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Bendix air dryer

Yahoo Message Number: 62133

In discussion with a Bendix tech, I find that CC used a modified version of the AD-9 air dryer. It is called AD-9 Drop-in #065647. This dryer has has an additional function of keeping the compressor crank case pressurized to prevent it from becoming a vacuum and drawing oiled air out of the crankcase. Has anyone rebuilt this unit. I am finding it is very expensive to replace.

Dave

aka Billy Byte (trusty hound)
2000 Allure #30443

Re: Bendix air dryer

Reply #1 –
Yahoo Message Number: 62137

My mistake, make that compressor cylinder not crank case.

Dave

aka Billy Byte (trusty hound)
2000 Allure #30443

Bendix air dryer

Reply #2 –
Yahoo Message Number: 62164
Hi Dave,

I was wondering when this question would come up. I can try to explain the system although I've slept since it all happened and I may have a couple of the details off.

Some Cummins engines used what is called a Holset Compressor. It is different than others by the need to keep air pressure on the cylinder head to keep the compressor from pumping oil. The compressor runs 100% of the time the engine is running. The governor senses the system pressure and controls the compressor valves to either pump air into the system or bypasses the air back to the intake air. Usually come on around 90 psi and cuts off in the neighborhood of 125 psi.

When you have a standard compressor, the governor senses the air pressure in the wet tank is at 90 psi, it closes the purge valve on the dryer, starts the compressor pumping and refills the system to 125 psi.
At cutoff the governor tells the compressor to bypass the air and to open the purge valve, clearing the air dryer . This is the air you hear discharging. The line between the compressor and the dryer goes to 0 psi , but the system air is held at 125 psi due to the main check valve on the discharge side of the dryer. When system air drops to 90 psi the process repeats itself. How long it takes to drop depends on use such as air brakes, suspension, pac brake operation, air horn and others.

If you have a Holset compressor, things change a little. The governor senses the air pressure in the wet tank is at 90 psi, it starts the compressor pumping and refills the system to 125 psi. At cutoff the governor tells the compressor to bypass the air and to open the purge valve, clearing the air dryer. Again, this is the air you hear discharging. This is where things change.

The line between the compressor and the dryer cannot drop to 0 psi because it has to maintain a pressure on top of the compressor. It does this by virtue of a small check valve that bypasses the air dryer from downstream of the dryer check valve and bleeds air back to the compressor head. The check valve is positioned so it will allow a back feed from the wet tank to the compressor but not allow air to go to the wet tank without going through the dryer first. This is a small heck valve that resides on the bottom next to the air dryer and is plumbed from the discharge side back to the inlet side and consists of the valve, a couple of fittings and a short piece of tubing.

There are two types of air dryer purge valves, a "hard seat" and a "soft seat". The hard seat is used on the normal compressors and stays open when purge takes place. The soft seat allows the dryer to purge, but closes and allows the air pressure to be maintained within the air dryer, thus allowing the check valve to build up pressure on the compressor head. When system air drops to 90 psi the process repeats itself.

The wet tank air is actually held by the compressor check valves and not the air dryer check valve. This makes a difference when troubleshooting air loss from the wet tank when you do an air brake test. Most would go directly to the air dryer check valve and replace it. But since the small bypass check valve allows air around the dryer, the compressor check valves may actually be at fault and you will get a slow continuous drop in wet tank pressure.
The question remains on what happens if you put a standard air dryer in place of the required dryer. The wet tank will function properly but the compressor will have no head pressure. The compressor will begin to pump oil and it will contaminate the air line between the compressor and air dryer and the air dryer itself will be contaminated with oil. When the purge valve opens, you will get oil dripping from the the bottom of the air dryer. Oil could possibly continue forward to the rest of the systems.

How do we know all this? We had our air dryer replaced by a commercial outfit and they did away with the check valve (actually handed it to me as an extra part) and said it wasn't needed. I noticed oil dripping from under the coach so we had to do it all over again. As I remember the fix was a standard AD-9 air dryer using a couple of Tee's and a new check valve, then replacing the standard purge valve with a "Soft Seat" valve. I'm going to guess about the prices, but I believe the material prices for the air dryer was about $200 and the purge valve was another $150. Labor is extra on top of that. The only difference in the dryers is that the one for the Holset compressor has a port tapped for 1/8" ips thread in the body downstream of the main check valve.
Hope this helps and not too confusing. If you need part numbers contact me on the back channel (off list) and I might be able to dig up the receipts and maybe even a photo or two.

Ron Gates

2000 Intrigue #10936
 

Re: Bendix air dryer

Reply #3 –
Yahoo Message Number: 62166
I got mine a NAPA, part #109493X. As I recall, the price was around $75?. I have seen another NAPA part #107794X talked about here.

Larry, 03 Allure 30856 W ISL 370