Trucks use air pressure in different ways. Air compressors are components of the truck’s AC system, and they’re also essential for the air brakes and other pneumatic systems.

If you’re having problems with your systems or notice it’s not building air pressure normally, it may indicate an issue with the compressor itself. 

Signs of a bad truck air compressor include: 

  • Truck not building air pressure 
  • The compressor won’t start
  • Compressor won’t stop
  • Unusual sounds
  • Warm air 
  • Air leaks
  • Electrical problems

These symptoms can mean big problems in the air system, so you’ll want to have it checked out as soon as possible. 

How to Tell If Your Truck’s Air Compressor is Bad

Problems with your truck’s air compressor can be stressful to deal with, but they’re not something you can ignore. There are some common signs and symptoms of an issue with the compressor, but they don’t all mean that it will need to be replaced. 

Common Red Flags of a Faulty Air Compressor:

  • Truck not building air pressure
  • Both air gauges stay still for minutes at a time
  • Fast cycling from the wet tank
  • Compressor won’t start
  • Compressor won’t stop
  • Too much pressure building up 
  • Sounds coming from safety valves
  • Air or fluid leaks
  • Air compressor cycling constantly

Often, there are smaller issues with different parts of the air system that cause the system to malfunction. If you can identify the component causing the problem, then a simple repair can get your truck back on the road quickly without a huge expense. 

Let’s discuss some of the most common problems you may find with the air compressor and what’s causing them. 

1. Truck Not Building Air Pressure

One of the first signs of a problem may be that your truck isn’t building air pressure like it should. Or, it may seem to be building pressure more slowly than normal. Both of these concerns could point to issues with the air compressor. 

It should normally take under 45 seconds for your truck to build up at least 80 to 135 PSI, so if it’s taking longer than that, it could mean trouble. 

Air pressure malfunctions could mean problems with the unloader valve or the air governor. 

The unloader valve is what releases air from the tank and allows the compressor’s motor to restart as needed. If the valve isn’t operating properly, it could cause the compressor to malfunction. 

In many cases, drivers find that the unloader malfunctions because it’s accumulated carbon buildup and other debris, causing the valve to stick. Luckily, this is an easy fix if you find that your issue is caused by a faulty unloader valve. 

Simply cleaning the valves and relubricating them will normally resolve this issue. 

The air brake governor is what controls the compression system and allows the air brakes to function properly. The governor is a very important part of the system. When it’s not working properly, the compressor can produce too much air or too little, either of which can be troublesome. 

In fact, a faulty air governor can be very dangerous, which is why they’re included in the pre-trip safety inspection. 

You can test your governor by following these steps: 

  • Pump the system down so it’s above 80 PSI
  • Engage the throttle a little to see if the PSI increases
  • If so, you know the compressor working properly in the load phase
  • Pump up to max pressure
  • When it reaches the highest point, watch for the needles to stop increasing
  • Air dryer should purge

If any of these steps don’t occur as they should, it could indicate a dangerous problem with the governor, and you should have it checked out right away. 

2. Air Compressor Leaks 

Leaks in the air system can cause all kinds of problems. They can result in the compressor failing to load or unload, excessive purging, and other issues. Before doing any other major work on the truck, it’s a good idea to check for leaks. 

Oil leaks can occur both inside and outside of the air compressor. The presence of excessive oil buildup in the air dryer is a sign that you may be dealing with an internal oil leak. 

An internal oil leak can also cause the system to build air slower than normal. When oil or other debris causes blockage within the compressor’s lines, it can result in noticeable problems. 

External leaks are more noticeable because you’ll see it dripping outside the compressor. Normally, these kinds of leaks are due to faulty seals or gaskets, or the leak could be coming from the oil pan or head gasket. These are simple to repair in most cases. 

However, if the crank seals have gone bad, you’ll probably have to replace the compressor.

Aside from oil, you may also experience coolant leaks from the air compressor. These can also be internal or external leaks. If you’re dealing with an internal coolant leak, the most likely cause is a cracked head. In some cases, these leaks may be due to a hole or crack in the cylinder wall. 

An internal coolant leak may be repaired, but sometimes it makes more sense to replace the compressor, especially if it’s older and has high mileage. 

External coolant leaks are typically attributed to a faulty head gasket. A cracked head may also result in this type of leak, but that’s a less common issue. The good news is that head gasket repairs are daily simple so you shouldn’t have to replace the whole compressor.

3. Problems With the ESS Piston

Near the compressor’s head is the ESS piston. This is what controls the airflow inside the compressor and allows for the tanks to pressurize. When the ESS piston is down and sealed, the system is in “loaded” mode, and air may enter through the air inlet valve. As the air is discharged, the pressure builds in the compressor. 

When the ESS piston is up (“unloaded” mode), the cylinder is open to the compressor inlet. This allows air to move between the two areas and the piston can continue to move without adding additional pressure. 

Because the ESS piston controls so many of the compressor’s basic functions, a faulty piston can result in big problems for the compressor. 

Cleaning the ESS piston or replacing it if needed is a simple repair that can correct many issues you may be dealing with. It’s always a good idea to check the piston after you’ve done some troubleshooting and haven’t resolved the problem. 

How the Air Brake System Works

The truck’s engine powers the air compressor, which creates air pressure to operate the vehicle’s air brakes. The compressor is cooled the same way as the engine, using coolant and oil for lubrication. 

The braking system is controlled using a governor and unloader mechanism to keep the pressure within the ideal ranges. When the pressure gets too high, the governor causes the unloader to stop compressing air and purge excess air if needed to relieve pressure. 

When more pressure is needed, the governor tells the compressor to go back to building air, and the air dryer kicks in to dry the air as it’s compressed. 

The system also includes a discharge line for moisture to escape. Water comes from the air as it’s dried and compressed, and sometimes oil droplets need to be expelled, too. An air dryer is used to collect oil droplets from the water vapors and filter them out from the discharge.  

How Much Does it Cost to Replace a Bad Truck Air Compressor?

Replacing the air compressor in your truck is a costly but necessary repair. In most cases, it should cost between $800 and $4,500 to have the air compressor replaced. The compressor itself may be anywhere from $450 to $1,300 and up depending on your make and model. 

On top of the parts cost, you’ll have to pay a mechanic for several hours of labor. That could be anywhere from $350 to $600 or more depending on the hourly rate. 

The good news is that in many cases you don’t have to replace the entire compressor when you have a problem. The issue could be due to a faulty component that would be less expensive to repair. 

For example, the dryer, governor, and various valves and seals are all parts that may need to be replaced if your air compressor isn’t working properly. 

In fact, if your compressor fails to load, fails to unload, or is purging excessively, it could indicate a leak in the governor’s control lines, a faulty governor, or loose fittings. These are all issues that are far less complex than having to replace the compressor altogether.

The Bottom Line

A truck relies on its air compressor to operate the air brakes and other pneumatic systems. This is a critical system and it must be in good working order to ensure safety on the road. If your compressor isn’t operating properly, it’s something you’ll want to have checked out as soon as possible. 

Understanding the signs and symptoms of a bad compressor will help you recognize the issue sooner so you can get it resolved and get back on the road quickly.