Cummins ISX11.9 CM2250 – Service Manual 012-999   Compressed Air System – Overview

The compressed air system normally consists of a gear-driven air compressor, an air governor, air tanks, and all necessary plumbing.

NOTE: The engine is supplied with the air compressor and related intake/coolant plumbing only. The rest of the compressed air system is the responsibility of the vehicle manufacturer.

Complete installation recommendations are found in Cummins Inc. publication, Automotive and Bus Installation Recommendations, Air Compressor Systems, Bulletin 3884946.

The components listed below are commonly used in a single cylinder air system:

1. Air governor
2. Discharge line
3. Air dryer
4. Supply tank
5. Check valve
6. Primary tank
7. Secondary tank
8. Check valve
9. Air compressor.

The components listed below are commonly used in a twin cylinder air system:

1. Air compressor
2. Air governor
3. Discharge line
4. Safety valve
5. Supply tank
6. Purge tank
7. Air dryer
8. Air governor
9. Supply tank
10. Check valve
11. Primary tank
12. Secondary tank.

The air compressor is naturally aspirated, where the air source is a separate, filtered, air source from the engine or before the turbocharger inlet of the engine. The air compressor inlet air is not pressurized.

The air compressor also has mounting provisions for driving an accessory, such as a power steering/hydraulic pump.

The key factor which determines the reliability and durability of an air compressor in an application is the amount of time the air compressor is supplying air during the vehicle/machine operation. This is known as the duty cycle of the air compressor.

Air compressors are not designed to pump continuously and generate a lot of heat when pumping. This heat is dissipated during the time the compressor is not pumping (known as unloaded operation).

Compressed air system maintenance/servicing can help minimize the air compressor duty cycle and improve the reliability and durability of the air compressor. These items include, but are not limited to:

1. Find and stop all leaks in the system. Air leaks historically can double or triple operating duty cycles. Close attention to correcting air system leaks is critical.
2. Check the air compressor exhaust port, discharge line, and fittings for carbon buildup. If there is any carbon buildup or restriction evident, clean or replace as needed.
3. Check the air lines and fittings between the outlet port of the air dryer and the first tank after the air dryer for any water or oil. The tank should be dry. If oil is present, replace the dryer desiccant and clean the downstream system and components as required.

Refer to the vehicle OEM service manual for service information for the compressed air system. For specific air compressor maintenance, see the Operation and Maintenance Manual and/or Owners Manual for the engine being serviced.

Air Compressor Cylinder Head – The air compressor cylinder head is cooled by engine coolant. The cylinder head contains intake and exhaust valves to regulate air flow in and out of the cylinder head.

Most air compressor cylinder heads can be serviced without removing the air compressor from the engine. This manual will cover service of the cylinder head with the compressor installed on the engine. If there is internal damage to the air compressor, the air compressor must be replaced.

NOTE: Prior to removing the air compressor cylinder head, make sure to check if replacement parts are available. Some air compressor cylinder heads are not able to be serviced separately from the air compressor.

Air Governor – The air compressor operates continuously but has a “loaded” and “unloaded” operating mode. The operating mode is controlled by a pressure activated air governor and the air compressor unloader assembly. The air governor can be located on the air compressor or remotely on the vehicle.

When the air system reaches a predetermined pressure, the governor applies an air signal to the unloader assembly, causing the unloader to either hold open or shut off the compressor’s intake valve, which causes compressed air to stop flowing into the compressed air system.

As the system is used, the pressure drops. At a predetermined pressure, the governor exhausts the air signal to the compressor unloader assembly, allowing the compressor to pump compressed air into the system.