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Air compressor problem
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By Geoff Weeks - Tuesday, April 13, 2010 9:01 AM
Nope, your looseing air in the line to the gov. Take some soapy water and spray the hoses to and from the compressor, air drier and wet tank. My guess is the line from the drier to the wet tank is bad. Seen it a buch of times.
By Geoff Weeks - Tuesday, April 13, 2010 3:24 PM
If the check valve is leaking, you should feel constant air comeing out of the dryer when the compressor un-loads, until it starts compressing again, since you didn't mention it was doing this, I assume the check isn't the problem. Normally when it "blows off" it will vent for a few seconds then stop. a good air system should run 15 miniutes or so between cycles while running down the highway (no brakes applications or other air uses) a really good system will do better. My 9670 use to go 45 miniutes between cycles! I am parked next to a truck doing the same thing... its bothersome.

  If the un loader is leaking, it will will also cause a loss of air in the wet tank, as it uses that to sense and unload. As Glenn said, put a guage in the "Res" line on the govenor and see what is happening. I have seen braided line that looked ok from the outside, but leaked like a sieve when sprayed with soapy water.

  you can temporarrly plug the unloader line to the air dryer and see if that stops it short cycleing, if so then the problem is at the dryer, if not then look elsewhere.

  Oh, and since both guages don't show a loss, (because the primary and secondary tanks both have check valves (which are working) between them and the wet tank) you know the problem isn't down stream from the wet tank.

  Many drivers see both gauges up and think their system is working fine... when actually it is in need of service. You should see it biuld and then fall back to 90 psi or so as the brakes are used, then biuld again to cut-out (120-125 psi). If it stays all the time at 120 (esp if you here it blow off every few sec) it is not funtioning properly.

By Mike Horrie - Tuesday, April 13, 2010 3:03 AM
A freind just bought a new used truck and the air compressor purges every 45 seconds even though both gauges are staying at 125 lbs.

He put a new governer on and that didn't help.

Could the purge valve be weak.

1999 n-14 truflo 550  bendix a9 I think

Thanks Mike

By glenn akers - Tuesday, April 13, 2010 2:13 PM
Or the check valve in the air dryer is draining the wet tank back into the dyer and letting the air supply to the air governor drop below the unloaded range and it starts to load up again. Also the unloader valve pistons may be leaking down the air governor signal line pressure and starting the compressor to pump again. Also if the o/rings on the purge valve is leaking the air governor signal pressure can drop below the unloaded range and starting the compressor to pump again.The Ad 4 has the check valve inside of the dyer and the AD 9 has it threaded in the bottom out let port.

First thing to do to check it is pinch off the air governor signal line to the dyer and se if the compressor stops unloading and loading.You may want to pull the intake hose to see if it stops sucking air when it unloads. If this stops the compressor from kicking on and off the the purge valve may be leaking by the o/rings. If this does not help then the unloader valves in the compressor air leaking off.But the other way to test is to install a prssure gauge in the res port of the governor and see if the air  pressure is leaking down to about 90 psi fast after the dyer purges. If it leaks down fast then the wet tank is not holding system pressure due to a air leak on the lines or the check valve in the dryer allowing the wet tank pressure drop and cause the govenor to let the compressor to charge again.If the wet tank loses it pressure then the air governor will do its job to let the compressor to charge again.

By Mike Horrie - Wednesday, April 14, 2010 2:59 AM
Thanks for all the info.

I sent him a link to this topic.

He lives in the Detroit area and I live up North