Air dyer update and additional help please (outcome)

Sounds like you are good to go, good for you for checking, testing, and noticing that there was a problem in the first place.

Brian

Ok I have an update And outcome. Many of you replied with great info and because of your input I started by searching for the Governor I found it on the side of the compressor. I reached down to view the tag and immediately noticed it was loose. After careful examination I discovered one of the 2 bolts holding the governor on , had come out and the other bolt a little loose. I live near a heavy truck parts store and they had the replacement governor in stock. I doubt there was anything wrong with the original but for $17.00 it was already off and so the new went on. I started the coach up and it cycled it built to just under 130 PSI on both gauges then purged. I let it set for a few minutes and it did did not cycle I pressed the brake a few times and the air built back up and purged at the same point as before. I think all is well now but I will service the dryer with a new kit in the near future. Again thanks to all. I hope the post serves to keep reminding us that servicing the air system is a must.

Thanks for the kind words. When I got my bus, I was very intimidated by the air brake system. In Ontario, where I lived, I had to take a commercial air brake course and get a full tilt commercial air brake license condition. I took that starting point and kind of ran with it - my bus is a 1980 MCI and uses a different type of air brake than current models, and I needed to fully understand it and how to test it, and repair it. It's taken a few years but I think I'm getting the hang of it now!

Brian

VintageRacer wrote:
The air governor controls the cut-in and cut-out pressures for the air compressor. It reads the pressure in the system and typically cuts in at 95 psi and cuts out at 120 psi. That 25 psi difference is not adjustable as it is set by the internal spring, however the range is adjustable - you can have 90 psi - 115, or 100 psi to 125, for example. The air dryer purges every time the compressor reaches the governors cut-out pressure point. When it purges, all of the air inside the air dryer and the line from the compressor to the air dryer is exhausted out to atmosphere, and the purge valve is left open until told to close when the system pressure reaches the governor's cut in pressure. So purge cycles are completely dependent on air pressure in the air system. If you have a rapid purge interval, or if the air dryer is purging out of sync with the rise and fall of air pressure in the system, then suspect a governor issue, although to be honest they are extremely reliable. If the governor seems to be operating normally as far as air pressure control is concerned, then the purge valve could be failing.

The normal test is, with engine running at 1,000 rpm, fan the brakes and observe the pressure falling with each press, until you reach 80 psi on the gauges. Observe the pressure begin to rise (governor cut in) then time the rise from 85 psi to 100 psi. This time should be around 30 seconds or less and is called the compressor recovery time). Then watch the pressure rise to 120 psi or so, and the governor reaches cut-out pressure and the air dryer should purge. That is the only time the air dryer should purge, after a compression cycle reaching cut out pressure.

Brian

This is the correct response and written so it's easy to understand. Anyone with a diesel coach should cut and paste this response for future reference.....Dennis

Thanks for all the prompt replies. I think it is the governor and I will start There. Also will do a rebuild kit just as a general maintenance.

The air governor controls the cut-in and cut-out pressures for the air compressor. It reads the pressure in the system and typically cuts in at 95 psi and cuts out at 120 psi. That 25 psi difference is not adjustable as it is set by the internal spring, however the range is adjustable - you can have 90 psi - 115, or 100 psi to 125, for example. The air dryer purges every time the compressor reaches the governors cut-out pressure point. When it purges, all of the air inside the air dryer and the line from the compressor to the air dryer is exhausted out to atmosphere, and the purge valve is left open until told to close when the system pressure reaches the governor's cut in pressure. So purge cycles are completely dependent on air pressure in the air system. If you have a rapid purge interval, or if the air dryer is purging out of sync with the rise and fall of air pressure in the system, then suspect a governor issue, although to be honest they are extremely reliable. If the governor seems to be operating normally as far as air pressure control is concerned, then the purge valve could be failing.

The normal test is, with engine running at 1,000 rpm, fan the brakes and observe the pressure falling with each press, until you reach 80 psi on the gauges. Observe the pressure begin to rise (governor cut in) then time the rise from 85 psi to 100 psi. This time should be around 30 seconds or less and is called the compressor recovery time). Then watch the pressure rise to 120 psi or so, and the governor reaches cut-out pressure and the air dryer should purge. That is the only time the air dryer should purge, after a compression cycle reaching cut out pressure.

Brian

Dog,

I had similar symptoms with my coach. I first rebuilt the dryer, which made no difference. (That action fixed the issue on my previous coach with the same symptom.) I then replaced the governor, which changed the purge interval from 45 seconds to several minutes.

Air suspension was mentioned. Does this affect the purge rate?

Everyone! Go buy an extra governor. It has never made sense to drive a $200K coach, and get stranded over an $18 part that nearly anyone could replace in 4 minutes. Our governor is so easy to access that it is almost crazy.

Matt B

The air governor on the compressor controls the purge function of the air dryer. It tells the air dryer to open it's purge valve when pressure reaches the governors cut-out point, and the purge valve is held open until the governor reaches it's cut in point at which time it tells the air dryer to close the purge valve. There is a one-way check valve at the output of the air dryer to the wet tank which locks air into the bus's system so that no air should escape while the purge valve is left open. The air governor acts like a control switch for the air dryer, the air dryer just does what it's told to.

An air leak of 10 lbs per hour is well within DOT specifications. I'm not sure exactly what your problem is, maybe you could re-state what you are worried about. Also, is your coach air suspension? What uses air other than brakes?

Brian

I also have a 1993 coach, and replaced my Midland dryer with a new Bendix just for preventive reasons some 8 or 9 years ago. The old dryer was clean.

Loosing five pounds over 30 minutes isn't that bad. The purging is something I haven't noticed, although I assume it functions on a regular basis. Rather than repair your old dryer, I'd suggest just replacing it like I did.

Good luck.

The Foretravel "foreforums" has a thread running in Recent Topics about 7 down "can't build past 40lbs." You may find it an interesting read related to the air system.