M11 Series – Service Manual – 012-999   Compressed Air System – Overview

Table of Contents

General Information

General Information


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 which supply compressed air to operate air activated devices. The compressor runs continuously, but has a loaded and unloaded operating mode.


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 data plate.

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.

Last Modified:  06-May-2003