Air Compressor Unloader and Valve Assembly (012-013)
B3.9 and B5.9 Series Engines
Section 12 - Compressed Air System - Group 12
Air Compressor Unloader and Valve
Initial Check (012-013-001)
Air pressure must be released from the system before
removing the air governor. The governor can be under
pressure and cause personal injury.
NOTE: The illustrations shown will be of the SS model
single-cylinder air compressor. Differences in procedures
for SS, QE, and ST model Holset air compressors will be
shown where necessary.
Remove the air governor or air governor hose from the air
compressor unloader body.
Operate the engine to activate the air compressor.
If the air compressor is not pumping, the unloader valve is
malfunctioning, and must be repaired or replaced.
The unloader valve body is installed with spring ten-
sion. Use care when removing to prevent personal in-
jury. Always wear protective eyewear.
Holset SS, E-Type, and ST Models
Hold the unloader valve body down, and remove the two
captive washer capscrews and the two plain washers.
Remove the unloader valve body.
Remove the o-ring seal.
Remove the rectangular ring seal.
Remove the unloader valve cap and the unloader valve
NOTE: Disassembly of the center unloader valve on Holset
two-cylinder air compressors is similar to the single-cylinder
Remove the intake valve seat and valve.
Remove the intake valve spring.