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Thread: M925 front axle will not lock in freezing temps

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    Default M925 front axle will not lock in freezing temps

    I am new to using the forums. I would like to discuss workarounds related to the front axle lock not engaging in freezing weather. Thank you.

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    Are your wipers and horn working? When you can not lock the front and in. If so the pressure protection valve is freezing up. Which means too much moisture in your system. Drain your tanks daily and if you have not serviced your air dryer lately you should or replace it.
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    Thank you for your prompt reply. I will try to develop the relevant issues, so we can build upon what we learn here.
    1) I always drain the air promptly but is seems like the air system for the front axle freezes anyway. From what you said I now assume that the air actuator system for the front axle, the wipers and the horn are on tank #3, separate from that for the air brakes and other hydraulics. I believe there are at least 3 air tanks (maybe 4).
    2) I am not currently frozen, so I do not know if the horn and wipers are working when I have this problem. I will check that next time I freeze up and also whether tank #3 bleeds). When the front axle switching is working in the warm you can hear the air release and the mechanism switch in the axle. When it is frozen, you hear nothing on switching the switch.
    3) I do not think the pressure protection valve is freezing up because I can operate the truck and it achieves 150 psi and bleeds as necessary even when the front axle actuator system is frozen.
    4) I like the idea of replacing the air dryer, although I already added a methanol injection system and it appears to keep the brake system from freezing up, but does not seem to affect the air system that powers the air actuator system for the front axle.
    5) I assume the methanol is getting to the air actuator system but I am not sure. It seems like what could be freezing is 1) air actuator switch on the dash, air actuator valve switch under the truck, air lines, or tank that holds the air for this system (#3?)f, any other items?
    6) Going to low range does not help because switching the axle in this way still depends on the same air actuator infrastructure to switch the front axle.
    7) Everything works fine unless it is freezing, once the truck warms up in the sun or the temp comes up outside, the front axle switches in and you can hear the air being switched by the switch. If it is really cold the system will not unfreeze.
    I wish the truck would stay with the front axle locked when you turn it off, but the air bleeds off and the front axle lock releases. After that, if you start it in the cold the air actuator system for the front axle is frozen again. If there was a way to lock in the front axle in without the air that would be great, but at this point it depends on the air system to maintain the front axle lock.

    Thank you for your help, looking for more ideas.

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    Your wipers. 6x6 and horn get all their air from the wet tank which has a pressure protection valve. It is the first tank in the system after the air dryer. Why it freezes up cause the wet tank catches water and the ppv is at a low spot. The whole air line could be freezing up from residue in it as well

    Last edited by simp5782; 12-23-2018 at 19:41.
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    Item #3 convinces me it's the PPV. Nothing downstream from the PPV is seeing air. This includes your air dryer expello valve and your air governor.

    The pressure relief valve venting at 150psi is separate from your air dryer which should sneeze at 120psi or so.

    The culprit is moisture in the system. Personally I'd ditch the methanol system and just make sure the air dryer is working and that the air is dry....

    Edit to add -

    PPV = pressure protection valve. Opens around 70 psi to feed air to noncritical systems.

    PRV = pressure relief valve. Pops at 150psi when the governor fails to stop building air. This protects the entire system from excessive pressure. This sounds similar to the air dryer venting but it's louder and will recur about every 15 seconds.
    Last edited by 98G; 12-23-2018 at 21:15. Reason: Autocorrupt cursed it

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    i appreciate you sending me the video and explaining things, very helpful.

    If I understand correctly the wet tank supplies the axle lock actuator system, is this true? What else other than the wipers and the horn does it supply?

    I don't understand why the truck roles and the air brakes function when the wet tank is frozen up and things downstream such as the air dryer are not getting air. The only symptom I experience is the front axle not engaging and making air noises.

    I think the methanol injector is connected to the air dryer, I don't know whether it goes to the wet tank.

    Why do you think the other tanks (two for the air brakes) and other air stuff continue to function independent of the frozen wet tank?

    I think you are helping me see that if the front axle actuator freezes up a workaround might be to warm the PPV and the wet tank to get things working.

    I am considering getting an upgraded air dryer (expensive), but it would be great to know what to heat to get things working.

    Thank you again for your ideas and suggestions.




    downstream from the PPV is seeing air. This includes your air dryer expello valve and your air governor.


    The culprit is moisture in the system. Personally I'd ditch the methanol system and just make sure the air dryer is working and that the air is dry....
    Last edited by SteelBrett; 12-23-2018 at 22:28.

