Compressor cycling

[Krink545]

I have a m923 A2. Just finished replacing one of the dp16 air chamber pancakes. Truck has no air leaks that I can hear with the truck off with the truck running it's hard to tell but I don't hear any leaks. Primary and secondary gauges never go below 120 PSI with the truck idling but the compressor Cycles on and off about every 30 seconds. I don't believe that is normal can somebody point me in the right direction maybe it's leaking back out through the compressor while the compressor is off?

 

[CARMAN]

I have chased this for years. SIMP has posted several ideas and I have never been able to make it stop. My tanks hold air, hear no leaks, but compressor continues to cycle at least once every minute for about 10-15 seconds then blows off. I have replaced the air dryer and rebuilt the top end of the compressor. Tapped on all valves and tanks to make sure nothing stuck...played with the regulator under the hood up by the windshield. Still cycles through.

 

[simp5782]

Put a gauge on the wet tank using the drain port. It is a 1/4" fitting

. Your compressor cycles off and on using the air governor. Your air governor sees air from the wet tank Via the pressure protection side. Not the primary and secondary tanks. Primary and secondary tanks have check valves that prevent back flow of air into the wet tank.


Your air leak is behind the dash, into the transfer case, or on your emergency gladhand hoses.

Hook up a gauge to the wet tank. Close off your primary and secondary tank shut off valves located on the tank. See what the guage is reading for the wet tank vs the compressor cycling off and on. If the compressor is cycling and the wet tank has not moved on pressure then replace the air governor. If the gauge is falling then report back and we will chase leaks from there

 

[Krink545]

Thanks for your reply. Both valves on the wet tank have been shut off I even plugged all of the lines on the wet tank except for the line from the air protection valve. I replaced the air governor and finally took the intake hose off of the air compressor and as soon as the compressor would switch to the unloaded mode I shut the truck off and I'm getting air coming through the compressor from the governor until it bleeds down to switch back to loaded mode. So I'm guessing the unloader valve in the top of the compressor is letting air past. Haven't tried yet and don't know if you can get the top of the compressor off with it being so close to the injection pump. Don't know if it's worth rebuilding or just buying a rebuilt one for $400.

 

[simp5782]

Thanks for your reply. Both valves on the wet tank have been shut off I even plugged all of the lines on the wet tank except for the line from the air protection valve. I replaced the air governor and finally took the intake hose off of the air compressor and as soon as the compressor would switch to the unloaded mode I shut the truck off and I'm still getting air coming through the compressor from the governor until it bleeds down to switch back to loaded mode. So I'm guessing the unloader valve in the top of the compressor is letting air past. Haven't tried yet and don't know if you can get the top of the compressor off with it being so close to the injection pump. Don't know if it's worth rebuilding or just buying a rebuilt one for $400.

Buy a rebuilt. Eli learned the hard way on those compressors. And do not squirt wd40 in the unloader

 

[CARMAN]

You can get the head off. Not fun but yes. Once the bolts are out and loose, bust it loose with a little tap of a hammer. Replace the head gasket and seal up the bolts as air will leak out of them. You can also clean up the pressure assembly. The back bolt has to come out with the head. Not enough clearance to lift the bolt out...and remember to put it back in the hole before getting the head back on. I have done this and still have the compressor cycling. You also must drain ALL coolant. If you try to skimp it will end up all over the floor...and it's a lot. Get a clean 5 gal. bucket. drain the radiator. (how am I doing SIMP)

 

[Elijah95]

Pulled my head many a times, unless it’s an emergency road side fix DO NOT spray any oil products down into the head when unit is assembled... lol.

Another tip, absolutely do not reinstall the compressor head without machining flat. Nothing hurts worse than doing a job twice, or 3x. I used a 12x12 glass pane on a flat surface with sandpaper taped to it to grind my head surface flat. It was bad.

Don’t remove the lower head with the coolant lines unless necessary, To perform a soft rebuilt, just pull the upper head and install a service kit with new soft parts such as reed, spring, gaskets and O rings.

Do not use just any petroleum based oil for reassembly, use air tool oil to lightly lube o rings.

But before you bother tearing it down, follow Wes’s advise on pin pointing your leak. I didn’t, and the high duty cycle of my compressor always running meant that I rebuilt it 3x whereas if I had of fixed my issues, rebuilt it 1 time and let it rip I would have been golden.


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[tobyS]

Happy, happy......me. I've fought air problems on the 929 for years. Beginning, I blew out the line from the regulator to the head and got lots of debris and some water. I used air tool oil in the line, but didn't take the head off.

Long story short...I paid some sharp guys to work on my truck, replaced the air drier (Haldex PURest) and regulator and now we go to 120 and stay there. Yippee.

 

[Krink545]

Here's a short video I just put on YouTube of my air leaking back through the compressor.



M923A2 compressor leak

 

[Elijah95]

Rebuild JUST the upper head of the compressor, no cooling system tampering required.

You’re leaking through your reed or o rings.

Buy these kits, or contact PAI on fleabay for a combo kit and save money. You are going to need the hard parts. Please resurface the head part no. 17 when doing this. I referenced that above in a prior reply, if you remove the lower cast iron head too you’ll have to drain the cooling system.

It’s tight but it can be done.





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[Elijah95]

Part no 17


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