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Work Packages 130-133 in the -23-1 TM might be helpful. If you haven't already, I'd be using shop air through the emergency gladhand while troubleshooting this so you can hear everything that's going on.
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Bigboy down again,M923 won't build air past 59psi
- Thread starter tbar123
- Start date
I tried a few things yesterday. First was I started the truck ran it up to 59 then applied the parking brake,no change. Released it and applied a few times no change. So I crawled until the truck and ran the air lines down. There are 2 valves on the transfer case. Looked 1 up in the TM and couldn't find the second. The one I did find rang a bell in my head.I remembered that soon after I got the truck I flipped the front axle lock in and heard the same sound in the intake. So I think it might be the valve I found in the TM called the "Front axle Engagement control valve". THe second unidentified valve sits about 6" to the drivers side of the a fore mentioned valve and has 2 wires coming from it,1 grounded to the frame and one to a breaker.
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There are two air cylinders on the transfer case, the shifter lock cylinder and the front axle engagement cylinder
The shifter lock cylinder locks the T-case shifter in place after the button is released and is on top of the T-case. It is air applied and spring released. Since it is air applied, a air leak from this cylinder would be constant until the button was pressed. It would leak air into the T-case which would then vent into the intake stack. Push the button on the T-case shifter when you hear the air in the intake stack and see if it stops. If it does then you can either rebuild/replace the cylinder or just cap the air lines and take it out of the system. I capped the lines on mine.
The front axle engagement cylinder in the back would only leak if you had the front axle engaged.
Ok, I did what Swamp Donkey suggested and **** has broken loose! I pushed the button in and no change, next I pulled the lever up with button in and still no change. I then lowered the shifter and sparks flew from one of the wires and soon after the horn started blaring again! So I shut the truck off and then pushed the shifter the reast of the way down and turned the batts back on and the horn was not sounding! But seriously WTF! The horn with the Hi-LO shifter?
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Well alrighty then.
Did the wires happen to get pinched at the pivot point for the shifter?
The first couple times you pressed the button with no change befor the "incident" should rule out the shift lock cylinder as your air leak. The horn going off during a direct short to ground probably wouldn't be abnormal given that the horn is activated by completing the ground.
Did the wires happen to get pinched at the pivot point for the shifter?
The first couple times you pressed the button with no change befor the "incident" should rule out the shift lock cylinder as your air leak. The horn going off during a direct short to ground probably wouldn't be abnormal given that the horn is activated by completing the ground.
I made this reply already once today, but for whatever reason it didn't post. SO the news is worse than what thought, All 4 wires around the shifter and the brake lever are screwed. Can anyone tell me what thing in the picture with the green arrow on the right is?
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Shift lock solenoid and shift lock cylinder.
A couple wires from the T-case shifter go to the solenoid. The solenoid sends air to operate the cylinder.
swede69
New member
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- concord nc
you can get rebuild parts for the unloader at Cummings,just 4 seals for the unloader head. pn# 211315,128085,128086,127936 about $15 for them ,nhc 250 eng.
The unloader valve is built into the top of the compressor. It has a spring which keeps it in a position to send the air generated by the compressor to the air dryer. When the air governor senses 120psi it sends air through a small line to the unloader valve overcoming the spring and changing the unloader valve position which dumps the air the compressor makes. These valves are known to stick in the unload position causing no/low air building.
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The unloader is part of the compressor. I think Swede69 was commenting on your air issue.
After seeing the state of those wires I question if pushing the button on the shifter actually worked the cylinder, making the test invalid. You can plug the airline going to the cylinder at the outlet from the solenoid and disconnect power to the solenoid. Everything will still work but the shifter won't have a positive lock. This is the way I did mine due to the cylinder leaking, minus terminating the solenoid wiring, and it shifts beautifully now.
The other wires go to the 5th Gear Lockup Solenoid on the transmission. I would repair those as this is necessary to ease shifting.
Oldworldtenacity
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- Charleston,WV
I had a similar problem except mine would not build past 40PSI and that much air was not coming out of my intake like yours. For me, I trouble shooted till I could not stand the truck anymore. I had just replaced the air dryer a few months before and also replaced the governor, plus capped the CTIS system off...also replumbed the lines for the new bigger Air horn I added.
