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    • #2714
      MyPrevost
      Keymaster

      Last night, during Hector’s presentation, Joe C. mentioned that he sets his customers’ air governors to 135 psi to help lift the rear of the bus when the tag axle is raised.

      Does anyone have any technical information or Prevost recommendations on whether this adjustment is advisable?

      #2716
      MyPrevost
      Keymaster

      Tom,

      Adding to the question, during one of our suspension sessions, a POG member mentioned that his tag axle didn’t lift. He said that when he moved the tag axle switch to the left of the driver’s seat, he could hear the tag air bags releasing but didn’t see the tag axle actually lift. He thought that in XLII models, the tag axle might not lift; instead, only the air would release from the air bags.

      Do XLII owners have any insights on this? On my XL, the tag axle lifts after a few minutes.

      Hector

      #2717
      MyPrevost
      Keymaster

      Hector,

      The 40’S will lift off the ground, but 45’s generally just unload the tag.

      #2718
      MyPrevost
      Keymaster

      The tag on my coach does not lift off the ground,I think that the switch only exhaust the bag.

      #2719
      MyPrevost
      Keymaster

      The Prevost Maintenance Manual specifies that the maximum pressure should be 125 PSI.

      Our coaches have many pressure-sensitive components, including brake chambers and air bags. Increasing the pressure to address a problem is like using aspirin to treat a brain tumor—it’s not a proper solution. I’ve heard various justifications for raising air pressure, but so far, every instance has been to fix an issue unrelated to pressure itself. For example, some coaches with excessive weight on the front axle have had pressures increased in an attempt to achieve proper driving height. The issue could be incorrect front air bags (a problem addressed in OKC) or excessive front axle weight.

      If the rear doesn’t rise when the tag axle is lifted, it might be due to too much weight in the rear or an issue with the hydraulic system designed to prevent rear drop, or perhaps unrealistic expectations. When the tag is lifted, the four air bags on the drive axle must carry the load. In my bus, they handle 8,000 pounds, which will lift the rear to the driving height, though it requires patience.

      If your tag axle isn’t lifting, it could mean the axle and wheels exceed 6,000 pounds, or there’s an issue with the system. Typically, if the tag isn’t designed to be raised, you won’t find a #30 brake chamber just above and behind the tag axle. Depending on the mechanical advantage of the lifting arm, the brake chambers can lift up to 3,000 pounds at 100 PSI each.

      Increasing system pressure beyond recommended limits can mask underlying issues. While some converters may ignore axle weight limits and some owners overload their coaches, be cautious with pressures around 135 to 140 PSI. These levels approach the air system’s overpressure limits, which are in place to protect both the bus and its components. Good luck to those of you pushing these limits.

      #2720
      MyPrevost
      Keymaster

      Interesting observations!

      Bluevost mentioned that Prevo Mira-Loma fixed his underperforming front axle issue by increasing the air pressure. From my experience, if Prevost sends you a “260” bag for the steer axle on an early ’90s chassis, the air dryer might cause the compressor to stop before the bus reaches ride height at 120 PSI. You’d need to pump the brakes to restart the compressor, and after doing this about three times, it might finally reach ride height. At 130 PSI, this issue usually goes away. With 130 PSI, the button responds better, and the ride height valve also reacts quicker.

      130 PSI isn’t crazy or dangerous; in fact, it might be necessary for heavy front axles or coaches with slides. Adjusting the regulator on an 8-V in an XL isn’t easy. My suggestion is to get the “259” bag, which will respond better at the lower 120 PSI. The “259” is the best bag for the front of these buses and works well for the tag axle too. It provides nearly 12 inches of travel compared to the 8 inches of the drive bags, which helps with lift and overall performance. The “259” doesn’t appear to be slightly overextended like the “104” or “105” bags used all around.

      For early ’90s and late ’80s XLs, you might need to enlarge the hole in the top plate of the bus to fit the bigger inlet of these air bags, but making the tool to do so is relatively simple.

      Jack 14R, do you know the PSI setting for the brake tanks on your XLIIDS when new?

      I’m planning to start a thread to clarify the condition I mentioned in Hector’s seminar, which was incorrectly stated in the first post. This will be useful for those with early ’90s vintage coaches and will help correct the record.

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