Post shutdown air release then pop

[Jake0147]

One of my deuces (the one I drive a lot) makes a sound after shutdown that none of the others do. You can hear air evacuating with authority for about 20 seconds and then a distinct air "pop" and then all is quiet. It would seam to be a pressure relief on the compressor or something. It sounds official but why don't the others have it?

You don't say how long after shutdown. There are a couple of places where a leak could make this happen immediately, too soon, or take a bit longer. Mine is about ten to twenty minutes after the engine is shut down. I think it leaks a touch at the governor or that control circuit, because once it "pops" the rate at which the tanks drop will decreases significantly. A severe leak in the governor, the signal line to the compressor, or even the unloader inside the compressor would make that sooner.

The air governor is on the firewall, and is essentially an air powered switch. When it senses that the tanks are properly charged, it sends a signal via the small air line to the compressor, which moves a mechanical valve, which by a mechanism which can vary from one to another, renders the compressor inoperative. It still runs, the piston(s) still move air, but the ability to contain or direct that air is defeated.

As a properly working truck sits, as the air in the tanks discharges, be it hours, days, weeks, etc... At some point you will hear that certain spot where the governor finally "kicks in", and sends (or actually "unsends") the air signal to the compressor. You can hear air "woosh" as the air is released from the signal line immediately followed by the "pop" when the unloader settles it's self back into it's "pumping" position.

So yes, the noise is pretty normal and to be expected. It's just not always noticed, as it can be anywhere from a few minutes to many hours or theoretically days...)_ before it happens, so it probably is not actually noticed on every truck.

 

[50shooter]

Thanks for all the input guys.

Jake, that would help explain the diff in my trucks. I checked 2 others this w/e and neither made a sound as soon as the truck was turned off. They may have one or six hours later but I wasn't around.

Thanks
Jason

 

[Lex_Ordo]

My truck has a two cylinder compressor. When I first bought it, and up to recently before, the air govenor on my truck would pop, a few minutes after shut down.

It turned out that my glad hands were leaking, and caused a lot of air dump, signaling the compressor to start pumping. Since getting the truck a year ago, it hasn't been driven that much, so I don't thnk that any unnecessary wear occured.
But...

Now that the glad hand valves have been replaced by No. 2 Diesel, and his brother, the govenor might dump after an hour or so. That is an air-tight system. I almost never hear the compressor click on now while driving.

 

[tm america]

pretty normal mine does it to