Air dryer help

[Jay Nepa]

I have a 1982 R model I changed the old style air dryer to a new style. Having trouble with the purge going off every 10-15 seconds. Pressure tested the signal line from compressor to dryer no leaks. Do I need a new unloader valve ? 

[Geoff Weeks]

Which AD did you install? 

No you most likely don't need a new air gov. You likely have either not installed the correct check valve after the A/D if it is the external purge volume type.

You could also have other check valve problems causing your wet tank pressure to fall off too quickly/

Why did you change? What problem were you trying to address with a new drier?

[Jay Nepa]

I had a freeze up and my air operated functions were not working properly. The old style drier was outrageous price for a direct replacement. So I retrofitted a new style dryer. The new air dryer came with a new valve does that need to be installed the parts guy I bought the new dryer from said if I don’t have one now I don’t need one when I put the new dryer on. The old air dryer was an AD2. I tried to post pic but website wouldn’t let me upload pics.  

[Geoff Weeks]

I can't help if you don't tell me what you put on!

How often was it cycling BEFORE you changed the air drier?

I suspect it was single check-protection valve and YES you do need it if you replace a AD-2 with an external purge volume drier.

In many cases if the air drier can't keep the air free of moisture, it isn't the drier but air system problems (leaks) that keep the system compressing too much of the time.

[Geoff Weeks]

Either of these?

Bendix AD-SP

Meritor Air Drier

[Jay Nepa]

I put on a wabco R 955205. I just put on the pressure controlled check valve on the air tank and it solved the problem.    The ad2 had it built into the dryer   Thanks for the help. 

[Geoff Weeks]

Ok, did you place it where they tell you in the instructions? It needs to be after the wet tank, in the line to the secondary tank. The original check valve in the secondary tank HAS to be removed and replaced with the valve provided. 

Anything less and there will not be enough purge air to correctly regenerate the drier bed. 

This is one of the reasons I don't like external purge driers. Too easy to install wrong.

[Jay Nepa]

I did install it between the dryer and the wet tank MY MISTAKE. I misunderstood the directions.  I will have to move it to the secondary tank tomorrow. 

[Geoff Weeks]

Read the instructions! You have to remove the existing check valve on the inlet of the secondary tank and replace with the valve provided. IT MUST be able to take air from the secondary tank back to the drier on purge to regenerate the drier bed. 

If there is some part you don't understand, post your question and I will try to help. 

I don't like these external purge volume driers. Too easy to be installed wrong or for the valve to fail. Internal purge driers like the AD-2, AD-4 and AD-9 don't need external source of air volume to purge the drier and regenerate the drier bed.