BENDIX® AD-HF® & AD-HFi™ AIR DRYER SERVICE REPLACEMENT KIT
Purge Reservoir
Standard 200 in3
Extended 300 in3
Extended Purge Port (AD-HF Only)

Purge Valve
Hard Seat
DLU

Bendix® PuraGuard® Desiccant
Cartridge 41mm Thread
P5 RES
1/4"-18 NPT
Bendix® D-2® Governor
(AD-HF Only)

1a IN
1/4"-18 NPT

Extended Purge
Port 1/4"-18 NPT
(AD-HF Only
if included)

22 SEC
21 PRI
1/4"-18 NPT 1/4"-18 NPT

Heater
12 Volt White
24 Volt Gray (AD-HF Only)

Cartridge Pressure
Protection Valve (CPPV)
103 Opening Pressure
112 Opening Pressure

Purge Reservoir
Lanyard Drain
Valve

Bendix AD-HFi Air Dryer

2-4 CON
1/4"-18 NPT

Safety Valve
23 Aux 2
1/4"-18 NPT

1 IN
1/2"-14 NPT

Heater 12v Only

24 Aux 1
1/4"-18 NPT

Solenoid
Governor

Note: All features of the AD-HFi dryer are identical
with the exception of those shown.

Figure 1 - Bendix® AD-HF® and AD-HFi™ PuraGuard® Oil Coalescing Air Dryers
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GENERAL SAFETY GUIDELINES

WARNING! PLEASE READ AND
FOLLOW THESE INSTRUCTIONS
TO AVOID PERSONAL INJURY OR DEATH:

When working on or around a vehicle, the following
guidelines should be observed AT ALL TIMES:
▲Park the vehicle on a level surface, apply the parking
brakes and always block the wheels. Always wear personal
protection equipment.
▲Stop the engine and remove the ignition key when working
under or around the vehicle. When working in the engine
compartment, the engine should be shut off and the ignition
key should be removed. Where circumstances require that
the engine be in operation, EXTREME CAUTION should be
used to prevent personal injury resulting from contact with
moving, rotating, leaking, heated or electrically-charged
components.
▲Do not attempt to install, remove, disassemble or
assemble a component until you have read, and thoroughly
understand, the recommended procedures. Use only the
proper tools and observe all precautions pertaining to use
of those tools.
▲If the work is being performed on the vehicle’s air brake
system, or any auxiliary pressurized air systems, make
certain to drain the air pressure from all reservoirs before
beginning ANY work on the vehicle. If the vehicle is
equipped with a Bendix® AD-IS® air dryer system, a Bendix®
DRM™ dryer reservoir module, a Bendix ® AD-9si ®, AD-HF®,
or AD-HFi™ air dryer, be sure to drain the purge reservoir.
▲Following the vehicle manufacturer’s recommended
procedures, deactivate the electrical system in a manner
that safely removes all electrical power from the vehicle.
▲Never exceed manufacturer’s recommended pressures.
▲Never connect or disconnect a hose or line containing
pressure; it may whip and/or cause hazardous airborne
dust and dirt particles. Wear eye protection. Slowly open
connections with care, and verify that no pressure is
present. Never remove a component or plug unless you are
certain all system pressure has been depleted.
▲Use only genuine Bendix ® brand replacement parts,
components and kits. Replacement hardware, tubing, hose,
fittings, wiring, etc. must be of equivalent size, type and
strength as original equipment and be designed specifically
for such applications and systems.
▲Components with stripped threads or damaged parts should
be replaced rather than repaired. Do not attempt repairs
requiring machining or welding unless specifically stated
and approved by the vehicle and component manufacturer.
▲Prior to returning the vehicle to service, make certain all
components and systems are restored to their proper
operating condition.
▲For vehicles with Automatic Traction Control (ATC), the ATC
function must be disabled (ATC indicator lamp should be
ON) prior to performing any vehicle maintenance where
one or more wheels on a drive axle are lifted off the ground
and moving.
▲The power MUST be temporarily disconnected from the
radar sensor whenever any tests USING A DYNAMOMETER
are conducted on a vehicle equipped with a Bendix ®
Wingman ® system.
▲You should consult the vehicle manufacturer's operating
and service manuals, and any related literature, in
conjunction with the Guidelines above.

