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Bendix compressor short cycling
For the past 2 years the compressor has been short cycling and has gotten progressively worse. Now I barely see a 2 pound drop on the dash gauges before the compressor charges, was about 5 pounds. Test drove it today and had the 2 pound cycles.
I have done all of the following in the past 6 months resulting in no improvement:
1) Installed new govenor on compressor in April.
2) Installed new govenor on compressor in July.
3) Installed new unloader mechanism on compressor in July.
4) Installed new purge valve on AD-9 dryer ( old one worked fine) in July.
5) Removed and bench tested delivery check valve on AD-9 Dryer (worked perfectly).
6) Put air gauge on wet tank, rate of pressure loss is less than one pound per minute ( 20 pounds in 30 minutes). Bendix spec of acceptability is less than one pound per minute.
Dash gauges are losing less than one pound per hour (20 pounds in 24 hours when starting at 120 psi).
My intuition is that something is wrong in the path of air to the govenor. But would like to hear some thoughts, thanks.
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13 Replies
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Do you have gauges for all three systems? Primary, secondary and accessory air? If so is the pressure drop you see on any particular circuit?
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Yes Jon. The primary and secondary circuit gauges are on the dash, and there is no pressure drop. The accessory gauge is in the steering compartment, and it too is not losing pressure. The only temporary gauge I installed was on the wet tank, and its pressure drop was less than one pound per minute.
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Since the wet tank supplies air to all the other tanks and it drops in pressure, but the other tanks do not drop is the petcock open? That is the pressure the compressor governor uses to control the cycle because that is the pressure it reads. Your compressor cycles might be because that tank is losing pressure that is not reflected on the dash to the extent the loss is at the wet tank. That could be a fitting, the petcock or even corrosion causing a leak in that tank.
That tank can literally be down to zero pressure and the check and protection valves in the system are insuring that loss of pressure does not also cause a pressure loss in the other tanks.
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The petcock has not been open. I did open it to check for water or oil residue, and there was none. Then I removed the petcock to screw in a temporary gauge. That wet tank needs thirty minutes to lose twenty pounds, which is the governors cut in point. But in fact the governor/compressor is cycling in a matter of probably three minutes.
So the governor thinks there is a twenty pound loss in three minutes while the three circuits are showing nothing close to this. Even the wet tank loss is within Bendix’s specs at less than one pound per minute. If twenty pounds are being lost in three minutes, I should be able to find it. Soapy (ivory) water and a stethoscope – not yet.
When I introduce shop air at the right rear, does that pressurize the system in front of the wet tank? If not, then I would not have exposed the leak.
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I THINK there is a small braided line going back to comp it can be leaking inside it self , it goes from wet tank back to comp but I do not have the air chart for your coach so take with gain of salt , also the big braided line from comp to dryer can be leaking to, hope that helps. I also think there are three lines leaving wet tank one line goes to comp and one too the primary tank and also one goes to front to feed aux tank and secondary tank but that is a big maybe check those things two
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If you pressurize the coach from the rear Schrader valve you are pressurizing all three air systems.
Peter raises a good point. You could be leaking between the governor and wet tank.
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Sorry about screwing up the thread with that wide post, but if you look at the diagram you can see how and where the governor is getting its signal.
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Mice have chewed a pin hole in one of your air lines. Had this happen to me but was more than a pin hole. to make it worse it was in the 4 inch PVC duct running front to back with about 8-to other hoses. I could barely hear it. Bob
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Logic. If the governor gets it’s signal from the small braided line from the wet tank, and the wet tank needs 30 minutes to lose 20 pounds (as mentioned above), then the governor cannot be causing the compressor to load/charge.
If the small braided line between the compressor and wet tank is leaking, that would show up on the wet tank gauge installed temporarily. There cannot be a check valve in that line as it would negate the purpose for the line.
Hope the compressor is robust enough to survive the OKC run (800 miles one way for us).
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True, but if that line from the wet tank to the governor has some blockage combined with a leak downstream that could be the short cycling. The pressure could bleed off the line downstream from the blockage and ahead of the governor. Remote possibility but given the new governors you have had I cannot think of anything else that makes sense especially with the leak down rate of the wet tank.
BTW, that is an exceptionally high leak rate. I know what Bendix and the DOT handbooks say is acceptable, but the standard really needs to be no leaks. I cannot say that about my axxessory system, but I can leave the bus sit for 3 months and when I start it the primary and secondary will both have over 100 PSI.
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Don’t know if I will get to it before OKC, but the next part swap out I guess is the wet tank to governor line. I’ve always prefered to throw new parts at my stuff rather than pay for labor. Thanks everyone, see you soon.
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Today I did some more tests.
1) Opened the wet tank petcock, zip tied a sandwich bag with a kleenex inside to it. Disconnected the RES line on the governor, shot 115 pounds of shop air through the line. Not a trace of anything on the kleenex or in the bag.
2) Disconnected the wet tank to governor line from the wet tank and attached a gauge to this end of the line, plummed in a valve and an air chuck on the governor end of this line. Clipped on shop air, filled the line, closed the valve, and the line held pressure (30 minute test). This says the line does not leak and is not blocked.
3) Did the same test with the same result on the governor (UNL) to air dryer line as 2) above.
I was going to just replace the two governor lines but they were suspended high up under the chassis with lots of stuff in the way of getting to all the zip ties and other suspension devices. So these tests seem to conclude that the lines are not the problem.
I believe the UNL line on the governor controls at what pressure the governor cuts out the compressor charge, and the RES line on the governor controls at what pressure the governor cuts in the compressor charge. The cut out has never been a problem. It’s always been the cut in pressure.
I have now completed all of the trouble shooting tests in the Bendix troubleshooting chart for the AD-9 air dryer using shop air. The next todo is replace the desiccant cartrage, even though there is no evidence that it is needed.
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Generally speaking an air dryer cartridge needs replacing when you see an increase in the moisture drawn from the wet tank when it is drained. I average around 5000 miles for air system draining and typically I get no moisture to perhaps a teaspoon full. Cartridges are changed about every 2 years.
Another clue the cartridge is in need of replacement is visible oil residue on the coach frame around the spitter on the air dryer. The element in the dryer tends to trap the oil (a small amount is always present from the compressor) but when it gets coated with oil it no longer functions as it should and then you begin to see the residue on the frame, along with some visible when draining the wet tank.
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