The Complete Guide for Air Dryers

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Proper care of your air dryer is vital. Learn more about functionality, maintenance and tips to keep your air system running effectively.
Sioux Falls Service Center

Chilly weather, snow, ice—it’s coming. And with dropping temps comes potential for air system freeze-ups. While you can’t do anything about the Mother Nature’s cooler side, you can do a lot to keep your air system working properly. With a little preventative maintenance, you limit those incidences of being stuck on the side of the road in the middle of nowhere. Before we dive deep into PM, common issues or even the brands we recommend, let’s go over how air dryers actually work.

How Does an Air Dryer Function?

An air dryer’s job is to remove moisture before it reaches the supply tank. Easy enough, right? So as warm, moist air comes off the compressor, it mixes with the cool walls of the air dryer. Some of this moist air will condense on the walls and drain down into the purge-valve. At that point, the air is forced through a filter that removes the moisture, collects solid particles and aerosol contaminates. This treats the air. Then it expels the treated air through the drying via a one-way valve. Then the governor will simultaneously signal to the air compressor to stop, at the same time it will signal the purge valve to open and force all the air and water contaminates out. This purge cycle also helps clean the filter!

Common Air Dryer Problems

So, what do you do when this purge cycle isn’t working properly and why? Well, other air contaminates, like oil and debris, can slow or stop the cycle, especially when the thermostat reading drops and oil tends to thicken, which would never happen in the warmer seasons. The problem? This can wreak havoc on your air brake system. If you notice your brakes are “sticky” this could be the culprit. Contaminates such as oil can cause premature damage to other components as well, with spring brake modulating valves, brake chamber diaphragms, oil seals, dash valves and more. Not to mention, if not treated, any system that relies on filtered, clean air is in jeopardy such as the engine, emission and drivetrain systems.

Preventative Maintenance

The solution, a little PM will keep your air system running smoothly. Twice a year, give or take, give the entire system a cleanout. Have the experts at Wheelco do this for you. Test your air compressions for excessive oil pumping, check your wet tank for excessive moisture, and service your air dryer or change the filter if needed. 

Daily Checks

Heavy Duty Diagram
  1. Drain your reservoir! On average, this takes about 10 minutes. It is the most basic step in limiting freezes. Water was designed to collect here in the reservoir, but it can jeopardize your air system if it’s not drained. Even a teaspoon of water can freeze in a matter of minutes on the highway. It if does, it can disable a portion of your air brake system, costing you time and money.
  2. Check your system for excessive air leakage. If your compressor is having to pump more air, it’s also pumping more moisture into the air brake system.
  3. Make sure your air compressor discharge line is angled downwards (away from the compressor). If it’s not, it can get backed up, pooling in kinked areas and end up freezing your lines.

Quick tip! If you have a PuraGuard oil-coalescing cartridge (as it is an OEM standard) you must replace it with a like unit.

 Why is a coalescing cartridge needed?

  • Today’s engines are more fuel efficient which generate more heat, use a lesser weight of oil and the oil can aerosol.  This oil aerosol can contaminate any components that are connected to the vehicle’s air system.
  • The Meritor WABCO oil coalescing cartridge removes 99.9% of contaminates from the air stream.
  • Vehicle manufacturers often require that an oil coalescing cartridge be maintained to avoid voiding a production warranty on different systems (i.e. emission controls, automated manual transmissions, etc.).

When did coalescing cartridges start to be used?

    • 2008 to today for Volvo & Mack.
    • 2010 to today for Freightliner, Western Star, Navistar, Kenworth and Peterbilt.

What cartridges does Meritor offer in oil coalescing technology?

    • Reman AD9 (R955107794PGX)
    • Reman ADIP (R955109493PGX)
    • New Spin-on (R950068A)
    • See the Meritor Air Systems catalog (PB1484), the Meritor WABCO ABS & Air Systems catalog (PB8857AS) or for additional details.

 What cartridges does Meritor offer in oil coalescing technology?

    • Reman AD9 (R955109685PGX or R955109689PGX)
    • Reman ADIP (R955109477PGX)
    • Reman ADIS (R9555004050PGX, etc.)
    • Reman ADSP (R955109991PGX)
    • Reman SS1200Plus (S432 471 101 0X)
    • See the Meritor Air Systems catalog (PB1484), the Meritor WABCO ABS & Air Systems catalog (PB8857AS) or for additional details.

 What about the desiccant?

    • All Meritor WABCO and Meritor All-Makes dryers and cartridges (new or reman) feature 100% new OE quality desiccant bead, not reclaimed desiccant bead like many reman dryers and cartridges feature.
    • Desiccant is an expensive material and while it can be reclaimed (washed, heated, cooled) but it does not retain the adsorption performance (attracting the water vapor) as 100% new desiccant bead has.
    • Lesser performance quality desiccant bead may exist in other dryers and cartridges which may not have the same adsorption performance or durability of new OE quality desiccant bead as featured in the Meritor WABCO and Meritor All-Makes dryers and cartridges.
    • If a dryer has white powder near the purge valve this is a sign that the desiccant is being crushed and no longer performance in an optimal range.  This powder can contaminate the air dryer, air tanks, brake control valves, suspension control valves, ABS valves, emission controls, automated manual transmissions, and other air accessories.

 How often should an oil coalescing cartridge be replaced?

  • Meritor WABCO cartridges should be replaced every two years for on-highway applications and annually for severe service applications.
  • Bendix design cartridges (new or reman) should be replaced annually for on-highway applications and every 6 months for severe service applications.

How often should a standard or traditional cartridge be replaced?

  • Meritor WABCO cartridges should be replaced every three years for on-highway applications and every two years for severe service applications.
  • As a general rule of thumb, if more than two tablespoons of water are found in the wet tank it is time to replace the cartridge.  

Can I place a Meritor WABCO oil coalescing cartridge in place of a Bendix cartridge?

    • A Meritor WABCO R950068A can replace a Bendix 5008414PG oil coalescing cartridge.
    • Both remove 99.9% of contaminants per their published literature ( or

Need Assistance?

If your brakes are sticky, you have excessive air leaking or if it’s just been a while since your last tune-up, our service technicians are ready to inspect and diagnose your vehicle. So, if you have any questions, or if there is anything we can help you with, don’t hesitate to give us a call today.

About Wheelco Truck & Trailer

At Wheelcobeing customer driven is more than just a motto we live by. It means that our employees will do everything we can to best serve you, our valued customer.

Customer driven is more than just offering you advice, giving time-honored service or contributing the latest tech-support. It’s about being there—whenever and wherever you need us—in-person, over the phone or online.

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