Bendix AD-9 Air Dryers

Extended Maintenance Intervals

Bendix AD-9 air dryers have a large capacity desiccant cartridge that allows for longer maintenance intervals. Air dryer rebuilds can be stretched to every 36 months when the AD-9 is installed on a typical over-the-road truck or tractor.

Easy To Service

Desiccant cartridge replacement is the only reason to ever completely disassemble the Bendix AD-9 air dryer. The check valve, turbo cut-off valve, heater, and purge valve assembly can all be serviced without disassembling the air dryer.

Troubleshooting the Bendix AD-9 Air Dryer

This troubleshooting documentation was compiled from Bendix's AD-9 Service Data Sheet. Download the data sheet for more detailed information.

Dryer is constantly "cycling" or purging. Dryer purges frequently (every 4 minutes or less while vehicle is idling)

Excessive system leakage IMPORTANT: Note whether air pressure loss is shown on dash gauge(s). Pressure loss shown on gauges is caused by service brake system or component leakage. Pressure loss NOT SHOWN on gauges is caused by supply system or component leakage.

Leaking purge valve housing assembly and/or O-rings in air dryer end cover.

Holset® "E" type compressor - When installing a Bendix Drop-In air dryer in a system equipped with a Holset E or QE compressor, remove the Holset ECON valve along with its feedback and governor control line.

Water and/or Oil in Supply or Service Reservoir

Desiccant cartridge needs to be replaced

Improper discharge line length or line material - Maximum air dryer inlet temperature is exceeded.

Air system charged from outside air source (outside air not passing through air dryer)

Air dryer not purging

Purge (air exhaust) time insufficient due to excessive system leakage

Excessive air usage, duty cycle too high

Compressor malfunction - Passing oil

Air bypasses desiccant cartridge assembly - If vehicle has a Holset compressor, inspect feedback check valve.

Oil present at air dryer purge exhaust or cartridge during maintenance

A small amount of oil in these areas is normal.

Safety valve on air dryer "popping off" or exhausting air

Restriction between air dryer and supply reservoir

Air dryer safety valve malfunction

Desiccant cartridge needs to be replaced

Governor malfunction. Missing or restricted governor control line installation

Constant exhaust of air at air dryer purge valve exhaust when air dryer is in Charge mode.

Air dryer purge valve leaking excessively

Compressor fails to unload (stop compressing air) and air dryer purge exhaust makes "sputtering" or "popping" sound

Purge control line connected to reservoir or exhaust port of governor. It must be connected to an unloader port on the governor.

Purge valve frozen open - malfunctioning heater and thermostat, wiring, blown fuse.

Excessive system leakage

Purge valve stays open - supply air leaks to control side - Replace purge valve and housing

Can not build system air pressure

Inlet and outlet air connections reversed

Check valve between air dryer and first reservoir malfunctioning

Kinked or blocked discharge line

Excessive bends in air compressor discharge line causing water to collect and freeze

Governor malfunction

Air dryer does not purge or exhaust air

Missing, broken, kinked, frozen, plugged or disconnected purge control line

Faulty air dryer purge valve

Air dryer safety valve malfunction

Desiccant material being expelled from air dryer purge valve exhaust

Air dryer not securely mounted. (Excessive vibration.)

Malfunctioning or saturated desiccant cartridge

Compressor passing excessive amount of oil

Desiccant cartridge not assembled properly to end cover. (Loose attachment)

"Pinging" noise excessive during compressor loaded cycle

Defective check valve assembly in air dryer end cover.

Leaking purge valve control piston o-ring.

Constant leaking of air at air dryer purge valve exhaust (non-charging mode.)

Defective check valve assembly in air dryer end cover.

Leaking purge valve control piston o-ring.

The air dryer purge piston cycles rapidly in the compressor unloaded (non-compressing) mode.

Compressor fails to "unload"

Typical Bendix AD-9 Plumbing Diagram