Air dryer

What model dryer do you have?
This for AD-9si
6. Air dryer does not purge or exhaust air. A. Faulty air dryer purge valve. A. After determining that air reaches the purge valve piston–by installing a Tee fitting with a pressure gauge into the control port –repair the purge valve if necessary. B. See Causes B, E, and F for Symptom #4. B. Refer to Symptom 4, Remedies B, E, and F

4. Constant exhaust of air at air dryer purge valve exhaust or unable to build system pressure. (Charge mode.) A. Air dryer purge valve leaking excessively. A. With the compressor loaded, apply a soap solution on the purge valve exhaust, to test for excessive leakage. Repair or replace the purge valve as necessary. B. Purge valve frozen open - faulty heater and thermostat, wiring, blown fuse. B. Refer to paragraph 5 of the Operation and Leakage Tests for the heater and thermostat test. C. Defective AD-9si air dryer delivery check valve. C. Refer to Symptom 1, Remedy B. D. Leaking turbo cut-off valve. D. Repair or replace the purge valve assembly. E. Defective governor. E. Check the governor at both “cut-in” and “cut-out” positions for proper pressures and excessive leakage at the exhaust. F. Leaking purge valve control piston seals. F. Repair or replace the purge valve assembly

 

Depends on the price in my opinion. An AD-9 is cheap. For a bit more than the purge valve you can get a whole dryer. Others are more expensive. Might have some debris in it. Try brake clean, pushing the valve in, see if it quits leaking. Chances are it’s shot. They have rebuild kits. I tried it once. Too much BS. It’s just a few bolts, snap ring , o rings, washer, spring. I couldn’t get past the snap ring. Fix it, you may get stranded when it starts leaking real bad, only building 20-30lbs if that. Keep a couple unions to bypass dryer all together if needed. Available in any plumbing dept.

 

When I have air dryer problems of any kind. I replace the whole unit. Usually is easier and less time spent wrenching, in the long run.
Good luck

 

I just replaced an AD9 purge valve while changing the cartridge in August with an aftermarket that cost $50. Authentic Bendix cost 2-3 times as much if I remember right. And it came fully assembled in the housing-no changing just the core. 3 bolts and putting on the O-rings was all the extra time. Unless your AD9 has some pitting from corrosion where the O-rings seat, I'd go this way. I still would keep fittings to connect the airlines to bypass the drier in case of an emergency (I do).

 

Wabco air dryer troubleshooting (alltroubleshooting.net)

Air flows out of purge valve entire time compressor is unloaded. Turbo cut-off valve not sealing. Outlet check valve not seating. Yes Yes Replace turbo cut-off valve. Replace outlet check valve.

The link also covers this.
Air dryer does not purge when compressor unloads (no blast of air from purge valve).

What model Wabco do you have?