B5.9G, B5.9LPG – Troubleshooting and Repair 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.

The Holset® and Cummins single- and two-cylinder air compressors are engine-driven, piston-type compressors that supply compressed air to operate air-activated devices. The compressor runs continuously but has a loaded and an unloaded operating mode.

The operating mode is controlled by a pressure-activated governor and the compressor unloading assembly. When the air system reaches a predetermined pressure, the governor applies an air signal to the air compressor unloader assembly, causing the unloader valve to hold the compressor intake valve open so compressed air stops flowing into the air system.

As the air in the air 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 again pump compressed air into the air system.

E-Type Air Compressor

Holset® SS and ST model air compressors built with the E-type unloader can be identified by the letter “E” (SS296E, SS338E, ST676E, and ST773E) and by the caution on the dataplate.

All QE (QE296 and QE338) model air compressors are equipped with the E-type unloader.

E-Type System with Air Dryer

Vehicles equipped with air dryers vented to the atmosphere during unloaded compressor operation, using the Holset® E-type air compressor, require the installation of an Econ valve to prevent excessive oil consumption.

NOTE: Some air dryers can have a built-in Econ valve. Check with the manufacturer as to which type is installed.

E-Type System without Air Dryer

Air systems without air dryers, or with air dryers not vented to the atmosphere during unloaded compressor operation, can use the Holset® E-type unloader valve without modifying the air system.

WARNING  The compressor air supply must not be plumbed so that an air/fuel mixture can enter the compressor. Personal injury or death can occur if the air/fuel mixture enters the compressor.

The Holset® QE air compressor (QE230 (10.0 cfm), QE296 (13.2 cfm), QE338 (15 cfm)) offered on the diesel will be available on the B5.9LPG/B5.9G.

The compressor air supply for the Holset® units can be turbocharged. The turbocharged compressor air supply is plumbed from the air intake elbow to the supply port on the compressor.