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    Quote Originally Posted by SteelBrett View Post
    Thank you for your prompt reply. I will try to develop the relevant issues, so we can build upon what we learn here.
    1) I always drain the air promptly but is seems like the air system for the front axle freezes anyway. From what you said I now assume that the air actuator system for the front axle, the wipers and the horn are on tank #3, separate from that for the air brakes and other hydraulics. I believe there are at least 3 air tanks (maybe 4).
    2) I am not currently frozen, so I do not know if the horn and wipers are working when I have this problem. I will check that next time I freeze up and also whether tank #3 bleeds). When the front axle switching is working in the warm you can hear the air release and the mechanism switch in the axle. When it is frozen, you hear nothing on switching the switch.
    3) I do not think the pressure protection valve is freezing up because I can operate the truck and it achieves 150 psi and bleeds as necessary even when the front axle actuator system is frozen.
    4) I like the idea of replacing the air dryer, although I already added a methanol injection system and it appears to keep the brake system from freezing up, but does not seem to affect the air system that powers the air actuator system for the front axle.
    5) I assume the methanol is getting to the air actuator system but I am not sure. It seems like what could be freezing is 1) air actuator switch on the dash, air actuator valve switch under the truck, air lines, or tank that holds the air for this system (#3?)f, any other items?
    6) Going to low range does not help because switching the axle in this way still depends on the same air actuator infrastructure to switch the front axle.
    7) Everything works fine unless it is freezing, once the truck warms up in the sun or the temp comes up outside, the front axle switches in and you can hear the air being switched by the switch. If it is really cold the system will not unfreeze.
    I wish the truck would stay with the front axle locked when you turn it off, but the air bleeds off and the front axle lock releases. After that, if you start it in the cold the air actuator system for the front axle is frozen again. If there was a way to lock in the front axle in without the air that would be great, but at this point it depends on the air system to maintain the front axle lock.

    Thank you for your help, looking for more ideas.
    1. Yes, all three of those are on the back side of the PPV. If your PPV sticks close, you will lose all 3 of these functions and the compressor will constantly run since the govenor no longer sees pressure.

    2. Not really critical to the problem but the front axle is engaged through the T-case, not the axle itself. Just so you know for future reference.

    3. The PPV is freezing. You are hearing the pressure relief valve purge, not the air dryer. The pressure relief valve purges at 150 psi and the air dryer purges around 125-130 psi depending on how the govenor is set.

    The govenor is also on the back side of the PPV. When the PPV freezes, the govenor doesn't see pressure so it cuts in the unloader to run the compressor. There actually is pressure in the system though, so pressure continues to build in the rest of the system. Once the pressure reaches 150 psi, the pressure relief valve releases pressure to save system components. The compressor is constantly running though which is why you're constantly seeing 150 psi on the gauges. When the PPV is working, normal operating pressure is around 120 psi.

    The secondary problem with the compressor running constantly is that you're creating even more eclxcessive moisture in the system. This just makes the problem worse.

    4. Is the methonal system plumbed in before or after the air dryer? If you want to run a methonal system, it should always be plumbed in after the air dryer. Running methonal through the air dryer destroys the desiccant in the filter and most of the methonal will be removed by the air dryer anyways.

    5. None of the items you listed are the problem. It's your PPV that's freezing. Grab a pack of those Hot Hands hand warmers, a zip-tie and a Zip lock bag. Activate the Hot Hands then place them in the bag. Position one warmer on each side of the PPV and zip-tie the bag in place. All three of these items are sold at almost any gas station and all three can be stored in definately in the truck. The bag just keeps the warmers from getting wet. This isn't a solution to the problem, just something to get you moving again.

    6. Correct, low range has nothing to with your problem. If you're truck is stock then low range is connected to the front axle engagement system in a way, but it's not related to the problem here.

    7. If this is strictly an offroad truck, there are ways to make this possible. It would no longer be selectable though...well not easily anyways. If you drive on the road at all, I'd leave it as is. Fix the moisture problem and you won't be looking to keep it locked in anyways because everything will work as it should.
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    Have you thought about taken air lines off to the different components and seen if air is present. While u have them off, you can add a couple drops of air tool oil to the valves, etc..... This soldiers sat a lot of times and things get stuck or dry up.
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    Quote Originally Posted by smoke View Post
    Have you thought about taken air lines off to the different components and seen if air is present. While u have them off, you can add a couple drops of air tool oil to the valves, etc..... This soldiers sat a lot of times and things get stuck or dry up.
    Thing is.... there's not really a reason to further troubleshoot. This is textbook (TM?) PPV failure due to moisture.

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