The air compressor is responsible for generating the compressed air used throughout an HGV’s pneumatic system.
If it begins to wear or fail, the vehicle may struggle to maintain the pressure required for braking, suspension and other systems.
However, compressor symptoms can overlap with air leaks, dryer faults and valve problems. Proper diagnosis is therefore important before replacing the compressor.
Here are eight signs worth investigating.
1. Air pressure takes longer to build
One of the most noticeable symptoms is an increase in the time required for the vehicle to reach normal operating pressure.
A worn compressor may simply be moving less air than it should.
However, significant leaks elsewhere in the system can produce exactly the same symptom.
2. The system struggles to reach normal pressure
A more advanced problem may result in pressure reaching a certain level and then struggling to climb further.
Possible causes include compressor wear, leaks, governor problems or faults within the air-treatment system.
3. The compressor appears to run excessively
If the compressor is working much more frequently than normal, it may be struggling to maintain system pressure.
This does not necessarily mean the compressor itself has failed.
A downstream leak may simply be forcing it to replenish lost air continuously.
4. You hear unusual compressor noise
Changes in mechanical noise should be investigated.
Knocking, rattling or other unusual sounds may indicate internal wear or another mechanical problem around the compressor drive.
Unusual noise should be assessed rather than treated as a definitive diagnosis on its own.
5. Oil is entering the air system
Compressed air can contain traces of compressor oil, which is one reason some commercial vehicle air-treatment systems incorporate oil separation.
Knorr-Bremse notes that oil residues can affect elastomers used within pneumatic valves, making effective air treatment important.
Unusual levels of oil downstream may justify investigation of the compressor and air-treatment system.
6. The compressor is running unusually hot
Compressing air naturally generates heat.
However, excessive operating temperature can indicate that the compressor is working harder than expected or experiencing a mechanical problem.
A significant air leak elsewhere may also increase compressor duty cycle and therefore temperature.
7. Air-system faults keep returning
Repeated problems with dryers, valves or other downstream components can sometimes indicate contamination originating earlier in the system.
Rather than repeatedly replacing the affected downstream component, the underlying source should be investigated.
8. Low-air warnings become more frequent
A vehicle that repeatedly struggles to maintain pressure may trigger low-air warnings more often.
Possible causes extend beyond the compressor, so the complete system should be checked.
DVSA guidance requires drivers to confirm that air builds correctly, warning systems operate and there are no air leaks.
Air compressor or air leak?
This is the critical distinction.
Before condemning the compressor, consider whether the system is losing the air it produces.
Check for:
- Air-line leaks
- Brake chamber leaks
- Dryer faults
- Reservoir leaks
- Protection valve faults
- Trailer leaks
- Air suspension leaks
Our guide to why an HGV loses air pressure covers these potential causes in more detail.
Need an HGV air compressor or related parts?
AutoFast supplies genuine, OEM and quality aftermarket HGV parts and truck parts, including compressors and related pneumatic components for leading commercial vehicle manufacturers.
Providing the vehicle registration or chassis details alongside the existing compressor number can help our team identify the correct replacement quickly and accurately.
