building too much air pressure

[bobthedeadguy]

hello my m925 just started building 150 psi and horn dosnt work wipers front diff engage ment dont work chased the problem down to the air dryer signal line at 150 psi thers no air coming out (then again theres no air there ever ) if i put shop air to that line compresser sounds like it stops building air. so am i correct in assumeing dryer crapped out or do i change filters and that will fix it or am i missing something some where ?

 

[fasttruck]

Adjust governor ? need wrench for the jamb nut and a straight screw driver to turn adjuster, in I imagine to reduce pressure. I expect governor to be on engine side of firewall as that is where it is on 809 series trucks.

 

[Scrounger]

The pressure protection valve (PPV) is not allowing air to pass. It is located on the air tank. You may be able to take it apart and clean it. Or to be on the safe side replace it. NSN 4820-01-114-7543 or Bendix 279505N.

 

[doghead]

And service your airdryer.

 

[sandcobra164]

The pressure protection valve (PPV) is not allowing air to pass. It is located on the air tank. You may be able to take it apart and clean it. Or to be on the safe side replace it. NSN 4820-01-114-7543 or Bendix 279505N.

View attachment 732651

Plus 1, this was my problem with the exact same symptoms. It feeds air to the "secondary" system once above 60 psi. Your air governor is on the secondary side so when this valve stops working the governor doesn't see system pressure and the compressor runs constantly. Luckily there is an overpressurization valve as well which vents around 150 psi. Just to confirm the diagnosis, check your windshield wipers. They are on the secondary side as well and will not function when the PPV valve is stuck closed.

 

[tobyS]

Follow the line up from the head on your compressor. That line operates the valve inside of the compressor head and when there is pressure on the line, it keeps the intake valve from going closed, thus regulates pressure made at the pump. If that valve is stuck, it makes the pressure go high (or more often stuck open and go low).

Take the line loose at the head and the control and blow air through that line to see if water and debris is in it. It's a good time to get a generous dose of pneumatic oil into the head and that line.

You can also take the control valve off and take it apart to clean it and re-lube it (I use waterproof red grease). It sounds like something is keeping the control line to the head from building pressure (which then relieves the pump's intake valve). The valve could be stuck in the closed position, so it cannot relieve pressure when it goes above the set point. I set mine at about 125-130 and then reset it about 118, which seems about right.