AIR DRYER TROUBLESHOOTING & MAINTENANCE GUIDE

The air dryer is your truck’s first line of defense against air system failure. It keeps your braking system safe by filtering out moisture, dirt, and oil vapor before air reaches your tanks.

Your braking and suspension systems depend on clean, dry, and reliable air. Contaminants like water, oil, and particles can freeze or corrode critical valves-causing uneven braking, poor suspension response, or total system failure.

Warning Signs of a Failing Air Dryer

• Frequent purge or constant exhaust from the purge valve
• Moisture or oil in the first reservoir tank
• Slow air pressure build-up after start-up
• Excessive compressor cycling
• Visible oil discharge or desiccant powder near the purge port

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Understanding Air System Maintenance

We all know the importance of maintaining the air system. The braking and suspension systems rely on a consistent air supply. Air system failures caused by water, particles, or oil can lead to truck-down situations, costly repairs, and lost productivity.

Air dryer maintenance should involve more than just swapping out the cartridge. Here’s why.

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Air Dryer System - Beyond the Cartridge

Most operators only replace the air dryer cartridge during service. But after seeing multiple air dryer assembly and purge valve failures, it’s clear that changing the cartridge alone isn’t enough.

After reviewing Bendix recommendations and consulting with experienced technicians and parts managers, the consensus is simple:

• Replacing the air dryer purge valve is just as critical as replacing the cartridge.

Skipping this step risks premature system failure-and a roadside breakdown no driver wants.

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Why the Purge Valve Matters

You might wonder, “Why bother changing the purge valve?”

Because if the valve sticks or fails inside the air dryer assembly, it can render the entire unit unusable. That means replacing the complete dryer-hundreds of dollars in parts and labor-all because a small purge valve wasn’t serviced. Spending a little now saves a truckload later.

Whenever you schedule air dryer service, insist on replacing the purge valve along with the desiccant cartridge. This minor addition can prevent pressure loss, moisture carry-over, and costly downtime.

No one wants to be stranded roadside with a load waiting-especially when routine maintenance could’ve prevented it.

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AIR DRYER TROUBLESHOOTING GUIDE

1. Pop-Off Valve Blowing at Air Dryer or Compressor

Symptoms: Pressure spike or continuous venting near the air dryer.
Possible Causes:

1. Restrictions between air dryer and supply reservoir

2. Dryer cartridge needs servicing

3. Discharge line from compressor to dryer plugged or frozen

4. Malfunctioning safety valve

Fix: Inspect and clear restrictions, replace the cartridge, and verify safety valve operation.

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2. Constant Exhaust from Purge Valve

Symptoms: Continuous air leakage at purge exhaust port.
Possible Causes:

1. Internal purge valve leak

2. Compressor not unloading

3. Incorrect purge control line connection

4. Frozen purge valve (heater not working)

Fix: Confirm plumbing, inspect seals, and test heater continuity.

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3. System Fails to Build Pressure

Symptoms: Air build-up too slow or nonexistent.
Possible Causes:

1. Inlet and outlet lines reversed

2. Restricted check valve between dryer and first tank

3. Carbon or ice blockage in discharge line

4. Kinked or collapsed discharge line

Fix: Verify routing, replace faulty check valve, and ensure discharge line slope.

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4. Oil Present at Air Dryer

Symptoms: Oil residue near purge exhaust or outlet ports.
Possible Causes:

1. Normal service interval reached

2. Compressor passing oil

3. Oil carry-over beyond dryer (compressor failure)

Fix: Replace the cartridge. If oil persists, rebuild or replace compressor and service the dryer.

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5. Desiccant Expelled or Paste-Like

Symptoms: Powder or paste expelled during purge.
Possible Causes:

1. Loose or vibrating dryer mount

2. Contaminated or saturated desiccant

3. Excessive oil from compressor

4. Use of chemicals (alcohol) in discharge line

Fix: Tighten mounting, replace desiccant cartridge, and eliminate chemical additives.

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6. Noise During Compressor Charging

Symptoms: Rattling, fluttering, or hissing from air dryer.
Possible Causes:

1. Faulty check valve

2. Leaking turbo cut-off valve

3. Damaged purge piston O-ring

Fix: Replace defective components and clean cut-off assembly.

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7. Constant Cycling or Purging

Symptoms: Frequent purge cycles even with stable air pressure.
Possible Causes:

1. Leaks in fittings or hoses (test with soapy water)

2. Accessory leaks at supply reservoir

3. Governor internal or external leakage

4. Incorrect dryer type for engine

5. Faulty outlet check valve

Fix: Seal leaks, replace governor or check valve, verify dryer model.

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8. Dryer Does Not Purge

Symptoms: No purge after compressor unload.
Possible Causes:

1. Blocked or broken purge control line

2. Faulty purge valve assembly

Fix: Inspect control line, replace purge valve.

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9. Purge Piston Cycles Rapidly During Unload

Cause: Compressor not unloading properly.
Fix: Inspect unloader and repair as needed.

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AIR DRYER TROUBLESHOOTING GUIDE
POP OFF VALVE IS BLOWING AT THE AIR DRYER OR THE AIR COMPRESSOR			CONSTANT EXHAUST OF THE PURGE VALVE
1	Restrictions between air dryer and supply reservoir.		1	Purge valve internal leak.
2	Dryer cartridge needs to be serviced.		2	Compressor failing to unload
3	Discharge line from the compressor to the air dryer plugged or frozen.		3	Purge control line connected to wrong place, reservoir or wrong port on governor.
4	Malfunction of the safety valve.		4	Purge valve frozen open, check heater is working.
CANNOT BUILD SYSTEM AIR PRESSURE			OIL PRESENT AT THE AIR DRYER
1	Inlet and outlet connections are reversed.		1	Time for regular maintenance.
2	The check valve between the dryer and first tank may be restricted.		2	Air compressor needs repair.
3	Excessive bends in discharge line, oil turns to carbon in line, water freezes in line between compressor and first tank.		3	Remember some oil is normal, if oil is getting past dryer to first air tank, compressor repair must be done and air dryer serviced.
4	Kinked or blocked discharge line.
DESICCANT MATERIAL BEING EXPELLED FROM AIR DRYER OR BECOMES PASTE LIKE APPERANCE			NOISE IN AIR DRYER DURING COMPRESSOR CHARGING CYCLE
1	Air dryer not securely mounted to frame causing vibration.		1	Defective check valve in air dryer.
2	Malfunction or saturated desiccant.		2	Leaking turbo cut-off valve.
3	Compressor passing excessive oil.		3	Leaking purge valve control piston o -ring.
4	Chemicals such as alcohol added to discharge line before the air dryer.
AIR DRYER IS CONSTANTLY CYCLING OR PURGING			AIR DRYER DOES NOT PURGE OR EXHAUST
1	Test fitting, hoses, lines and connections with a soapy water. Tighten or replace as necessary.		1	Missing, broken, kinked, frozen, plugged or disconnected purge valve control line.
2	Test accessories that are connected to supply reservoir.		2	Faulty air dryer purge valve.
3	Test governor for leakage, either internal of external leak.
4	Be sure dryer application is correct if you have QE or E-type Cummins Engine.		AIR DRYER PURGE PISTON CYCLES RAPIDLY DURING COMPRESSOR UNLOADING CYCLE
5	Test outlet check valve on air dryer.		1	Compressor is failing to unload, check unloader.

Air dryer maintenance is preventive maintenance. Replace the cartridge and purge valve together, inspect lines for restriction or oil, and test your heater. This small habit keeps your air system dry, your brakes reliable, and your fleet on the move. Explore our entire range of Air Dryer Systems here.