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  3. Goodysnap

    Goodysnap Road Train Member

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    The dryer/ dryer mounted governoer signals the conpressor to turn off via the unloader line at 130psi air is on the line. This is when the purge occurs inside the dryer. At cut in or 95 or so psi the air is off the line and the compressor builds.

    Start by checkiing this unloader line from the comressor to the governor for leaks. With the engine running it may be hard to hear it leaking. If you disconnect the unload line at the governor does it still leak? Did this gradually get worse or did it start after a repair event?

    Bendix is a great help- 1800AIRBRAKE
     
    Last edited: Sep 25, 2021
    Flint1 and Dave_in_AZ Thank this.
  4. Rockslyde

    Rockslyde Bobtail Member

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    The leaking air from the purge valve is pretty loud, I think You might be suggesting that there is a different leak. I will check on that and try disconnecting the unloader line at the governor. As for recent repairs, the pins on the leaf springs were changed just a couple of days before this leak started. Also, I should mention that the purge valve has been occasionally making the "rattlesnake sound" for a month or so before it turned into a full blown leak.
     
  5. Rockslyde

    Rockslyde Bobtail Member

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    I disconnected the unloader line at the governor. It still leaked until the compressor kicked on. Then it stops leaking until the purge. Just the same as it has been doing. .
     
  6. Goodysnap

    Goodysnap Road Train Member

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    With the unload line disconeected the compressor should be running all the time. Did air leak out of the governor unload port ( where you disconnected the line) at cut out pressure of 130psi when the dryer purged? Did you replace with a Bendix unit? Came with a governor on it right? Is the exhaust port open on the governor?
     
  7. Rockslyde

    Rockslyde Bobtail Member

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    When the dryer purged,the air came out of the governor unload port, at high pressure, much more than a mere leak it was at full power. It was not a Bendix unit, but yeah, the governor was already on it. Not sure which is the exhaust port, I'll look on a diagram and let you know on that.
     
  8. Rockslyde

    Rockslyde Bobtail Member

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    The exhaust port is not open on either the old or the new dryer. A black plug on both.
     
  9. Goodysnap

    Goodysnap Road Train Member

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    Some IDIS dyer are exteded purge and may continue to leak after initial purge. I would suggest double checking that the dryer purge leaks until cut in pressure. Inspect the entire truck system for other air leaks. If you can, charge the system to full cut out pressure with shop air so you might hear other leaks.

    The purge process all takes place from within the dryer. This is why the governor is mounted to the dryer itself. Which you have allready replaced. Can you remove the discharge line at the dryer, is the compressor still pumping after the compressor is unloaded, immeadiately after purge.

    If you can apply air to the unloader line at the governor the compressor should not build. Your original suspect of the unloader may be accurate but is not common. Was the replacement a Paccar compressor?
     
  10. Rockslyde

    Rockslyde Bobtail Member

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    The replacement compressor is a Wabco, which I believe is OEM. The compressor stops pumping after the purge until the air pressure drops to about 96 or 97 lbs. It then kicks in, at which time the purge valve stops leaking. It starts leaking again immediately after the next purge. I think I can rig up a way to charge the system with shop air to check for other
     
  11. Rockslyde

    Rockslyde Bobtail Member

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    However the main leak coming thru the purge valve stops when the engine is turned off.
     
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