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The function of both the Bendix

®

 AD-9si

™ 

air dryer and the 

Bendix

®

 AD-9si

™

 PuraGuard

®

 oil coalescing air dryer is to 

collect and remove air system contaminants in solid, liquid, 

and aerosol form before they enter the brake system. The 

AD-9si includes a spin-on style cartridge for easy servicing, 

and most include a fully integrated governor to control air 

system charging.
The air dryer provides clean, dry air to the components of 

the brake system, which increases the life of the system 

and reduces maintenance costs.  The need for daily manual 

draining of the reservoirs is eliminated.
The Bendix AD-9si PuraGuard oil coalescing air dryer has 

an identical appearance to the standard Bendix AD-9si air 

dryer, but contains a coalescing media at the inlet of the 

desiccant bed.  This coalescing media provides a higher 

level of oil removal over the standard AD-9si air dryer.  

The AD-9si PuraGuard oil coalescing air dryer has all of 

the same functions as the standard AD-9si air dryer and is 

used in applications where lower oil concentration levels 

are required.
IMPORTANT! When servicing, note that standard AD-9si 

air dryers or air dryer cartridges may be serviced with 

PuraGuard oil coalescing air dryers or cartridges, however, 

PuraGuard oil coalescing air dryers or cartridges 

MUST 

only be serviced with like replacements.
Note: Unless otherwise stated in this document, AD-9si 

coalescing air dryer.
The AD-9si air dryer consists of a “spin on” desiccant 

cartridge secured to a base assembly.  The base assembly 

contains a delivery check valve assembly, safety valve, 

heater and thermostat assembly, integrated governor (in 

most assemblies), threaded air connections, internal purge 

volume, and purge valve assembly. 
The removable purge valve assembly incorporates the 

purge valve mechanism and a turbocharger cut-off feature 

that is designed to prevent loss of engine “turbo” boost 

pressure during the purge cycle of the AD-9si air dryer. For 

ease of maintenance, all replaceable assemblies should 

be serviceable without removal of the air dryer from its 

mounting on the vehicle — provided adequate clearance 

exists between the air dryer and the vehicle components.  

Inlet Port (air in)

1

Delivery Port Out

2

Extended Purge

1

Governor Unloader

1

Governor Exhaust

1

 (Refer to Figures 2 through 5.)

The Bendix AD-9si air dryer is designed to receive 

compressed air from the vehicle air compressor, clean and 

dry the air, deliver air to the vehicle’s air reservoirs and 

control the compressor/dryer charge cycle.  The AD-9si 

air dryer is available with an internal governor (Figures 2 
and 3) or without an internal governor (Figures 4 and 5).

AIR DRYER OPERATION: GENERAL

The Bendix AD-9si air dryer alternates between two modes 

or “cycles” during operation: the Charge Cycle and the 

Purge Cycle. The following describes these “cycles” of 

operation.