Tech Tips: 4 Most Common Diesel Brake Air Compressor Failures

If you're reading this you probably already know at least a little something about diesel brake air compressors. So we won't bore you with the simple stuff.

What you really need to know is that the compressor is built of two main sections:

1. The Engine Block

2. The Cylinder Head

If you're in the market, browse our quality selection of new air compressor drives.

The diesel engine block holds the crankshaft and piston. The cylinder head holds the valves and unloader. If you're having problems with the compressor, here are some simple fixes to common issues.

The 4 Most Common Diesel Air Brake Compressor Issues

"My air compressor won't build air pressure."

"My air compressor is slow to build air pressure."

These are the most common complaints we hear when it comes to diesel engine air brake compressors.

There are a few things that can cause this:

• Compressor Drive

• Piston Rings

• Air Governor

• Unloader

• Valves

• Head Gasket

1. Diesel Air Brake Compressor Drive Failure

If the compressor drive failed, in most cases the compressor won't put out any air at all. It is possible that the drive catches intermittently, but that is rare. If the piston rings / cylinder wall are worn out, depending on the severity, the compressor either won't pump at all, or it will pump slowly.

The easiest ways to test both the drive and rings at once are to disconnect the discharge line and push a compression tester equipped with a rubber tip into the outlet fitting.

Note: Do not thread a tester onto the discharge line as it could cause extreme pressure and damage. If the pump is capable of building at least 120psi, this means the drive and rings are fine.

2. The Air Compressor Doesn't Build to at Least 120psi:

If your diesel air compressor doesn't build to at least 120psi, remove the air governor and test again.

If it builds air with the governor removed, verify that the air dryer is not sending a signal to the governor to unload the compressor.

If no air is coming from the air dryer signal line, change the governor. If there is air present, repair the dryer.

3. The Air Brake Compressor Still Won't Build Air Pressure with the Governor Removed:

If your air brake compressor still won't build air pressure after removing the governor, the next step is to disassemble the unloader.

Inspect the parts for damage and correct operation. If a problem is found, replace the damaged pieces and re-test with the compression gauge.

4. No Problems Were Found with the Uploader:

Remove the parts necessary to inspect the cylinder head valves and head gasket. Repair or replace any damaged parts and use the compression gauge to test again.

What If No Problems Were Found?

At this point, either the drive is bad, or you need to change the compressor. Extensive troubleshooting may or may not reveal the cause, but factoring time and labor rates could easily exceed the cost of replacing the compressor.

We offer a wide range of new and remanufactured air brake compressors at Highway and Heavy Parts, the leading diesel engine parts supplier.

If at any time during these tests the compressor is found to be producing at least 120 psi, the compressor should be considered fine. The problem of your air compressor building air pressure slowly (or not at all) will be found somewhere in the air system.

The most common issues are a blocked discharge line or a malfunctioning air dryer.

Visually inspect the discharge line for blockage or kinks. You can use a shop air compressor to blow through the line, ensuring it's clear of obstruction. Replace the line if any problem is found.

The easiest way to test the air dryer is to bypass it. If the system builds air with the dryer bypassed, rebuild or replace the dryer.

For more information on issues you may be experiencing in your engine, check out our past blog, Common Diesel Engine Problems.

Other Common Problems with Air Brake Compressors

Why Does My Air Brake Compressor Cycle Constantly?

This problem usually points back to the air dryer, air governor, sensing line, or the air compressor unloader. Here are some easy ways to troubleshoot:

• Remove the air governor sensing line at the governor and use regulated shop air to cycle the compressor. If supplying and removing the shop air cycles the compressor normally, it indicates the air compressor unloader and governor are working normally. If not, inspect and replace the unloader parts or governor as needed.

• Check the air governor sensing line and fittings for leaks. Leaks in this line will let the pressure bleed off and cause the compressor to cycle. If a leak is found fix or replace the line. 

• If no problems are found, check the air dryer for correct operation. Rebuild or replace it if any problems are found.

Summary

These checks and tests are meant to be a quick and simple way to test a diesel brake air compressor and determine its functionality. There are always exceptions and rare occurrences, but these should be able to diagnose most issues. Highway and Heavy Parts has an extensive selection of compressors for nearly all applications.

Let us help you get the right air brake compressor! We have ASE Certified Techs who can help make sure you're getting the best compressor for the job. Call them at 844-215-3406, or request a quote online.

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