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Air Compressor Troubleshooting

Air Compressor Troubleshooting

Compressors that are passing excessive oil as evidenced by the presence of oil at valve exhaust ports or seeping from air inlets is usually a result of:

  1. Restricted air intake – Check the compressor air filter and replace as necessary. Check the compressor air intake hose for kinks, excessive bends, and a minimum ID of 5/8″.
  2. Restricted oil return – Do not use sealers on mounting gaskets as this could result in diminished oil return on some compressors. Check the bottom oil drain line on compressors that use this feature to ensure that there are no bends where oil could pool and that the minimum ID is 1/2″.

Causes of slow air pressure build times are:

  1. Dirty intake filter or restricted intake line
  2. Restricted discharge line or compressor discharge cavity
  3. improperly functioning unloaders or governor

Common causes of high head pressure and resulting failure are:

  1. Discharge line is kinked or clogged with carbon
  2. Water traps in discharge line causing line freeze-up
  3. Discharge line smaller than the minimum recommended 1/2″ ID
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D-2 Air Governor

D-2 Air Governor

Never condemn or adjust the governor pressure settings unless you have checked pressure with an accurate test gauge or a dash gauge that is registering accurately. Standard dash gauges are only required to be accurate to within +/- 10 percent.


Remember the following if adjustment is necessary:

  1. Turn the adjusting screw counter-clockwise to lower the air pressure setting.
  2. Turn the addjusting screw clockwise to increase the air pressure setting.
  3. Be careful not to overadjust. Each quarter turn of the adjusting screw raises or lowers the pressure setting approximately 4 psi.
  4. The cut-in and cut-out range is not adjustable.

The most common cause of governor failure is contamination. Blow out all attaching lines, hoses, etc when replacing a governor. Make sure the reservoir pressure sensing line is routed from the reservoir so that no contamination may enter the line and pass into the governor. A replacement D-2 air governor can be found here.

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General Air System Valve Troubleshooting

Valve leakage is commonly caused by, but not restricted to:

  1. Contamination caused wear, damage, or sticking of the valve’s operating parts
  2. Application of air pressure to a valve’s delivery port from another device in the air system

Determine the port(s) improperly emitting air pressure and perform the following service checks:

  1. Exhaust port leakage at rest – Disconnect the delivery lines. If the leak stops, inspect the device at the other end of the delivery line for leakage. If the leakage continues, inspect and repair or replace the leaking valve.
  2. Delivery port leakage at rest – Consult the appropriate service manual for specific test procedures.
  3. Exhaust port leakage during application – Consult the appropriate service manual for valve specific test procedures.

Always blow out all attaching lines and reservoirs when installing a replacement valve to purge any contamination from the system. Avoid using thread sealant or tape as excess material can itself contaminate the air device.

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Air Valve Part Number References

There is no application guide that tells us what air brake valves the manufacturer used on your truck or trailer. The only people who can tell you what valve the vehicle manufacturer used based on a VIN are the vehicle dealers. With that said, there are other ways to determine which valve you need. You may find a label or a metal tag on the valve that has the OEM part number printed on it. Sometimes, the number is stamped right on the valve body. Raised casting numbers are usually no help at all. You’re looking for stamped numbers here.

If you can’t find any numbers, you can look for a match based on the pictures we provide here on This is easiest if you know at least what type of valve you’re working with. Is it a relay valve or push-pull valve? If you have no idea what kind of brake valve you’re replacing, you will probably benefit from looking over the Bendix Air Brake Handbook.

We try to provide a comprehensive cross reference lists for each air brake valve we offer here, but there are always OE numbers we haven’t learned about yet. The list of pages below contains useful part number references from OE valve manufacturers. You can try your part number at the sites listed or just give us a call or drop us an email for help.

  • Bendix Part Number Search Use this if you have a Bendix part number and want to find their current number.
  • Bendix Cross Reference Cross reference a vehicle manufacturer’s part number to a Bendix number.
  • Haldex Cross Reference If Haldex made it, they should be able to cross reference for your number to the current number. Look at the right side of the page for the cross reference search box.
  • Meritor Parts Online Use the search box at the top of the page.