Air dryer purging

Lee Bradley · May 08, 2015, 09:24:01 AM

1991 Neoplan Cityliner with 8v-92. The air pressure comes up to 120 psi and the air compressor shuts down as it should and the air dryer purges but then the air dryer continues to purge every 30 to 60 seconds as long as the engine is running. Shut the engine off and the purging stops and the system holds air; was on vacation for two weeks and the air pressure was still over 100 psi. If I connect the bus to regulated shop air, the system is fine up to about 125 psi but at 128 psi the air dryer starts purging every 30 to 60 seconds as long as air is supplied at that pressure. I have replaced the governor and varied the bus system pressure from 110 psi to 140 psi and the dryer starts purging when the governor shuts down the compressor. I don't think the dryer should be purging that often but it doesn't seem to drop the system pressure. Is this something I need to fix?

eagle19952 · May 08, 2015, 09:56:16 AM

serviced the dryer yet ?

Jim Eh. · May 08, 2015, 10:07:54 AM

What make and model of air dryer?

Lee Bradley · May 08, 2015, 10:34:58 AM

AD-4

John316 · May 08, 2015, 11:08:12 AM

I'm betting on the unloaders. On top of the air compressor (as in built in), there are little pistons. Ours were close to the the air governor. When ours had issues the dryer would always be cycling, just as you describe.

TomC · May 08, 2015, 02:33:18 PM

That is the air dryer telling you it is time to service it. Get a rebuild kit with filter and purge valve. Easy to do. If you don't rebuild it, the next step will be for the air system to stop working. Good Luck, TomC

luvrbus · May 08, 2015, 03:11:53 PM

Check the 1/4 in line from the governor to the dryer if it has a leak or a crack it will purge continuously

Raymond smith · May 08, 2015, 06:32:04 PM

On my transport that i use to drive , that 1/4" line had a pinhole in it like Luvrbus said it will purge way to much

buswarrior · May 11, 2015, 07:19:27 AM

There isn't one of those turbo feeds to the air system on that?

Another complication as newer stuff gets into the conversionists' hands...

Happy coaching!
buswarrior

Lee Bradley · May 13, 2015, 08:26:10 AM

It was time to service the air dyer but there was a broken part in the air dryer. There is a check valve on the output port, the one that supplies air to the bus. That check valve had completely failed, the plastic flap was completely gone. So when the system reached shut-off pressure the governor signaled the compressor to shut-off and the air dryer to purge. When the purge valve opened to purge the air dryer it also started draining the air tank and as so as it dropped air pressure 20 psi the governor signaled the air compressor to start and the purge to stop. So there was never a complete purge of the air dryer and the air compressor was running to much of the time. I installed a new desiccate cartage and a purge valve kit and all is good again. Thanks to everyone for their input.

bevans6 · May 13, 2015, 08:51:20 AM

Could you see the drop in air pressure on the gauge? If there was a check valve between the wet tank and the dry tank, I bet you would never see that leak.

Edit: On my MCI the sense line to the governor is after the dry tank, the pressure protection valve and the E-filter. It actually comes from the accessory system. There is a check valve between the wet tank and the dry tank, so that failure at the air dryer would have caused the air dryer to purge out the whole wet tank, but would not have caused the same rapid cycle since the rest of the air system would have remained at normal pressure. But recovery would have taken longer since the wet tank would have had to fill to a pressure greater than the dry tank before the system started to refill. You would have seen a big delay after the compressor should have kicked in until the gauge started to rise. Isn't that interesting to think about, and think of how to trouble shoot such a failure.

Brian

Lee Bradley · May 13, 2015, 09:38:35 AM

There was no drop on the bus pressure gauges. In the system you described, the purge time be longer than the time limits for the air dryer. You could add a test gauge to the wet tank.

bevans6 · May 13, 2015, 09:41:35 AM

Yep, you would probably hear it purge for about 10 minutes as it emptied the wet tank!

Edit: This is how you learn stuff, folks! The next time I get a 10 minute long purge and an empty wet tank after purge I will say to myself "that is a failed check valve on the air dryer, I will fix that straight away!"

Brian