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VEHICLE APPLICATION REQUIREMENTS
GENERAL
The basic application requirements presented here
apply to a standard air dryer installation. The majority of
vehicles in use today will meet these basic requirements,
however, some may not. Examples of vehicles that may
not meet the requirements include, bulk trailer unloading
operations and other high air consumption/continuous flow
systems. While the Bendix® AD-HF® air dryer or the Bendix®
AD-HFi™ air dryer can be used on these vehicles, the
standard installation procedure presented in this manual
may require modification to ensure proper operation and
service life. Consult your local authorized Bendix parts
outlet or sales representative for additional information.
•

harge Cycle Time – The AD-HF air dryer and
C
AD-HFi air dryer are designed to provide clean, dry
air for the brake system. When a vehicle's air system
is used to operate non-brake air accessories, it is
necessary to determine that – during normal, daily
operation – the compressor should recover from
governor "cut-in" to governor "cut-out" (usually 110
psi to 130 psi) in 90 seconds or less at engine RPMs
commensurate with the vehicle vocation. If the recovery
time consistently exceeds this limit, it may be necessary
to "by-pass" the air accessory responsible for the high
air usage. Consult your local authorized Bendix parts
outlet or sales representative for additional information.

•

urge Cycle Time – During normal vehicle operation,
P
the air compressor remains unloaded for 35 seconds for
the standard AD-HF & AD-HFi air dryer or 55 seconds
for the extended purge model. These purge times
are required to ensure complete regeneration of the
desiccant material. If the purge time is occasionally
shorter than the times specified, no permanent ill
effect should be expected, however, if the purge time
is consistently less, an accessory by-pass system must
be installed.

•

uropean Air Brake Systems – Brake systems that
E
incorporate compressors without integral unloading
mechanisms and/or utilize a compressor discharge line
unloader valve have special AD-HF & AD-HFi air dryer
installation requirements. Consult your local authorized
Bendix parts outlet or sales representative for additional
information.

•

•

•

ir Compressor Size – No restriction (for application
A
information regarding non-Bendix compressors,
consult the compressor manufacturer). For Bendix®
compressors, refer to Bendix Compressor Application
Guideline, BW2625. For assistance concerning
compressor size compatibility, contact an authorized
Bendix parts outlet or Bendix marketing representative.
olset "E or QE" Type Air Compressors – The
H
Bendix® AD-HF® & AD-HFi™ air dryer is not compatible
with this compressor type.
se of Standard or Extended Purge AD-HF & AD-HFi
U
Air Dryer – Refer to the Bendix Air Dryer Application
Guideline, BW2600.

VEHICLE PREPARATION
1. Park the vehicle on a level surface and prevent
movement by all means other than the brakes.
2. Drain all the reservoirs to 0 psi.

Always depressurize the air dryer purge reservoir,
and all other reservoirs on the vehicle to 0 psi before
servicing the air dryer.
Air
Connection
Port ID

Function/
Connection

Port Size
(NPT)

Qty.

1
IN

Inlet Port (air in)

1/2" - 14

1

1a
IN

External Fill
(through desiccant cartridge)

1/4" - 18

1

21
PRI

Delivery Port Out
(to Primary Reservoir)

1/4" - 18

1

Do not install the air dryer included in this kit if it does
not match the one being removed.

22
SEC

Delivery Port Out
(to Secondary Reservoir)

1/4" - 18

1

This kit is to be used to replace the AD-HF and AD-HFi
air dryer. It is not a retrofit kit. This kit cannot be installed
on vehicles that do not have an existing AD-HF or
AD-HFi air dryer. As shown in Figures 1 through 4, there
are several variants of AD-HF & AD-HFi air dryers. Be sure
to verify the air dryer included in this kit is identical to the
air dryer you are replacing. Ensure both air dryers have the
same port configuration, governor type and cut-in/cut-out
pressure settings, heater voltage, purge reservoir type,
mounting configuration, and pressure protection valve
opening pressure settings in the same locations.

