Possibly a bad air dryer?

[bald1]

I recently purchased a '98 Dutchstar DP with a 325hp Cummins and a Midland Grau air dryer. When I start the motorhome, the pressure builds on both gauges. After the pressure is at max, the air dryer cycles like it should. However, it will continue to cycle every 30 seconds. The governor seems to be working correctly on the compressor, and I can't find any air leaks anywhere in the system. I replaced the canister filter on the air dryer, and it stills acts the same. Is it possible that the air dryer is malfunctioning? I tried to contact Spartan to order a factory service manual for my coach, but they said it's too old and no longer available. Any help would be appreciated.

[bald1]

I recently purchased a '98 Dutchstar DP with a 325hp Cummins and a Midland Grau air dryer. When I start the motorhome, the pressure builds on both gauges. After the pressure is at max, the air dryer cycles like it should. However, it will continue to cycle every 30 seconds. The governor seems to be working correctly on the compressor, and I can't find any air leaks anywhere in the system. I replaced the canister filter on the air dryer, and it stills acts the same. Is it possible that the air dryer is malfunctioning? I tried to contact Spartan to order a factory service manual for my coach, but they said it's too old and no longer available. Any help would be appreciated.

[Cruzer]

The air dryer "spits" when the compressor governor reaches it's maximum prssure and begins the unload cycle at it's "cut out" point. As the air bags, brakes, or whatever gradually use air, the air pressure drops to the "cut in" point, at which time the compressor valves close and it begins to pump air again. Once the air once again reaches the cut out point, it will unload and the air dryer will receive a signal and spit.

So, if the air compressor is cutting out and in every 30 seconds the problem lies in your air compressor governor, which is short cycling. It's not the dryer's fault that it's getting these "unload" signals and causing it to spit.

However, if the compressor is not short cycling, and the dryer still spits, then the problem lies in the dryer. Although, the dryer generally responds to air pressure being applied. I tend to think that it wouldn't spit unless air was sent to it from the governor. If anything, it would fail in the "never spit" position. Therefore, my odds are on the air governor as the culprit.

[Cruzer]

Note, you might want to check out the thread "Air Bags Leaking Down" in this Spartan Forum. That air governor line might be the one you're looking for.

[bald1]

Thanks for the explanation Mark. I understand now how it operates. I'll check it tomorrow and see what's going on with the governor and unloader line. I wish I could get a service manual from the factory. I've been an Industrial truck mechanic(fork trucks,crane,etc.)for over 20 years, and I'm sure if I had the correct information on the system I could figure it out. Unfortunately, that last time I worked on air brakes was about 29 years ago.

[bald1]

Well, I did some checking the last couple of nights and have determined that the air dryer is faulty. I hooked up a pressure gauge to the unload side of the governor(the line that goes to the dryer). What is happening is that instead of the dryer just "spitting", it gets the signal from the unload side of the governor and releases air until the pressure drops so much that the compressor turns back on and dumps the pressure from the unload circuit. Then it takes approximately 30 seconds for it to pump up to the unload pressure(120 lbs.) and it does it all over again. The compressor is constantly pumping air to recover from the dryer dumping it. I unhooked the dryer and capped the unload fitting on the governor, and the compressor works like it should. I guess it's time for a new dryer.

[bald1]

Just thought I'd give an update and some information that I found out. I ended up rebuilding my existing Midland air-dryer. The master rebuild kit cost $50 and was very easy to install. There is a "turbo saver" valve in the dryer, and mine had a severely deteriorated seat on it which was causing the problem. I also found out that these rigs that use a Holset "E" type compressor need a special dryer if anybody's thinking of replacing theirs. I went to a local Spartan repair facility, and they were in the process of replacing a dryer in a Newmar motorhome. Somebody had already replaced it with the wrong dryer, and the motorhome ended up with engine oil throughout the braking system.