• Park the vehicle, apply the parking brakes, and chock the wheels.
• Drain the air system.
• For systems that do not use an air dryer, a pressure gauge added to the air governor unloader port will aid with the leak test.
• Build system pressure to governor cut-out.
• With the engine still running, observe the dash gauges and the gauge added to the air governor for ten minutes.
If the vehicle is equipped with an air dryer, listen for air dryer purge.
NOTE: Do not cycle air accessories such as seats, doors, wipers, air bags, etc.
If there is any noticeable decrease of the air gauge readings or the air dryer cycled during the ten minute test, repair
the leaks.
NOTE: Leaks in systems that hold pressure for 5 to 10 minutes may be hard to find.
Air Compressor Diagnostic Test
• Park the vehicle, apply the parking brakes, and chock the wheels.
• Drain the air system.
• Start the engine and run at idle speed for 5 to 10 minutes
• Make sure there are no air demands on the air system.
• Use the brake pedal to bring the air system pressure to 276 to 414 kPa [40 to 60 psi] and press the throttle to the
floor. (Max RPM)
• Start timing when the system pressure reaches 586 kPa [85 psi] and stop timing at 689 kPa [100 psi].
• The pressure rise from 586 to 689 kPa [85 to 100 psi] should take 25 seconds or less. If the time is over 25 seconds,
the vehicle may have excessive tank volume or leaks. Confirm the compressor size and total tank volume using
the reference charts below.
• If possible test a second vehicle of the same make and model to help rule out excessive tank volume.
