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Thread: Transmission issue, help!

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    Default Transmission issue, help!

    I searched the forum, got some good reading in but have a strange symptom with the trans and truck is disabled. I have the old school 3 speed 3L80.I will do some troubleshooting but maybe someone can narrow down my search. I was driving the truck and it started slowing while the rpm's increased. Pretty soon it acted like it was in neutral and would not move forward or reverse. I checked the fluid, it's red, full and no burnt smell. I checked the the transfer case by shifting into different ranges, no change. The drive lever linkage is connected and the rod underneath is being actuated normally with the lever. The symptom that someone might make sense of is that if I shut down the truck and restart (either hot or cold) it shifts normally and will move normally (in either forward or reverse) a short time... but will immediately not work if I shift from drive to reverse or the opposite. I can then shut it off, start it again right away, it goes into gear normally then the trouble will start again. once the issue has started it doesnt matter how much I move the lever it doesnt catch the slightest in any position. Shutdown and restart motor, acts normal again for short time! I know little about trannys but could the fact that it works on start but quickly fades be some symptom that enough fluid is pooled somewhere on start to function but not replenished somehow by the system? Thank you so much for any inputs!
    Last edited by Navy dude; 05-14-2019 at 18:20.

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    Which transmission?

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    So sorry, I have the old school 3 speed. TH400 I believe.... I'll edit above.

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    4 Star General Wire Fox's Avatar
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    Run your engine, shift though all the gears, then place it in park while on a level surface. With the engine still running, tell us what the fluid level is like. This really sounds like once you get running and the fluid passages fill up, your transmission is starved for fluid and loses engagement. This would make for a delightfully easy fix if it's the case.

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    Quote Originally Posted by Wire Fox View Post
    Run your engine, shift though all the gears, then place it in park while on a level surface. With the engine still running, tell us what the fluid level is like. This really sounds like once you get running and the fluid passages fill up, your transmission is starved for fluid and loses engagement. This would make for a delightfully easy fix if it's the case.

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    It's full when running, if anything slightly overfull. Nice pinkish red color.... My brother in law the mechanic says he will test the pressures says it looks like some type of "apply" issue. He has never worked on a HMMWV but says the th400 is simple and easy. My next plan is to download the TM for troubleshooting so that anything specific to a HMMWV could be helpful to him. I think I will first drop the pan and service the filter to start, even I can do that. He said that he traced the trans lines to the cooler and opened the purge valve (my interpretation) and no fluid sprayed out as he expected it would (once again my interpretation over the phone... I limped the truck into my father in laws driveway and left it, and my brother in law stopped by quickly to take a look). He also just reported that even when it first starts it barely goes into gear.
    Last edited by Navy dude; 05-14-2019 at 18:45.

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    Unless it's been added there is no PARK in a HMMWV TH400 transmission. Here's a link to a You Tube video on pressure testing a TH400 trans: It could be something as simple as the pressure relief spring on the pump breaking but I don't think it's going to get fixed without dropping the trans.
    Last edited by papakb; 05-15-2019 at 16:54.

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    I've seen a partially clogged filter have these symptoms.

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    I'll put my wager on clutch pack piston seal giving up. The more you try, especially if you give it RPM, the more damage it will do to the clutches and particularly the aluminum piston. Check for grit in the pan oil from the decaying frictions and (hope not) aluminum balls from the melting aluminum piston. This would seem to have the smell of burnt oil, so if tranny oil is not carmel, it may not be the problem. Pressure testing will tell if the seal(s) is leaking or worse.
    Last edited by tobyS; 05-15-2019 at 14:08.
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    Thanks so much, guys. I have a filter on order will see whatís in the pan once it gets here. Iíll follow up with results....

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    to me it sounds like a bad pump or torque converter but the lack of burned oil leads me to think pump also if the pump goes bat there will me no circulation through the cooler

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