Finally traced the problem to the Compressor. Took the unloader valve off and it wasnt stuck either.
It ended being the whole compressor that was bad or the internal valves/cylinder were so full of carbon that the compressor just could not build enough air anymore. The Compressor place in Chicago that I got my new one from said that it is a very common problem with these trucks that have the single cylinder design compressors in them. He said if I run the truck a lot to not be surprised if the new one only lasted for 3 years or so before it happens again.
Only thing that makes me think it might not be your compressor being shot though and just a leaking valve is how much air is coming out of your intake after shutting the truck off.
I've searched the forums and repair parts TM for the 5th gear lock up solenoid and can't find it. Does it have another name? Or does anyone have a picture? Thanks for the help already and I know that I'm not taking the advice really well, But I do appreciate it.
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Actual page #199 in the -24P-1 TM. Item #15 in the diagram.
From the -10 Operators Manual
Alright I decided to start with the small stuff and work my way up. SO I replaced the transfer lever switch and wires. Then I tried to start BigPOS and of course as soon as 59 psi is reached the horns start blowing. I unplugged the horn switch and horns kept blowing, Then I removed the switch from the parking brake and made sure nothing was grounding there and guess what! YEP THE DAG GONE HORNS ARE STILL BLOWING!!!!!!!
, I'm getting pretty darn sick of having an extremely expensive 22,000lb paper weight!
, I'm getting pretty darn sick of having an extremely expensive 22,000lb paper weight!
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Alright I decided to start with the small stuff and work my way up. SO I replaced the transfer lever switch and wires. Then I tried to start BigPOS and of course as soon as 59 psi is reached the horns start blowing. I unplugged the horn switch and horns kept blowing, Then I removed the switch from the parking brake and made sure nothing was grounding there and guess what! YEP THE DAG GONE HORNS ARE STILL BLOWING!!!!!!!
Yeah? I think we've all been there at one time or another! I find it best to step away. Don't want to "break" anything else on paperweight or my body. Give it a day or two.... (easier typed than done!) Looks like you've gotten some help. It's just a matter of "finding" the right thing. I hope ya find the answer, either from another member here on S.S. or yourself. WHEN it's fixed, you'll look back and laugh. (not until then though) Good luck and God's speed!
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It's either part of the joy of owing one, or it's the bane of owning one. Sorry for your luck! Don't know if you'd be better of with "another one" instead of this one? Some vehicles are rolling gremlin collectors! LOL Murphy always seems to have a seat somewhere for the long haul. Others? They've never heard of "murphy" and drive through a flood, tornado, and blizzard with just their rust and such holding them together. I truly wish you well! I pray that you find the cause, get it fixed, and are trouble free for the rest of your ownership!
It sounds to me like the air valve on the horn is always allowing air past it to the horns regardless of wheather or not the horn switch is activated. The horns probably do not produce sound at less than 59psi. The constant air drain from the horn air valve is keeping the pressure from building.
Plumber I appreciate the motivational words, Very kind of you good sir! Which Is why I started looking at the schematics today.
Hulk I agree with why the pressure isn't building. At this point I can't tell if the original problem still exists. As soon as there is enough air in the system you can hear the horn blowing although it's a slight sound this where the air is escaping now.
So I wanted to look at the schematics and see if there is a connection between the transfer case switch and the horns and the only thing I could find is in the picture. Does anyone know if when the lever shorted out that could've shorted the air horn solenoid? Or maybe a cheaper circuit breaker? The yellow line is circuit 585 that turns into 37 which ends up as 10. The green line is 26 from the horn that ends up 10 as well. Is this the connection that is causing the horn problem? Thanks again for any and all help.
Hulk I agree with why the pressure isn't building. At this point I can't tell if the original problem still exists. As soon as there is enough air in the system you can hear the horn blowing although it's a slight sound this where the air is escaping now.
So I wanted to look at the schematics and see if there is a connection between the transfer case switch and the horns and the only thing I could find is in the picture. Does anyone know if when the lever shorted out that could've shorted the air horn solenoid? Or maybe a cheaper circuit breaker? The yellow line is circuit 585 that turns into 37 which ends up as 10. The green line is 26 from the horn that ends up 10 as well. Is this the connection that is causing the horn problem? Thanks again for any and all help.