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Possible air compressor failure M925A2
- Thread starter Russ Knight
- Start date
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- 113
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- Perry, FL
Pretty certain the unloader is stuck. Won’t build air past about 20psi. I hooked shop air to the truck and couldn’t hear a leak anywhere.
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You could also have a stuck one way valve. Take the flared fitting that comes off the compressor off. Then take the flared/pipe fitting out of the compressor head. It will have a 1/2" NPT on the compressor side. you can then screw your gladhand fitting on to that and then attach your shop air via a glad hand onto that. Then charge the system. This runs air thru the whole system including the charge line and air dryer. Unlike going off the emergency port you are only filling the tanks up and not thru the one way valve in the dryer. This makes it easier to diagnose a compressor issue. The air systems on these things will drive you crazy.
You can also put a 1/2" npt to 1/2" DOT line fitting in to the head of the compressor and rig up an air pressure gauge there to see what exactly you have coming out of the compressor at it. This will show if there are any obstructions in the charge line. Sometimes they can get gunked up and down at the elbow near the cab and frame rail it will all settle right there and only allow so much pressure to go by.
A stuck primary tank one way valve (Kn23000) can cause both tanks to not get over a certain PSI cause it will only open so far. I had a gremlin doing that. wouldn't build past 40, ended up being the one way valve was not opening but so far and pressurizing the wet tank to 120psi and on the PPV side telling the governor to shut the compressor off when the primary and secondary tanks never got above 40.
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If you get into the compressor and find more issues, also search the ss296 number that Wes posted earlier, lots of other kits and parts pop up.
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Pretty sure it’s the unloader. Pulled the intake fitting off and the air is pulsating there. Little if any vacuum. The question now is how to get the back two head bolts out? I can loosen them but haven’t figured out a way to unscrew them.
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The compressor needs to come off to get them out.
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- Perry, FL
I was afraid of that.....
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- 1,117
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- Location
- Perry, FL
One more question. Why is the compressor intake connected to the head assembly of the engine?
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Gets its supply air to compress from there. It is filtered already.
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I know that’s where it gets it’s supply from, but why there?
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Cause its filtered by the air filter already. Its good clean air. Rather than having another filter to replace that would be under the hood and exposed to other elements and hot air. Plus it doesn't have to work too hard to pull it down that short tube when the engine is already forcing air into the engine to draw from.
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If you loosen up the bolts, they should come with the head. It can be done without removal of the compressor. I'm sure there are some good threads and wes referenced a TM.
Clean the control also and make sure that tube is clear...it can hold a lot of debris.
Clean the control also and make sure that tube is clear...it can hold a lot of debris.
74M35A2
Well-known member
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Because 8.3’s run supercharged air compressors. 
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Pulled the compressor yesterday. Found the head gasket brittle and broken. Everything else looked good. I think all my air was escaping through the broken gasket. Hope to have everything buttoned up tomorrow and I'll know for sure.
Good for you! Sometimes we feel like we crawl and other times we make significant progress.
I red primer paint what I can on the inside to keep rust down (or high heat) and use red grease liberally, but not globs. It seems to help, since some moisture is inevitable. Maybe we should have line lubricators and run a little Marvel Mystery Oil.
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suction pressure
Think sort of manifold pressure
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Reinstalled the compressor after installing he top end rebuild kit, fired the truck up, and the compressor is pumping better than ever. The compressor head gasket was the issue. What an aggravating job though. Had the compressor been engineered to be installed on about a 30 degree angle, the top end could be serviced without pulling the entire compressor / power steering pump assembly.