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BobThompson · 07-21-2015, 12:16 PM

I have a Cummins 8.3 in a 1999 DSDP. The air compressor keeps cycling, even though there is no loss of air in the primary and secondary tanks. The loss is readable at the air chuck inside the generator compartment. The air dryer is a fresh rebuild and I've inserted a check valve between it and the governor. Air can be heard leaking back through the intake manifold and I have been told there is a check valve in the air compressor cylinder head that can stick or malfunction, causing the air to leak out. Since this would cause the governor to see low air pressure, it then cycles the compressor and upon reaching the governors cutoff setting, unloads the air dryer. If anyone has had a similar problem, I would appreciate their thoughts as well as their eventual outcome. Thanks, Bob T

NHRA225 · 07-21-2015, 02:29 PM

Does your Air System purge @ 125 psi ?
How long does it take for your gauge to drop and which tank drops 1st.
Mine will purge about every 3 to 5 min sitting in the driveway.
I would read this as it will answer most of your questions
>RV Air Systems

twinboat · 07-21-2015, 04:29 PM

Follow the 5/8 air line from the dryer to the first tank it goes to. There should be a check valve right in that connection. Pull it apart and see if it's jammed open, that will cause your issue.

There is no check valve in the compressor head. The governor lifts the valves in the compressor. The air is stopped at the wet (first) tank by the check valve. The governor also send air to the dryer to unload.

Also, make sure the 1/4 line from the wet tank to the governor is not leaking. That causes excessive cycling of the compressor and loss of air, in the only tank with no gauge.

twinboat · 07-21-2015, 04:34 PM

Quote:

Originally Posted by NHRA225

Does your Air System purge @ 125 psi ?
How long does it take for your gauge to drop and which tank drops 1st.
Mine will purge about every 3 to 5 min sitting in the driveway.
I would read this as it will answer most of your questions
>RV Air Systems

Your system should not purge that often, unless you are using or leaking air. If you shut it off, how long before your tanks drop to 90 lbs?

NHRA225 · 07-21-2015, 04:36 PM

Mine takes a day and holds @ 90 for weeks, that's if I level with the Jacks down - if I just park it it will drop to 50 or so a few days later.

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twinboat · 07-21-2015, 05:02 PM

You may want to check the pressure drop on the wet tank. Get it up to 125, shut it off and listen for a leak. The wet tank should hold, as we'll as the others. That tank is the one, suppling the governor.

NHRA225 · 07-21-2015, 05:07 PM

I'm gonna stay in touch with you if you don't mind ! My Air bags are 23 yrs old and I'm having them changed next week, then I'm going through the whole system.

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twinboat · 07-21-2015, 05:28 PM

No problem, here.

BobThompson · 07-21-2015, 07:38 PM

To attempt to reply to as many of the questions asked, the gauges in the primary and secondary tanks do not drop, even a little bit. I installed a check valve in the line from the compressor to the air dryer and had no effect. There is a line that runs from somewhere to the front of the coach inside the generator compartment and that shows a drop in pressure from the normal top of 120 +- to 90 when the compressor kicks in again. I have tried listening for a leak but even with the engine shut off, or with a portable compressor hooked into the line at the front of the coach we could not hear anything one would describe as a leak except at the area of the engine. I have been told by people at Cummins that this is an internal problem in the compressor itself. The system purges as normal, 120 psi. I wouldn't want to have to remove the compressor cylinder head as it is way down in there somewhere. Thanks, Bob T

twinboat · 07-21-2015, 08:58 PM

If you have a check valve, in the big supply line from the compressor to the dryer, correctly, no air can back feed to the compressor. Correct?

If not, there is supposed to be one in the tank, where the air dryer line hooks to the tank. Did you find and check it?

Your first post said "between air dryer and governor". That a small line. That should not be there.

Have you moved any of the governor lines around?

Is this a problem, that happened after air dryer rebuild?

Here is another way to check for leaks, pull the tank drain for the wet tank (If you only have 2 tanks, it would be the short end, of large tank, look for the weld seam ) if 3 tanks, usually the smaller one.

Hook the air supply there and listen for leaks.

BobThompson · 07-22-2015, 04:36 AM

You caught my mistaken info, twinboat. The line to the air dryer comes directly from the compressor. This makes me think there is no wet tank. The leak started before the air dryer rebuild, which had precipitated it's rebuild. The introduction of the check valve has actually caused the cycling to increase in speed, I think. I assume there is less capacity to drain down. It's hard to find where everything is in such tight spaces and I have been unable to trace the air line from the front to it's source. I think this would be key because the reading at the front matches what is happening. I will talk to Spartan today to see if they can shed some light on this. Thanks for your help. If I get this figured out I will post it. Bob T

twinboat · 07-22-2015, 05:39 AM

The wet tank is the first tank AFTER the air dryer. They all have wet tanks, they collect the oil and moisture that gets past the air dryer.

The governor signal, that causes the cycling, comes from the wet tank. When that tank drops, it turns on the compressor.

You should not, or have to, add components to a air brake system.

You can jump out the air dryer to test. Hook the big hoses together and plug the small one.

Good luck

twinboat · 07-22-2015, 05:47 AM

A thought.

Since you have a check valve, move it to the side of the air dryer going to the tanks, after the air dryer. If you notice a change , that would be a clue of a stuck check valve, down near the tank.

BobThompson · 07-22-2015, 10:00 AM

I just got a diagram from Spartan, twinboat, that shows exactly what you spoke of.

The good thing is it assures me that the expensive repair of replacing the wet tank will be unnecessary. The check valve in the wet tank/primary tank is internal and requires the tank to be replaced in it's entirety if the primary leaks. I am hoping the problem may be in the governor. I'm wondering if it could be leaking internally, causing it to send the signal to the compressor to cycle. Your thoughts? Bob T