The air dryer purge is an important aspect of a semi truck's air brake system, as it helps to remove moisture and contaminants from the compressed air used to activate the brakes, suspension, transmission etc. In this blog, we will explain what controls the air dryer purge, how it works, and some potential causes for your semi truck to keep purging air. Remember this is general information for all makes and models of semi trucks, Volvo, Freightliner, Kenworth, Peterbilt, Mack, International, Western Star etc.
What controls the air dryer purge?
Ultimately the air compressor governor controls the purge, because that's where the process starts. When the air pressure becomes greater than that of the preset “cut-out” (of the governor).
How does the air dryer purge work?
The air dryer purges when the governor causes the compressor to stop pumping air. While this is happening, the line connecting the governor unloader port sends air through the the air dryer control port. The air pressure then causes the air dryer purge valve to open.
How often does an air dryer purge?
This really depends on a lot, such as: how sealed your air system is (older trucks tend not hold air so well), the health of your air compressor, your air compressor governors preset cut-off pressure, if there are any existing air leaks throughout the air systems. There are trucks that purge every 5 minutes at idle without any air activated functioning active, there are some newer truck especially specific makes and models that purge once every 30 minutes to 1 hour if sitting at idle without any air activated functions active.
What are some potential causes for a semi truck to keep purging air?
There are several potential issues that can cause a semi truck's air dryer to keep purging. These include:
-
Faulty air dryer filter: The air dryer filter is responsible for removing moisture and contaminants from the compressed air as it passes through the purge system. If the filter is clogged or otherwise faulty, it may cause the purge system to initiate more frequently.
-
Air leaks in the brake system: If there are air leaks in the brake system, it may cause the pressure in the air tanks to drop, which can trigger the purge system.
-
Faulty air compressor: The air compressor is responsible for generating the compressed air used in the brake system. If the compressor is faulty or not functioning properly, it may cause the purge system to initiate more frequently.
-
Faulty purge valve: The purge valve is responsible for allowing a small amount of air to flow through the air dryer filter during the purge cycle. If the purge valve is faulty or not functioning properly, it may cause the air dryer to initiate the purge more frequently or for longer periods of time than necessary.
-
Faulty turbo cut-off valve: The turbo cut-off is located in the air dryer base as well, along with the purge valve and works in sync with the other valves, if any of the parts inside of the air dryer base are malfunctioning it can cause frequent purges.
-
Faulty air dryer base: The air dryer base is responsible for supporting the air dryer, containing all of the valves, ports, sensors and tubing, connecting it to the brake system. If the base is faulty, clogged or not functioning properly, it may cause the air dryer to initiate the purge more frequently.
-
Faulty or failed air compressor governor: The air compressor governor is responsible for controlling the output of the air compressor. If the governor is faulty or has failed, it may cause the air dryer to initiate the purge more frequently.
-
Blocked or leaking air compressor signal line: The air compressor signal line is usually a small tube that connects the air compressor to the air dryer. If this line is blocked or leaking, it may cause the air dryer to initiate the purge more frequently.
-
Blocked or leaking air compressor discharge hose: The air compressor discharge hose is responsible for carrying compressed air from the compressor to the air dryer. If this hose is blocked or leaking, it may cause the air dryer to initiate the purge more frequently.
-
Air leaks in the suspension system: If there are air leaks in the suspension system, it may cause the pressure in the air tanks to drop, which can trigger the purge system.
-
Air leaks in the transmission system (if applicable): If your semi truck is equipped with an air-assisted transmission system, there may be air lines that run from the air tanks to the transmission. If these lines are leaking, it may cause the air dryer to initiate the purge more frequently.
In summary, the air dryer purge is an important aspect of a semi truck's air brake system, suspension, transmission etc. Properly maintaining and replacing the air dryer filter is important to ensure that the purge system is working effectively. There are several potential issues that can cause your semi truck's air dryer to keep purging, including problems with the air dryer filter, air leaks in the brake system, faulty air compressor, faulty purge valve, faulty release valve, faulty air dryer base, faulty air compressor governor, blocked or leaking air compressor signal line, blocked or leaking air compressor discharge hose, air leaks in the suspension system, and air leaks in the transmission system (if applicable). It is important to properly diagnose and address any issues that may be causing the air dryer to initiate the purge more frequently in order to ensure that the air brake system is operating at its best.
If you would like to read about properly maintaining your air dryer filter read our blog: Semi Truck Air Dryer Filter Troubleshooting & Tips