23
AUX 2

Auxiliary Delivery Port
(air out)

1/4" - 18

1

24
AUX 1

Auxiliary Delivery Port
(air out)

1/4" - 18

4

2-4
CON

Unloader Control Air

1/4" - 18

1

P5
RES

External Fill
(bypass desiccant cartridge)

1/4" - 18

1

DESCRIPTION
ATTENTION

Table 1 – AD-HF and AD-HFi Configuration

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2-4
Compressor
Unloader
(1/4" NPT)

PS External Fill (1/4" NPT)
21 Primary (1/4" NPT)

1a Supply (1/4" NPT)
1 Supply (1/2" NPT)

22 Secondary (1/4" NPT)
23 Auxiliary 2 (1/4" NPT)

3 Purge

24 Auxiliary 1 (1/4" NPT)
24 Auxiliary 1 (1/4" NPT)
24 Auxiliary 1 (1/4" NPT)
24 Auxiliary 1 (1/4" NPT)

Figure 2 - Bendix ® AD-HF ® ISO Pneumatic Schematic
2-4
Compressor
Unloader
(1/4" NPT)

To ECU
PS External Fill (1/4" NPT)
21 Primary (1/4" NPT)

1a Supply (1/4" NPT)
1 Supply (1/2" NPT)

22 Secondary (1/4" NPT)
23 Auxiliary 2 (1/4" NPT)

3 Purge

24 Auxiliary 1 (1/4" NPT)
24 Auxiliary 1 (1/4" NPT)
24 Auxiliary 1 (1/4" NPT)
24 Auxiliary 1 (1/4" NPT)

Figure 3 - Bendix ® AD-HFi ™ ISO Pneumatic Schematic

DISASSEMBLY

CLEANING & INSPECTION

Identify and mark all connections on the air dryer for ease
of installation.

1. Using a clean rag, wipe the air dryer mounting surface.

1. D
isconnect all air line connections and the heater
connector to the air dryer.
2. Remove and retain the air dryer mounting bolts.
3. Remove the air dryer.

4

2. Inspect for physical damage to the air dryer mounting
surface.

Standard Purge with
Mounting Holes

Extended Purge Port with
Mounting Holes

Extended Purge with
Mounting Holes

Standard Purge with
Mounting Studs

Extended Purge Port with
Mounting Studs

Extended Purge with
Mounting Studs

Figure 4 - Bendix ® AD-HF® and AD-HFi ™ Air Dryer Purge Volume and Mounting Configurations

ASSEMBLY

TESTING THE AIR DRYER

1. S
ecure the air dryer to the vehicle using the mounting
hardware that was removed during disassembly.
Torque to 300-320 in-lbs.

Before placing the vehicle in service, perform the following
tests:

NOTE: Maximum thread engagement for mounting bolts
is .75” (See Figure 5).
2. C
onnect all air and electrical connections to the new
air dryer as marked during the disassembly process.

1. Close all reservoir drain valves.
2. B
uild up system pressure to governor cut-out and
note that the AD-HF® or AD-HFi™ air dryer purges
with an audible burst of air, followed immediately by
approximately 30 seconds of air flowing out of the
purge valve.
3. " Fan” the service brakes to reduce system air pressure
to governor cut-in. Note that the system once again
builds to full pressure and is followed by a purge at the
AD-HF or AD-HFi air dryer exhaust.
4. It is recommended that the total air system be tested
for leakage to ensure that the air dryer(s) do not cycle
excessively.
For additional information refer to the Bendix Air Brake
Handbook, BW5057.

Figure 5 - Maximum Thread Engagement

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S-1663 © 2021 Bendix Commercial Vehicle Systems LLC, a member of Knorr-Bremse • 7/21 • All Rights Reserved

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