High air pressure on 923 Driving me crazy

[dodgebroke]

Hey guys, any help would be appreciated. I rebuilt my dryer 6 months ago and all has been great. Then out of nowhere my air pressure gos to 150 plus and my relief valve keeps popping. The spitter on the dryer is not activating however. I pulled it apart and it seems fine. I'm completely stumper and I need to use the truck for an event tomorrow. Any ideas?

 

[porkysplace]

It sounds more like the governor or unloader valve giving you issues .
Post #2 of this thread gives you a link to the TM and the section number for thr air system.

[h=1]Location of air compressor & governor[/h]

 

[Scrounger]

It is the PPV. Pressure protection valve that is causing the problem. You should also notice that the horn, wipers, and front axle won’t engage. The PPV is preventing air to the secondary system, which is where the governor receives its signal from. It is an easy fix. It will be on an air tank. Some people remove them, some try to clean them. I try to keep things original. To each his own.

 

[porkysplace]

In post # 3 0f this thread Scrounger posted the part numbers.
[h=1]building too much air pressure[/h]

 

[dodgebroke]

Thanks guys, I replaced the governor (again) today and it helped, but now I've got several leaks to hunt down, I'm assuming from over pressure. The old girl keeps me busy.

 

[dodgebroke]

I will definately be replacing the PPV as well.

 

[rosco]

My experience with over-pressure was finally cured by replacing the "unloader valve" in the head of the compressor. It lets the compressor to go into a "free wheeling"/coasting mode, where it doesn't pump air. In the meantime, I replaced a bunch of stuff, as suggested above. The only thing that will give you 150# air pressure is the compressor unloader valve sticking. Its not a bad fix, and can often be done on the truck, depending on what else might be in the way.

 

[simp5782]

My experience with over-pressure was finally cured by replacing the "unloader valve" in the head of the compressor. It lets the compressor to go into a "free wheeling"/coasting mode, where it doesn't pump air. In the meantime, I replaced a bunch of stuff, as suggested above. The only thing that will give you 150# air pressure is the compressor unloader valve sticking. Its not a bad fix, and can often be done on the truck, depending on what else might be in the way.

If the pressure protection valve is not opening the governor does not see any air pressure source in order to tell the governor to send air to the unloader at it's set pressure. So the compressor just keeps running

 

[tobyS]

Follow the line from the unloader valve to the control on the firewall. Remove it and clean it. Take the line loose at the compressor and blow air through it. Replace and put some Marvel Mystery Oil (pneumatic oil) in the line to free up the unloader valve. That control collects gunk. Once cleaned, use a light coat of red (waterproof) grease to lube it, helps keep moisture from rusting it inside.

The pressure is set by the small screw and locknut under the plastic cap on the control. Back it off about 2 turns and watch the pressure. I was running about 125 and it was a bit high so set it about 118#...which seems about right.

 

[tobyS]

My experience with over-pressure was finally cured by replacing the "unloader valve" in the head of the compressor. It lets the compressor to go into a "free wheeling"/coasting mode, where it doesn't pump air. In the meantime, I replaced a bunch of stuff, as suggested above. The only thing that will give you 150# air pressure is the compressor unloader valve sticking. Its not a bad fix, and can often be done on the truck, depending on what else might be in the way.

The unloader valve usually sticks open making it so it won't build pressure. I guess it could be rusted so it won't open and release but first I would try getting some pneumatic oil in it. Taking the line loose and blowing air through it will usually tell if there is crud (oil, water and rust) stopping the signal from opening the unloader valve. (top of comp is a pain to get apart).

 

[dodgebroke]

Done and Done. Thank you gentlemen for your knowledge. Everthing is working great with the air system, now the alternator quit charging. I'm going to bed.