Air compressor governor

[J_E]

Short version:  Assuming my air pressure gauge is working correctly, what should my air pressure be doing going down the road.

The longer version starts with me looking through the brakes section of the bus maintenance manual.  I had started looking through it to see what the procedure was for checking the brake system and adjusting the brakes.  Seeing a section on an air compressor governor caught me off guard.  I haven't seen anything to make me thing that there is a governor on my air system.  When I saw the spec section for the governor, I knew why.  The maintenance has probably deferred to the point that it is non functional.  Sonofacrap, something else not mentioned from the inspection.  If I read the book correctly, I should see the gauge stay between 90 and 120 psig.  What I have seen is the gauge go up to the 130-140 psig area then vent air down to the 90-100 area.  Basically, I hit the relief and blow down until the relief reseats.  I timed it once while coming back from Texas and it took about 15 minutes from the time I heard the relief open until I heard it open again, but that depended on engine rpm and where/how I was driving.

[bevans6]

What your air pressure should be doing is varying between 95 and 120 psi as you drive and the brake/suspension system uses air.  When air pressure reaches around 120 psi the governor will cut out, and signal the air dryer to purge, which is the sound you hear.   When the governor cuts out it operates the unloader valves in the air compressor, effectively turning it off.  When air pressure reaches 95 psi the governor cuts in and re-activates the compressor, which raises the air pressure back up to 120 psi.  It would be pretty normal for the air pressure to take 10 - 15 minutes of driving to lower itself from 120 psi down to 95 or so, then take about 30 seconds to recover from 95 back up to 120.

[chessie4905]

I had the same problem on a tractor once. Did the same thing. I would pump the brakes frequently to keep it from going over 140. Replaced governor and all was well. Governor's are cheap either rebuilt or new. Repair kits are cheap too. If it is remotely mounted, make sure the signal line from the compressor is connected and not plugged or pinched off. Most governor's are mounted directly to the compressor, however.

[Zephod]

Air pressure....
150 psi - emergency pressure release cuts in.
125 psi - governor cuts in to maintain 125 psi
100 psi - recommended minimum for moving the bus.
65 psi - low air light should come on and low air buzzer will sound. Pull over and stop immediately!!!
25 psi emergency brakes come on. You're not going anywhere.


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[buswarrior]

Your goal is to chase all the air leaks so the air pressure only goes down when you use the brakes.

Perfection is rarely achieved, but worthy to pursue?

happy coaching!
buswarrior

[Zephod]

Your goal is to chase all the air leaks so the air pressure only goes down when you use the brakes.

Perfection is rarely achieved, but worthy to pursue?

happy coaching!
buswarrior

The work bus has air leaks. If I don't stomp the foot brake after releasing the parking brake, it leaks air.


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[gumpy]

If I read the book correctly, I should see the gauge stay between 90 and 120 psig.  What I have seen is the gauge go up to the 130-140 psig area then vent air down to the 90-100 area.  Basically, I hit the relief and blow down until the relief reseats. 

Yeah, sounds like your governor is stuck. They're $15-$20 at a truck parts house. It's located on the air compressor. Very easy to change. They can be taken apart, cleaned, and greased and may well give you years of service again. I did that on mine and it's been working fine for about 15 years, but I now carry a spare.